Configuring and managing logical volumes

Red Hat Enterprise Linux 9

Configuring and managing the LVM on RHEL

Red Hat Customer Content Services

Abstract

Logical volume management (LVM) creates a layer of abstraction over physical storage to create a logical storage volume, which is a virtual block storage device that a file system, database, or application can use. The physical volume (PV) is either a partition or a whole disk. By using these PVs, you can create a volume group (VG) to create a pool of disk space for the logical volumes (LV) from the available storage.
You can create a logical volume (LV) by combining physical volumes into a volume group. LV provides more flexibility than using physical storage, and the created LVs can be extended or reduced without repartitioning or reformatting the physical device.
You can also perform several advanced operations with the LVM, such as creating thin-provisioned logical volumes, snapshots of the original volume, RAID volumes, cache volumes, and striped logical volumes.

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Chapter 1. Overview of logical volume management

Logical volume management (LVM) creates a layer of abstraction over physical storage, which helps you to create logical storage volumes. This provides much greater flexibility in a number of ways than using physical storage directly.

In addition, the hardware storage configuration is hidden from the software so it can be resized and moved without stopping applications or unmounting file systems. This can reduce operational costs.

1.1. LVM architecture

The following are the components of LVM:

Physical volume
A physical volume (PV) is a partition or whole disk designated for LVM use. For more information, see Managing LVM physical volumes.
Volume group
A volume group (VG) is a collection of physical volumes (PVs), which creates a pool of disk space out of which logical volumes can be allocated. For more information, see Managing LVM volume groups.
Logical volume
A logical volume represents a mountable storage device. For more information, see Managing LVM logical volumes.

The following diagram illustrates the components of LVM:

Figure 1.1. LVM logical volume components

LVM Logical Volume Components

1.2. Advantages of LVM

Logical volumes provide the following advantages over using physical storage directly:

Flexible capacity
When using logical volumes, you can aggregate devices and partitions into a single logical volume. With this functionality, file systems can extend across multiple devices as though they were a single, large one.
Convenient device naming
Logical storage volumes can be managed with user-defined and custom names.
Resizeable storage volumes
You can extend logical volumes or reduce logical volumes in size with simple software commands, without reformatting and repartitioning the underlying devices. For more information, see Modifying the size of a logical volume.
Online data relocation

To deploy newer, faster, or more resilient storage subsystems, you can move data while your system is active using the pvmove command. Data can be rearranged on disks while the disks are in use. For example, you can empty a hot-swappable disk before removing it.

For more information on how to migrate the data, see the pvmove man page and Removing physical volumes from a volume group.

Striped Volumes
You can create a logical volume that stripes data across two or more devices. This can dramatically increase throughput. For more information, see Extending a striped logical volume.
RAID volumes
Logical volumes provide a convenient way to configure RAID for your data. This provides protection against device failure and improves performance. For more information, see Configuring RAID logical volumes.
Volume snapshots
You can take snapshots, which is a point-in-time copy of logical volumes for consistent backups or to test the effect of changes without affecting the real data. For more information, see Snapshot of logical volumes.
Thin volumes
Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available physical space. For more information, see Creating and managing thin provisioned volumes (thin volumes).
Cache volumes
A cache logical volume uses a fast block device, such as an SSD drive to improve the performance of a larger and slower block device. For more information, see Enabling caching to improve logical volume performance.

Additional resources

Chapter 2. Managing local storage using RHEL System Roles

To manage LVM and local file systems (FS) using Ansible, you can use the storage role, which is one of the RHEL System Roles available in RHEL 9.

Using the storage role enables you to automate administration of file systems on disks and logical volumes on multiple machines and across all versions of RHEL starting with RHEL 7.7.

For more information about RHEL System Roles and how to apply them, see Introduction to RHEL System Roles.

2.1. Introduction to the storage RHEL System Role

The storage role can manage:

  • File systems on disks which have not been partitioned
  • Complete LVM volume groups including their logical volumes and file systems
  • MD RAID volumes and their file systems

With the storage role, you can perform the following tasks:

  • Create a file system
  • Remove a file system
  • Mount a file system
  • Unmount a file system
  • Create LVM volume groups
  • Remove LVM volume groups
  • Create logical volumes
  • Remove logical volumes
  • Create RAID volumes
  • Remove RAID volumes
  • Create LVM volume groups with RAID
  • Remove LVM volume groups with RAID
  • Create encrypted LVM volume groups
  • Create LVM logical volumes with RAID

2.2. Parameters that identify a storage device in the storage RHEL System Role

Your storage role configuration affects only the file systems, volumes, and pools that you list in the following variables.

storage_volumes

List of file systems on all unpartitioned disks to be managed.

storage_volumes can also include raid volumes.

Partitions are currently unsupported.

storage_pools

List of pools to be managed.

Currently the only supported pool type is LVM. With LVM, pools represent volume groups (VGs). Under each pool there is a list of volumes to be managed by the role. With LVM, each volume corresponds to a logical volume (LV) with a file system.

2.3. Example Ansible playbook to create an XFS file system on a block device

The example Ansible playbook applies the storage role to create an XFS file system on a block device using the default parameters.

Warning

The storage role can create a file system only on an unpartitioned, whole disk or a logical volume (LV). It cannot create the file system on a partition.

Example 2.1. A playbook that creates XFS on /dev/sdb

---
- hosts: all
  vars:
    storage_volumes:
      - name: barefs
        type: disk
        disks:
          - sdb
        fs_type: xfs
  roles:
    - rhel-system-roles.storage
  • The volume name (barefs in the example) is currently arbitrary. The storage role identifies the volume by the disk device listed under the disks: attribute.
  • You can omit the fs_type: xfs line because XFS is the default file system in RHEL 9.
  • To create the file system on an LV, provide the LVM setup under the disks: attribute, including the enclosing volume group. For details, see Example Ansible playbook to manage logical volumes.

    Do not provide the path to the LV device.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.4. Example Ansible playbook to persistently mount a file system

The example Ansible applies the storage role to immediately and persistently mount an XFS file system.

Example 2.2. A playbook that mounts a file system on /dev/sdb to /mnt/data

---
- hosts: all
  vars:
    storage_volumes:
      - name: barefs
        type: disk
        disks:
          - sdb
        fs_type: xfs
        mount_point: /mnt/data
        mount_user: somebody
        mount_group: somegroup
        mount_mode: 0755
  roles:
    - rhel-system-roles.storage
  • This playbook adds the file system to the /etc/fstab file, and mounts the file system immediately.
  • If the file system on the /dev/sdb device or the mount point directory do not exist, the playbook creates them.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.5. Example Ansible playbook to manage logical volumes

The example Ansible playbook applies the storage role to create an LVM logical volume in a volume group.

Example 2.3. A playbook that creates a mylv logical volume in the myvg volume group

- hosts: all
  vars:
    storage_pools:
      - name: myvg
        disks:
          - sda
          - sdb
          - sdc
        volumes:
          - name: mylv
            size: 2G
            fs_type: ext4
            mount_point: /mnt/data
  roles:
    - rhel-system-roles.storage
  • The myvg volume group consists of the following disks:

    • /dev/sda
    • /dev/sdb
    • /dev/sdc
  • If the myvg volume group already exists, the playbook adds the logical volume to the volume group.
  • If the myvg volume group does not exist, the playbook creates it.
  • The playbook creates an Ext4 file system on the mylv logical volume, and persistently mounts the file system at /mnt.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.6. Example Ansible playbook to enable online block discard

The example Ansible playbook applies the storage role to mount an XFS file system with online block discard enabled.

Example 2.4. A playbook that enables online block discard on /mnt/data/

---
- hosts: all
  vars:
    storage_volumes:
      - name: barefs
        type: disk
        disks:
          - sdb
        fs_type: xfs
        mount_point: /mnt/data
        mount_options: discard
  roles:
    - rhel-system-roles.storage

Additional resources

2.7. Example Ansible playbook to create and mount an Ext4 file system

The example Ansible playbook applies the storage role to create and mount an Ext4 file system.

Example 2.5. A playbook that creates Ext4 on /dev/sdb and mounts it at /mnt/data

---
- hosts: all
  vars:
    storage_volumes:
      - name: barefs
        type: disk
        disks:
          - sdb
        fs_type: ext4
        fs_label: label-name
        mount_point: /mnt/data
  roles:
    - rhel-system-roles.storage
  • The playbook creates the file system on the /dev/sdb disk.
  • The playbook persistently mounts the file system at the /mnt/data directory.
  • The label of the file system is label-name.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.8. Example Ansible playbook to create and mount an ext3 file system

The example Ansible playbook applies the storage role to create and mount an Ext3 file system.

Example 2.6. A playbook that creates Ext3 on /dev/sdb and mounts it at /mnt/data

---
- hosts: all
  vars:
    storage_volumes:
      - name: barefs
        type: disk
        disks:
          - sdb
        fs_type: ext3
        fs_label: label-name
        mount_point: /mnt/data
        mount_user: somebody
        mount_group: somegroup
        mount_mode: 0755
  roles:
    - rhel-system-roles.storage
  • The playbook creates the file system on the /dev/sdb disk.
  • The playbook persistently mounts the file system at the /mnt/data directory.
  • The label of the file system is label-name.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.9. Example Ansible playbook to resize an existing file system on LVM using the storage RHEL System Role

The example Ansible playbook applies the storage RHEL System Role to resize an LVM logical volume with a file system.

Warning

Using the Resizing action in other file systems can destroy the data on the device you are working on.

Example 2.7. A playbook that resizes existing mylv1 and myvl2 logical volumes in the myvg volume group

---

- hosts: all
   vars:
    storage_pools:
      - name: myvg
        disks:
          - /dev/sda
          - /dev/sdb
          - /dev/sdc
        volumes:
            - name: mylv1
              size: 10 GiB
              fs_type: ext4
              mount_point: /opt/mount1
            - name: mylv2
              size: 50 GiB
              fs_type: ext4
              mount_point: /opt/mount2

- name: Create LVM pool over three disks
  include_role:
    name: rhel-system-roles.storage
  • This playbook resizes the following existing file systems:

    • The Ext4 file system on the mylv1 volume, which is mounted at /opt/mount1, resizes to 10 GiB.
    • The Ext4 file system on the mylv2 volume, which is mounted at /opt/mount2, resizes to 50 GiB.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.10. Example Ansible playbook to create a swap volume using the storage RHEL System Role

This section provides an example Ansible playbook. This playbook applies the storage role to create a swap volume, if it does not exist, or to modify the swap volume, if it already exist, on a block device using the default parameters.

Example 2.8. A playbook that creates or modify an existing XFS on /dev/sdb

---
- name: Create a disk device with swap
- hosts: all
  vars:
    storage_volumes:
      - name: swap_fs
        type: disk
        disks:
          - /dev/sdb
	size: 15 GiB
        fs_type: swap
  roles:
    - rhel-system-roles.storage
  • The volume name (swap_fs in the example) is currently arbitrary. The storage role identifies the volume by the disk device listed under the disks: attribute.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.11. Configuring a RAID volume using the storage System Role

With the storage System Role, you can configure a RAID volume on RHEL using Red Hat Ansible Automation Platform and Ansible-Core. Create an Ansible playbook with the parameters to configure a RAID volume to suit your requirements.

Prerequisites

  • The Ansible Core package is installed on the control machine.
  • You have the rhel-system-roles package installed on the system from which you want to run the playbook.
  • You have an inventory file detailing the systems on which you want to deploy a RAID volume using the storage System Role.

Procedure

  1. Create a new playbook.yml file with the following content:

    ---
    - name: Configure the storage
      hosts: managed-node-01.example.com
      tasks:
      - name: Create a RAID on sdd, sde, sdf, and sdg
        include_role:
          name: rhel-system-roles.storage
        vars:
        storage_safe_mode: false
        storage_volumes:
          - name: data
            type: raid
            disks: [sdd, sde, sdf, sdg]
            raid_level: raid0
            raid_chunk_size: 32 KiB
            mount_point: /mnt/data
            state: present
    Warning

    Device names might change in certain circumstances, for example, when you add a new disk to a system. Therefore, to prevent data loss, do not use specific disk names in the playbook.

  2. Optional: Verify the playbook syntax:

    # ansible-playbook --syntax-check playbook.yml
  3. Run the playbook:

    # ansible-playbook -i inventory.file /path/to/file/playbook.yml

Additional resources

2.12. Configuring an LVM pool with RAID using the storage RHEL System Role

With the storage System Role, you can configure an LVM pool with RAID on RHEL using Red Hat Ansible Automation Platform. You can set up an Ansible playbook with the available parameters to configure an LVM pool with RAID.

Prerequisites

  • The Ansible Core package is installed on the control machine.
  • You have the rhel-system-roles package installed on the system from which you want to run the playbook.
  • You have an inventory file detailing the systems on which you want to configure an LVM pool with RAID using the storage System Role.

Procedure

  1. Create a new playbook.yml file with the following content:

    - hosts: all
      vars:
        storage_safe_mode: false
        storage_pools:
          - name: my_pool
            type: lvm
            disks: [sdh, sdi]
            raid_level: raid1
            volumes:
              - name: my_volume
                size: "1 GiB"
                mount_point: "/mnt/app/shared"
                fs_type: xfs
                state: present
      roles:
        - name: rhel-system-roles.storage
    Note

    To create an LVM pool with RAID, you must specify the RAID type using the raid_level parameter.

  2. Optional. Verify playbook syntax.

    # ansible-playbook --syntax-check playbook.yml
  3. Run the playbook on your inventory file:

    # ansible-playbook -i inventory.file /path/to/file/playbook.yml

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.13. Configuring a stripe size for RAID LVM volumes using the storage RHEL System Role

With the storage System Role, you can configure a stripe size for RAID LVM volumes on RHEL using Red Hat Ansible Automation Platform. You can set up an Ansible playbook with the available parameters to configure an LVM pool with RAID.

Prerequisites

  • The Ansible Core package is installed on the control machine.
  • You have the rhel-system-roles package installed on the system from which you want to run the playbook.
  • You have an inventory file detailing the systems on which you want to configure an LVM pool with RAID using the storage System Role.

Procedure

  1. Create a new playbook.yml file with the following content:

    hosts: all
      vars:
        storage_safe_mode: false
        storage_pools:
          - name: my_pool
            type: lvm
            disks: [sdh, sdi]
            volumes:
              - name: my_volume
                size: "1 GiB"
                mount_point: "/mnt/app/shared"
                fs_type: xfs
                raid_level: raid1
                raid_stripe_size: "256 KiB"
                state: present
      roles:
        - name: rhel-system-roles.storage
  2. Optional. Verify playbook syntax:

    # ansible-playbook --syntax-check playbook.yml
  3. Run the playbook on your inventory file:

    # ansible-playbook -i inventory.file /path/to/file/playbook.yml

Additional resources

  • Managing RAID
  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

2.14. Example Ansible playbook to compress and deduplicate a VDO volume on LVM using the storage RHEL System Role

The example Ansible playbook applies the storage RHEL System Role to enable compression and deduplication of Logical Volumes (LVM) using Virtual Data Optimizer (VDO).

Example 2.9. A playbook that creates a mylv1 LVM VDO volume in the myvg volume group

---
- name: Create LVM VDO volume under volume group 'myvg'
  hosts: all
  roles:
    -rhel-system-roles.storage
  vars:
    storage_pools:
     - name: myvg
       disks:
         - /dev/sdb
       volumes:
         - name: mylv1
           compression: true
           deduplication: true
           vdo_pool_size: 10 GiB
           size: 30 GiB
           mount_point: /mnt/app/shared

In this example, the compression and deduplication pools are set to true, which specifies that the VDO is used. The following describes the usage of these parameters:

  • The deduplication is used to deduplicate the duplicated data stored on the storage volume.
  • The compression is used to compress the data stored on the storage volume, which results in more storage capacity.
  • The vdo_pool_size specifies the actual size the volume takes on the device. The virtual size of VDO volume is set by the size parameter. NOTE: Because of the Storage role use of LVM VDO, only one volume per pool can use the compression and deduplication.

2.15. Creating a LUKS2 encrypted volume using the storage RHEL System Role

You can use the storage role to create and configure a volume encrypted with LUKS by running an Ansible playbook.

Prerequisites

  • Access and permissions to one or more managed nodes, which are systems you want to configure with the crypto_policies System Role.
  • An inventory file, which lists the managed nodes.
  • Access and permissions to a control node, which is a system from which Red Hat Ansible Core configures other systems. On the control node, the ansible-core and rhel-system-roles packages are installed.
Important

RHEL 8.0-8.5 provided access to a separate Ansible repository that contains Ansible Engine 2.9 for automation based on Ansible. Ansible Engine contains command-line utilities such as ansible, ansible-playbook, connectors such as docker and podman, and many plugins and modules. For information about how to obtain and install Ansible Engine, see the How to download and install Red Hat Ansible Engine Knowledgebase article.

RHEL 8.6 and 9.0 have introduced Ansible Core (provided as the ansible-core package), which contains the Ansible command-line utilities, commands, and a small set of built-in Ansible plugins. RHEL provides this package through the AppStream repository, and it has a limited scope of support. For more information, see the Scope of support for the Ansible Core package included in the RHEL 9 and RHEL 8.6 and later AppStream repositories Knowledgebase article.

Procedure

  1. Create a new playbook.yml file with the following content:

    - hosts: all
      vars:
        storage_volumes:
          - name: barefs
            type: disk
            disks:
             - sdb
            fs_type: xfs
            fs_label: label-name
            mount_point: /mnt/data
            encryption: true
            encryption_password: your-password
      roles:
       - rhel-system-roles.storage

    You can also add the other encryption parameters such as encryption_key, encryption_cipher, encryption_key_size, and encryption_luks version in the playbook.yml file.

  2. Optional: Verify playbook syntax:

    # ansible-playbook --syntax-check playbook.yml
  3. Run the playbook on your inventory file:

    # ansible-playbook -i inventory.file /path/to/file/playbook.yml

Verification

  1. View the encryption status:

    # cryptsetup status sdb
    
    /dev/mapper/sdb is active and is in use.
    type: LUKS2
    cipher: aes-xts-plain64
    keysize: 512 bits
    key location: keyring
    device: /dev/sdb
    [...]
  2. Verify the created LUKS encrypted volume:

    # cryptsetup luksDump /dev/sdb
    
    Version:       	2
    Epoch:         	6
    Metadata area: 	16384 [bytes]
    Keyslots area: 	33521664 [bytes]
    UUID:          	a4c6be82-7347-4a91-a8ad-9479b72c9426
    Label:         	(no label)
    Subsystem:     	(no subsystem)
    Flags:       	allow-discards
    
    Data segments:
      0: crypt
    	offset: 33554432 [bytes]
    	length: (whole device)
    	cipher: aes-xts-plain64
    	sector: 4096 [bytes]
    [...]
  3. View the cryptsetup parameters in the playbook.yml file, which the storage role supports:

    # cat ~/playbook.yml
    
        - hosts: all
          vars:
            storage_volumes:
              - name: foo
                type: disk
                disks:
                 - nvme0n1
                fs_type: xfs
                fs_label: label-name
                mount_point: /mnt/data
                encryption: true
                #encryption_password: passwdpasswd
                encryption_key: /home/passwd_key
                encryption_cipher: aes-xts-plain64
                encryption_key_size: 512
                encryption_luks_version: luks2
    
          roles:
           - rhel-system-roles.storage

Additional resources

2.16. Example Ansible playbook to express pool volume sizes as percentage using the storage RHEL System Role

The example Ansible playbook applies the storage System Role to enable you to express Logical Manager Volumes (LVM) volume sizes as a percentage of the pool’s total size.

Example 2.10. A playbook that express volume sizes as a percentage of the pool’s total size

---
- name: Express volume sizes as a percentage of the pool's total size
  hosts: all
  roles
    - rhel-system-roles.storage
  vars:
    storage_pools:
    - name: myvg
      disks:
        - /dev/sdb
      volumes:
        - name: data
          size: 60%
          mount_point: /opt/mount/data
        - name: web
          size: 30%
          mount_point: /opt/mount/web
        - name: cache
          size: 10%
          mount_point: /opt/cache/mount

This example specifies the size of LVM volumes as a percentage of the pool size, for example: "60%". Additionally, you can also specify the size of LVM volumes as a percentage of the pool size in a human-readable size of the file system, for example, "10g" or "50 GiB".

2.17. Additional resources

  • /usr/share/doc/rhel-system-roles/storage/
  • /usr/share/ansible/roles/rhel-system-roles.storage/

Chapter 3. Managing LVM physical volumes

The physical volume (PV) is a partition or whole disk designated for LVM use. To use the device for an LVM logical volume, the device must be initialized as a physical volume.

If you are using a whole disk device for your physical volume, the disk must have no partition table. For DOS disk partitions, the partition id should be set to 0x8e using the fdisk or cfdisk command or an equivalent. If you are using a whole disk device for your physical volume, the disk must have no partition table. Any existing partition table must be erased, which will effectively destroy all data on that disk. You can remove an existing partition table using the wipefs -a <PhysicalVolume>` command as root.

3.1. Overview of physical volumes

Initializing a block device as a physical volume places a label near the start of the device. The following describes the LVM label:

  • An LVM label provides correct identification and device ordering for a physical device. An unlabeled, non-LVM device can change names across reboots depending on the order they are discovered by the system during boot. An LVM label remains persistent across reboots and throughout a cluster.
  • The LVM label identifies the device as an LVM physical volume. It contains a random unique identifier, the UUID for the physical volume. It also stores the size of the block device in bytes, and it records where the LVM metadata will be stored on the device.
  • By default, the LVM label is placed in the second 512-byte sector. You can overwrite this default setting by placing the label on any of the first 4 sectors when you create the physical volume. This allows LVM volumes to co-exist with other users of these sectors, if necessary.

The following describes the LVM metadata:

  • The LVM metadata contains the configuration details of the LVM volume groups on your system. By default, an identical copy of the metadata is maintained in every metadata area in every physical volume within the volume group. LVM metadata is small and stored as ASCII.
  • Currently LVM allows you to store 0, 1, or 2 identical copies of its metadata on each physical volume. The default is 1 copy. Once you configure the number of metadata copies on the physical volume, you cannot change that number at a later time. The first copy is stored at the start of the device, shortly after the label. If there is a second copy, it is placed at the end of the device. If you accidentally overwrite the area at the beginning of your disk by writing to a different disk than you intend, a second copy of the metadata at the end of the device will allow you to recover the metadata.

The following diagram illustrates the layout of an LVM physical volume. The LVM label is on the second sector, followed by the metadata area, followed by the usable space on the device.

Note

In the Linux kernel and throughout this document, sectors are considered to be 512 bytes in size.

Figure 3.1. Physical volume layout

LVM Physical Volume Layout

Additional resources

3.2. Multiple partitions on a disk

You can create physical volumes (PV) out of disk partitions by using LVM.

Red Hat recommends that you create a single partition that covers the whole disk to label as an LVM physical volume for the following reasons:

Administrative convenience
It is easier to keep track of the hardware in a system if each real disk only appears once. This becomes particularly true if a disk fails.
Striping performance
LVM cannot tell that two physical volumes are on the same physical disk. If you create a striped logical volume when two physical volumes are on the same physical disk, the stripes could be on different partitions on the same disk. This would result in a decrease in performance rather than an increase.
RAID redundancy
LVM cannot determine that the two physical volumes are on the same device. If you create a RAID logical volume when two physical volumes are on the same device, performance and fault tolerance could be lost.

Although it is not recommended, there may be specific circumstances when you will need to divide a disk into separate LVM physical volumes. For example, on a system with few disks it may be necessary to move data around partitions when you are migrating an existing system to LVM volumes. Additionally, if you have a very large disk and want to have more than one volume group for administrative purposes then it is necessary to partition the disk. If you do have a disk with more than one partition and both of those partitions are in the same volume group, take care to specify which partitions are to be included in a logical volume when creating volumes.

Note that although LVM supports using a non-partitioned disk as physical volume, it is recommended to create a single, whole-disk partition because creating a PV without a partition can be problematic in a mixed operating system environment. Other operating systems may interpret the device as free, and overwrite the PV label at the beginning of the drive.

3.3. Creating LVM physical volume

This procedure describes how to create and label LVM physical volumes (PVs).

In this procedure, replace the /dev/vdb1, /dev/vdb2, and /dev/vdb3 with the available storage devices in your system.

Prerequisites

  • The lvm2 package is installed.

Procedure

  1. Create multiple physical volumes by using the space-delimited device names as arguments to the pvcreate command:

    # pvcreate /dev/vdb1 /dev/vdb2 /dev/vdb3
      Physical volume "/dev/vdb1" successfully created.
      Physical volume "/dev/vdb2" successfully created.
      Physical volume "/dev/vdb3" successfully created.

    This places a label on /dev/vdb1, /dev/vdb2, and /dev/vdb3, marking them as physical volumes belonging to LVM.

  2. View the created physical volumes by using any one of the following commands as per your requirement:

    1. The pvdisplay command, which provides a verbose multi-line output for each physical volume. It displays physical properties, such as size, extents, volume group, and other options in a fixed format:

      # pvdisplay
      --- NEW Physical volume ---
        PV Name               /dev/vdb1
        VG Name
        PV Size               1.00 GiB
      [..]
      --- NEW Physical volume ---
        PV Name               /dev/vdb2
        VG Name
        PV Size               1.00 GiB
      [..]
      --- NEW Physical volume ---
        PV Name               /dev/vdb3
        VG Name
        PV Size               1.00 GiB
      [..]
    2. The pvs command provides physical volume information in a configurable form, displaying one line per physical volume:

      # pvs
        PV         VG  Fmt    Attr    PSize      PFree
      /dev/vdb1        lvm2           1020.00m   0
      /dev/vdb2        lvm2           1020.00m   0
      /dev/vdb3        lvm2           1020.00m   0
    3. The pvscan command scans all supported LVM block devices in the system for physical volumes. You can define a filter in the lvm.conf file so that this command avoids scanning specific physical volumes:

      # pvscan
        PV  /dev/vdb1                      lvm2 [1.00 GiB]
        PV  /dev/vdb2                      lvm2 [1.00 GiB]
        PV  /dev/vdb3                      lvm2 [1.00 GiB]

Additional resources

  • pvcreate(8), pvdisplay(8), pvs(8), pvscan(8), and lvm(8) man pages

3.4. Removing LVM physical volumes

If a device is no longer required for use by LVM, you can remove the LVM label by using the pvremove command. Executing the pvremove command zeroes the LVM metadata on an empty physical volume.

Procedure

  1. Remove a physical volume:

    # pvremove /dev/vdb3
    Labels on physical volume "/dev/vdb3" successfully wiped.
  2. View the existing physical volumes and verify if the required volume is removed:

    # pvs
      PV         VG   Fmt    Attr    PSize      PFree
    /dev/vdb1  	    lvm2           1020.00m   0
    /dev/vdb2  	    lvm2           1020.00m   0

If the physical volume you want to remove is currently part of a volume group, you must remove it from the volume group with the vgreduce command. For more information, see Removing physical volumes from a volume group

Additional resources

  • pvremove(8) man page

3.5. Additional resources

Chapter 4. Managing LVM volume groups

A volume group (VG) is a collection of physical volumes (PVs), which creates a pool of disk space out of which logical volumes (LVs) can be allocated.

Within a volume group, the disk space available for allocation is divided into units of a fixed-size called extents. An extent is the smallest unit of space that can be allocated. Within a physical volume, extents are referred to as physical extents.

A logical volume is allocated into logical extents of the same size as the physical extents. The extent size is therefore the same for all logical volumes in the volume group. The volume group maps the logical extents to physical extents.

4.1. Creating LVM volume group

You can create an LVM volume group (VG) myvg using the /dev/vdb1 and /dev/vdb2 physical volumes (PVs). By default, when physical volumes are used to create a volume group, its disk space is divided into 4MB extents. This extent size is the minimum amount by which the logical volume can be increased or decreased in size. The extent size can be modified using the -s argument of the vgcreate command and large numbers of extents have no impact on I/O performance of the logical volume. You can put limits on the number of physical or logical volumes the volume group can have using the -p and -l arguments of the vgcreate command.

Prerequisites

  • The lvm2 package is installed.
  • One or more physical volumes are created. For more information about creating physical volumes, see Creating LVM physical volume.

Procedure

  1. Create a myvg VG using any of the following methods:

    • Without specifying any options:

      # vgcreate myvg /dev/vdb1 /dev/vdb2
       Volume group "myvg" successfully created.
    • By specifying the volume group extent size using the -s argument:

      # vgcreate -s 2 /dev/myvg /dev/vdb1 /dev/vdb2
      Volume group "myvg" successfully created.
    • By limiting the number of physical or logical volumes the VG can have using the -p and -l arguments:

      # vgcreate -l 1 /dev/myvg /dev/vdb1 /dev/vdb2
      Volume group "myvg" successfully created.
  2. View the created volume groups by using any one of the following commands according to your requirement:

    • The vgs command provides volume group information in a configurable form, displaying one line per volume group:

      # vgs
        VG    #PV #LV #SN  Attr  VSize   VFree
       myvg   2    0   0   wz-n  159.99g 159.99g
    • The vgdisplay command displays volume group properties such as size, extents, number of physical volumes, and other options in a fixed form. The following example shows the output of the vgdisplay command for the volume group myvg. To display all existing volume groups, do not specify a volume group:

      # vgdisplay myvg
        --- Volume group ---
        VG Name               myvg
        System ID
        Format                lvm2
        Metadata Areas        4
        Metadata Sequence No  6
        VG Access             read/write
      [..]
    • The vgscan command scans all supported LVM block devices in the system for volume group:

      # vgscan
        Found volume group "myvg" using metadata type lvm2
  3. Optional: Increase a volume group’s capacity by adding one or more free physical volumes:

    # vgextend myvg /dev/vdb3
    Physical volume "/dev/vdb3" successfully created.
    Volume group "myvg" successfully extended
  4. Optional: Rename an existing volume group:

    # vgrename myvg myvg1
    Volume group "myvg" successfully renamed to "myvg1"

Additional resources

  • vgcreate(8), vgextend(8), vgdisplay(8), vgs(8), vgscan(8), vgrename(8), and lvm(8) man pages

4.2. Combining LVM volume groups

To combine two volume groups into a single volume group, use the vgmerge command. You can merge an inactive "source" volume with an active or an inactive "destination" volume if the physical extent sizes of the volume are equal and the physical and logical volume summaries of both volume groups fit into the destination volume groups limits.

Procedure

  • Merge the inactive volume group databases into the active or inactive volume group myvg giving verbose runtime information:

    # vgmerge -v myvg databases

Additional resources

  • vgmerge(8) man page

4.3. Removing physical volumes from a volume group

To remove unused physical volumes (PVs) from a volume group (VG), use the vgreduce command. The vgreduce command shrinks a volume group’s capacity by removing one or more empty physical volumes. This frees those physical volumes to be used in different volume groups or to be removed from the system.

Procedure

  1. If the physical volume is still being used, migrate the data to another physical volume from the same volume group :

    # pvmove /dev/vdb3
      /dev/vdb3: Moved: 2.0%
     ...
      /dev/vdb3: Moved: 79.2%
     ...
      /dev/vdb3: Moved: 100.0%
  2. If there are not enough free extents on the other physical volumes in the existing volume group:

    1. Create a new physical volume from /dev/vdb4:

      # pvcreate /dev/vdb4
        Physical volume "/dev/vdb4" successfully created
    2. Add the newly created physical volume to the myvg volume group:

      # vgextend myvg /dev/vdb4
        Volume group "myvg" successfully extended
    3. Move the data from /dev/vdb3 to /dev/vdb4:

      # pvmove /dev/vdb3 /dev/vdb4
        /dev/vdb3: Moved: 33.33%
        /dev/vdb3: Moved: 100.00%
  3. Remove the physical volume /dev/vdb3 from the volume group:

    # vgreduce myvg /dev/vdb3
    Removed "/dev/vdb3" from volume group "myvg"

Verification

  • Verify that the /dev/vdb3 physical volume is removed from the myvg volume group:

    # pvs
      PV           VG    Fmt   Attr   PSize        PFree      Used
      /dev/vdb1 myvg  lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb2 myvg  lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb3   	    lvm2   a--    1020.00m   1008.00m    12.00m

Additional resources

  • vgreduce(8), pvmove(8), and pvs(8) man pages

4.4. Splitting a LVM volume group

If there is enough unused space on the physical volumes, a new volume group can be created without adding new disks.

In the initial setup, the volume group myvg consists of /dev/vdb1, /dev/vdb2, and /dev/vdb3. After completing this procedure, the volume group myvg will consist of /dev/vdb1 and /dev/vdb2, and the second volume group, yourvg, will consist of /dev/vdb3.

Prerequisites

  • You have sufficient space in the volume group. Use the vgscan command to determine how much free space is currently available in the volume group.
  • Depending on the free capacity in the existing physical volume, move all the used physical extents to other physical volume using the pvmove command. For more information, see Removing physical volumes from a volume group.

Procedure

  1. Split the existing volume group myvg to the new volume group yourvg:

    # vgsplit myvg yourvg /dev/vdb3
      Volume group "yourvg" successfully split from "myvg"
    Note

    If you have created a logical volume using the existing volume group, use the following command to deactivate the logical volume:

    # lvchange -a n /dev/myvg/mylv

    For more information about creating logical volumes, see Managing LVM logical volumes.

  2. View the attributes of the two volume groups:

    # vgs
      VG     #PV #LV #SN Attr   VSize  VFree
      myvg     2   1   0 wz--n- 34.30G 10.80G
      yourvg   1   0   0 wz--n- 17.15G 17.15G

Verification

  • Verify that the newly created volume group yourvg consists of /dev/vdb3 physical volume:

    # pvs
      PV           VG      Fmt   Attr   PSize        PFree      Used
      /dev/vdb1 myvg   lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb2 myvg   lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb3 yourvg lvm2   a--    1020.00m   1008.00m    12.00m

Additional resources

  • vgsplit(8), vgs(8), and pvs(8) man pages

4.5. Moving a volume group to another system

You can move an entire LVM volume group (VG) to another system using the following commands:

vgexport
Use this command on an existing system to make an inactive VG inaccessible to the system. Once the VG is inaccessible, you can detach its physical volumes (PV).
vgimport
Use this command on the other system to make the VG, which was inactive in the old system, accessible in the new system.

Prerequisites

  • No users are accessing files on the active volumes in the volume group that you are moving.

Procedure

  1. Unmount the mylv logical volume:

    # umount /dev/mnt/mylv
  2. Deactivate all logical volumes in the volume group, which prevents any further activity on the volume group:

    # vgchange -an myvg
    vgchange -- volume group "myvg" successfully deactivated
  3. Export the volume group to prevent it from being accessed by the system from which you are removing it.

    # vgexport myvg
    vgexport -- volume group "myvg" successfully exported
  4. View the exported volume group:

    # pvscan
      PV /dev/sda1    is in exported VG myvg [17.15 GB / 7.15 GB free]
      PV /dev/sdc1    is in exported VG myvg [17.15 GB / 15.15 GB free]
      PV /dev/sdd1   is in exported VG myvg [17.15 GB / 15.15 GB free]
      ...
  5. Shut down your system and unplug the disks that make up the volume group and connect them to the new system.
  6. Plug the disks into the new system and import the volume group to make it accessible to the new system:

    # vgimport myvg
    Note

    You can use the --force argument of the vgimport command to import volume groups that are missing physical volumes and subsequently run the vgreduce --removemissing command.

  7. Activate the volume group:

    # vgchange -ay myvg
  8. Mount the file system to make it available for use:

    # mkdir -p /mnt/myvg/users
    # mount /dev/myvg/users /mnt/myvg/users

Additional resources

  • vgimport(8), vgexport(8), and vgchange(8) man pages

4.6. Removing LVM volume groups

You can remove an existing volume group using the vgremove command.

Prerequisites

Procedure

  1. If the volume group exists in a clustered environment, stop the lockspace of the volume group on all other nodes. Use the following command on all nodes except the node where you are performing the removal:

    # vgchange --lockstop vg-name

    Wait for the lock to stop.

  2. Remove the volume group:

    # vgremove vg-name
      Volume group "vg-name" successfully removed

Additional resources

  • vgremove(8) man page

Chapter 5. Managing LVM logical volumes

A logical volume is a virtual, block storage device that a file system, database, or application can use. To create an LVM logical volume, the physical volumes (PVs) are combined into a volume group (VG). This creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated.

5.1. Overview of logical volumes

An administrator can grow or shrink logical volumes without destroying data, unlike standard disk partitions. If the physical volumes in a volume group are on separate drives or RAID arrays, then administrators can also spread a logical volume across the storage devices.

You can lose data if you shrink a logical volume to a smaller capacity than the data on the volume requires. Further, some file systems are not capable of shrinking. To ensure maximum flexibility, create logical volumes to meet your current needs, and leave excess storage capacity unallocated. You can safely extend logical volumes to use unallocated space, depending on your needs.

Important

On AMD, Intel, ARM systems, and IBM Power Systems servers, the boot loader cannot read LVM volumes. You must make a standard, non-LVM disk partition for your /boot partition. On IBM Z, the zipl boot loader supports /boot on LVM logical volumes with linear mapping. By default, the installation process always creates the / and swap partitions within LVM volumes, with a separate /boot partition on a physical volume.

The following are the different types of logical volumes:

Linear volumes
A linear volume aggregates space from one or more physical volumes into one logical volume. For example, if you have two 60GB disks, you can create a 120GB logical volume. The physical storage is concatenated.
Striped logical volumes

When you write data to an LVM logical volume, the file system lays the data out across the underlying physical volumes. You can control the way the data is written to the physical volumes by creating a striped logical volume. For large sequential reads and writes, this can improve the efficiency of the data I/O.

Striping enhances performance by writing data to a predetermined number of physical volumes in round-robin fashion. With striping, I/O can be done in parallel. In some situations, this can result in near-linear performance gain for each additional physical volume in the stripe.

RAID logical volumes
LVM supports RAID levels 0, 1, 4, 5, 6, and 10. RAID logical volumes are not cluster-aware. When you create a RAID logical volume, LVM creates a metadata subvolume that is one extent in size for every data or parity subvolume in the array.
Thin-provisioned logical volumes (thin volumes)
Using thin-provisioned logical volumes, you can create logical volumes that are larger than the available physical storage. Creating a thinly provisioned set of volumes allows the system to allocate what you use instead of allocating the full amount of storage that is requested
Snapshot volumes
The LVM snapshot feature provides the ability to create virtual images of a device at a particular instant without causing a service interruption. When a change is made to the original device (the origin) after a snapshot is taken, the snapshot feature makes a copy of the changed data area as it was prior to the change so that it can reconstruct the state of the device.
Thin-provisioned snapshot volumes
Using thin-provisioned snapshot volumes, you can have more virtual devices to be stored on the same data volume. Thinly provisioned snapshots are useful because you are not copying all of the data that you are looking to capture at a given time.
Cache volumes
LVM supports the use of fast block devices, such as SSD drives as write-back or write-through caches for larger slower block devices. Users can create cache logical volumes to improve the performance of their existing logical volumes or create new cache logical volumes composed of a small and fast device coupled with a large and slow device.

5.2. Creating LVM logical volume

Prerequisites

Procedure

  1. Create a logical volume:

    # lvcreate -n mylv -L 500M myvg
    Logical volume "mylv" successfully created.

    Use the -n option to set the LV name to mylv, and the -L option to set the size of LV in units of Mb, but it is possible to use any other units. The LV type is linear by default, but the user can specify the desired type by using the --type option.

    Important

    The command fails if the VG does not have a sufficient number of free physical extents for the requested size and type.

  2. View the created logical volumes by using any one of the following commands as per your requirement:

    1. The lvs command provides logical volume information in a configurable form, displaying one line per logical volume:

      # lvs
        LV   VG   Attr         LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
       mylv myvg -wi-ao----   500.00m
    2. The lvdisplay command displays logical volume properties, such as size, layout, and mapping in a fixed format:

      # lvdisplay -v /dev/myvg/mylv
        --- Logical volume ---
        LV Path                /dev/myvg/mylv
        LV Name                mylv
        VG Name                myvg
        LV UUID                YTnAk6-kMlT-c4pG-HBFZ-Bx7t-ePMk-7YjhaM
        LV Write Access        read/write
      [..]
    3. The lvscan command scans for all logical volumes in the system and lists them:

      # lvscan
       ACTIVE                   '/dev/myvg/mylv' [500.00 MiB] inherit
  3. Create a file system on the logical volume. The following command creates an xfs file system on the logical volume:

    # mkfs.xfs /dev/myvg/mylv
    meta-data=/dev/myvg/mylv       isize=512    agcount=4, agsize=32000 blks
             =                       sectsz=512   attr=2, projid32bit=1
             =                       crc=1        finobt=1, sparse=1, rmapbt=0
             =                       reflink=1
    data     =                       bsize=4096   blocks=128000, imaxpct=25
             =                       sunit=0      swidth=0 blks
    naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
    log      =internal log           bsize=4096   blocks=1368, version=2
             =                       sectsz=512   sunit=0 blks, lazy-count=1
    realtime =none                   extsz=4096   blocks=0, rtextents=0
    Discarding blocks...Done.
  4. Mount the logical volume and report the file system disk space usage:

    # mount /dev/myvg/mylv /mnt
    
    # df -h
    Filesystem               1K-blocks  Used   Available Use% Mounted on
    
    /dev/mapper/myvg-mylv   506528    29388  477140     6%   /mnt

Additional resources

  • lvcreate(8), lvdisplay(8), lvs(8), lvscan(8), lvm(8) and mkfs.xfs(8) man pages

5.3. Creating a RAID0 striped logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size. The following procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks.

Prerequisites

  1. You have created three or more physical volumes. For more information about creating physical volumes, see Creating LVM physical volume.
  2. You have created the volume group. For more information, see Creating LVM volume group.

Procedure

  1. Create a RAID0 logical volume from the existing volume group. The following command creates the RAID0 volume mylv from the volume group myvg, which is 2G in size, with three stripes and a stripe size of 4kB:

    # lvcreate --type raid0 -L 2G --stripes 3 --stripesize 4 -n mylv my_vg
      Rounding size 2.00 GiB (512 extents) up to stripe boundary size 2.00 GiB(513 extents).
      Logical volume "mylv" created.
  2. Create a file system on the RAID0 logical volume. The following command creates an ext4 file system on the logical volume:

    # mkfs.ext4 /dev/my_vg/mylv
  3. Mount the logical volume and report the file system disk space usage:

    # mount /dev/my_vg/mylv /mnt
    
    # df
    Filesystem             1K-blocks     Used  Available  Use% Mounted on
    /dev/mapper/my_vg-mylv   2002684     6168  1875072    1%   /mnt

Verification

  • View the created RAID0 stripped logical volume:

    # lvs -a -o +devices,segtype my_vg
      LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices Type
      mylv my_vg rwi-a-r--- 2.00g mylv_rimage_0(0),mylv_rimage_1(0),mylv_rimage_2(0) raid0
      [mylv_rimage_0] my_vg iwi-aor--- 684.00m /dev/sdf1(0) linear
      [mylv_rimage_1] my_vg iwi-aor--- 684.00m /dev/sdg1(0) linear
      [mylv_rimage_2] my_vg iwi-aor--- 684.00m /dev/sdh1(0) linear

5.4. Renaming LVM logical volumes

This procedure describes how to rename an existing logical volume mylv to mylv1.

Procedure

  1. If the logical volume is currently mounted, unmount the volume:

    # umount /mnt

    Replace /mnt with the mount point.

  2. Rename an existing logical volume:

    # lvrename myvg mylv mylv1
    Renamed "mylv" to "mylv1" in volume group "myvg"

    You can also rename the logical volume by specifying the full paths to the devices:

    # lvrename /dev/myvg/mylv /dev/myvg/mylv1

Additional resources

  • lvrename(8) man page

5.5. Removing a disk from a logical volume

This procedure describes how to remove a disk from an existing logical volume, either to replace the disk or to use the disk as part of a different volume.

In order to remove a disk, you must first move the extents on the LVM physical volume to a different disk or set of disks.

Procedure

  1. View the used and free space of physical volumes when using the LV:

    # pvs -o+pv_used
      PV          VG    Fmt    Attr   PSize      PFree     Used
     /dev/vdb1 myvg  lvm2   a--    1020.00m    0         1020.00m
     /dev/vdb2 myvg  lvm2   a--    1020.00m    0         1020.00m
     /dev/vdb3 myvg  lvm2   a--    1020.00m   1008.00m   12.00m
  2. Move the data to other physical volume:

    1. If there are enough free extents on the other physical volumes in the existing volume group, use the following command to move the data:

      # pvmove /dev/vdb3
        /dev/vdb3: Moved: 2.0%
       ...
        /dev/vdb3: Moved: 79.2%
       ...
        /dev/vdb3: Moved: 100.0%
    2. If there are no enough free extents on the other physical volumes in the existing volume group, use the following commands to add a new physical volume, extend the volume group using the newly created physical volume, and move the data to this physical volume:

      # pvcreate /dev/vdb4
        Physical volume "/dev/vdb4" successfully created
      
      # vgextend myvg /dev/vdb4
        Volume group "myvg" successfully extended
      
      # pvmove /dev/vdb3 /dev/vdb4
        /dev/vdb3: Moved: 33.33%
        /dev/vdb3: Moved: 100.00%
  3. Remove the physical volume:

    # vgreduce myvg /dev/vdb3
    Removed "/dev/vdb3" from volume group "myvg"

    If a logical volume contains a physical volume that fails, you cannot use that logical volume. To remove missing physical volumes from a volume group, you can use the --removemissing parameter of the vgreduce command, if there are no logical volumes that are allocated on the missing physical volumes:

    # vgreduce --removemissing myvg

Additional resources

  • pvmove(8), vgextend(8), vereduce(8), and pvs(8) man pages

5.6. Removing LVM logical volumes

This procedure describes how to remove an existing logical volume /dev/myvg/mylv1 from the volume group myvg.

Procedure

  1. If the logical volume is currently mounted, unmount the volume:

    # umount /mnt
  2. If the logical volume exists in a clustered environment, deactivate the logical volume on all nodes where it is active. Use the following command on each such node:

    # lvchange --activate n vg-name/lv-name
  3. Remove the logical volume using the lvremove utility:

    # lvremove /dev/myvg/mylv1
    
    Do you really want to remove active logical volume "mylv1"? [y/n]: y
    Logical volume "mylv1" successfully removed
    Note

    In this case, the logical volume has not been deactivated. If you explicitly deactivated the logical volume before removing it, you would not see the prompt verifying whether you want to remove an active logical volume.

Additional resources

  • lvremove(8) man page

5.7. Managing LVM logical volumes using RHEL System Roles

Use the storage role to perform the following tasks:

  • Create an LVM logical volume in a volume group consisting of multiple disks.
  • Create an ext4 file system with a given label on the logical volume.
  • Persistently mount the ext4 file system.

Prerequisites

  • An Ansible playbook including the storage role

5.7.1. Example Ansible playbook to manage logical volumes

The example Ansible playbook applies the storage role to create an LVM logical volume in a volume group.

Example 5.1. A playbook that creates a mylv logical volume in the myvg volume group

- hosts: all
  vars:
    storage_pools:
      - name: myvg
        disks:
          - sda
          - sdb
          - sdc
        volumes:
          - name: mylv
            size: 2G
            fs_type: ext4
            mount_point: /mnt/data
  roles:
    - rhel-system-roles.storage
  • The myvg volume group consists of the following disks:

    • /dev/sda
    • /dev/sdb
    • /dev/sdc
  • If the myvg volume group already exists, the playbook adds the logical volume to the volume group.
  • If the myvg volume group does not exist, the playbook creates it.
  • The playbook creates an Ext4 file system on the mylv logical volume, and persistently mounts the file system at /mnt.

Additional resources

  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

5.7.2. Additional resources

5.8. Removing LVM volume groups

You can remove an existing volume group using the vgremove command.

Prerequisites

Procedure

  1. If the volume group exists in a clustered environment, stop the lockspace of the volume group on all other nodes. Use the following command on all nodes except the node where you are performing the removal:

    # vgchange --lockstop vg-name

    Wait for the lock to stop.

  2. Remove the volume group:

    # vgremove vg-name
      Volume group "vg-name" successfully removed

Additional resources

  • vgremove(8) man page

Chapter 6. Modifying the size of a logical volume

After you have created a logical volume, you can modify the size of the volume.

6.1. Extending a logical volume and file system

You can extend a logical volume (LV) using the lvextend command. You can specify by how much you want to extend the LV, or how large you want the LV to be after you extend it. Use the -r option of the lvextend command to grow the underlying file system along with the LV.

Warning

You can also extend logical volumes using the lvresize command, but this command does not guarantee against accidental shrinkage.

Prerequisites

  • You have an existing logical volume (LV) with a file system on it. Determine the file system type and size using the df -Th command. For more information about creating a logical volume and a file system, see Creating LVM logical volume.
  • You have sufficient space in the volume group to grow your LV and file system. Use the vgs -o name,vgfree command to determine the available space. For more information about creating volume groups, see Creating LVM volume group.

Procedure

  1. Optional: If the volume group has insufficient space to grow your LV, add a new physical volume to the volume group:

    # vgextend myvg /dev/vdb3
    Physical volume "/dev/vdb3" successfully created.
    Volume group "myvg" successfully extended.
  2. Extend the LV and the file system:

    Note

    Using the lvextend command without the -r argument extends the LV only. To extend an underlying XFS file system, see Increasing the size of an XFS file system, for a GFS2 file system, see Growing a GFS2 filesystem and for an ext4 file system, see Resizing an ext4 file system.

    Note

    Use the -L option to extend the LV to a new size and the -l option to specify the number of extents depending on the size of the logical volume that you want to increase.

    # lvextend -r -L 3G /dev/myvg/mylv
    fsck from util-linux 2.32.1
    /dev/mapper/myvg-mylv: clean, 11/131072 files, 26156/524288 blocks
      Size of logical volume myvg/mylv changed from 2.00 GiB (512 extents) to 3.00 GiB (768 extents).
      Logical volume myvg/mylv successfully resized.
    resize2fs 1.45.6 (20-Mar-2020)
    Resizing the filesystem on /dev/mapper/myvg-mylv to 786432 (4k) blocks.
    The filesystem on /dev/mapper/myvg-mylv is now 786432 (4k) blocks long.

    You can also extend the mylv logical volume to fill all of the unallocated space in the myvg volume group:

    # lvextend -l +100%FREE /dev/myvg/mylv
     Size of logical volume myvg/mylv changed from 10.00 GiB (2560 extents) to 6.35 TiB (1665465 extents).
     Logical volume myvg/mylv successfully resized.

Verification

  • Verify that the file system and the LV has grown:

    # df -Th
    Filesystem            Type      Size  Used Avail Use% Mounted on
    devtmpfs              devtmpfs  1.9G     0  1.9G   0% /dev
    tmpfs                 tmpfs     1.9G     0  1.9G   0% /dev/shm
    tmpfs                 tmpfs     1.9G  8.6M  1.9G   1% /run
    tmpfs                 tmpfs     1.9G     0  1.9G   0% /sys/fs/cgroup
    /dev/mapper/rhel-root xfs        45G  3.7G   42G   9% /
    /dev/vda1             xfs      1014M  369M  646M  37% /boot
    tmpfs                 tmpfs     374M     0  374M   0% /run/user/0
    /dev/mapper/myvg-mylv xfs       2.0G   47M  2.0G   3% /mnt/mnt1

Additional resources

  • vgextend(8), lvextend(8), and xfs_growfs(8) man pages

6.2. Reducing a logical volume and file system

You can reduce a logical volume and its file system by using the lvreduce command and the resizefs option.

If the logical volume you are reducing contains a file system, to prevent data loss you must ensure that the file system is not using the space in the logical volume that is being reduced. For this reason, use the --resizefs option of the lvreduce command when the logical volume contains a file system.

When you use --resizefs, lvreduce attempts to reduce the file system before shrinking the logical volume. If shrinking the file system fails because it is full or does not support shrinking, then the lvreduce command fails and does not attempt to reduce the logical volume.

Warning

In most cases, the lvreduce command warns about possible data loss and asks for confirmation. However, you should not rely on these confirmation prompts to prevent data loss because in some cases you will not see these prompts, such as when the logical volume is inactive or the --resizefs option is not used.

Note that using the --test option of the lvreduce command does not indicate if the operation is safe because this option does not check the file system or test the file system resize.

Prerequisites

  • File system of the logical volume supports shrinking. Determine the file system type and size using the df -Th command.

    Note

    For example, the GFS2 and XFS filesystems do not support shrinking.

  • Underlying file system is not using the space in the LV that is being reduced.

Procedure

  1. Shrink the mylv logical volume and its filesystem in the myvg volume group using one of the following options:

    • Reduce the LV and its file system to a desired value:

      # lvreduce --resizefs -L 500M myvg/mylv
      File system ext4 found on myvg/mylv.
      File system size (2.00 GiB) is larger than the requested size (500.00 MiB).
      File system reduce is required using resize2fs.
      ...
      Logical volume myvg/mylv successfully resized.
    • Reduce 64 megabytes from the logical volume and filesystem:

      # lvreduce --resizefs -L -64M myvg/mylv
      File system ext4 found on myvg/mylv.
      File system size (500.00 MiB) is larger than the requested size (436.00 MiB).
      File system reduce is required using resize2fs.
      ...
      Logical volume myvg/mylv successfully resized

Additional resources

  • lvreduce(8) man page

6.3. Extending a striped logical volume

You can extend a striped logical volume (LV) by using the lvextend command with the required size.

Prerequisites

  1. You have enough free space on the underlying physical volumes (PVs) that make up the volume group (VG) to support the stripe.

Procedure

  1. Optional: Display your volume group:

    # vgs
      VG      #PV #LV #SN Attr   VSize   VFree
      myvg     2   1   0 wz--n- 271.31G 271.31G
  2. Optional: Create a stripe using the entire amount of space in the volume group:

    # lvcreate -n stripe1 -L 271.31G -i 2 myvg
      Using default stripesize 64.00 KB
      Rounding up size to full physical extent 271.31 GiB
  3. Optional: Extend the myvg volume group by adding new physical volumes:

    # vgextend myvg /dev/sdc1
      Volume group "myvg" successfully extended

    Repeat this step to add sufficient physical volumes depending on your stripe type and the amount of space used. For example, for a two-way stripe that uses up the entire volume group, you need to add at least two physical volumes.

  4. Extend the striped logical volume stripe1 that is a part of the myvg VG:

    # lvextend myvg/stripe1 -L 542G
      Using stripesize of last segment 64.00 KB
      Extending logical volume stripe1 to 542.00 GB
      Logical volume stripe1 successfully resized

    You can also extend the stripe1 logical volume to fill all of the unallocated space in the myvg volume group:

    # lvextend -l+100%FREE myvg/stripe1
      Size of logical volume myvg/stripe1 changed from 1020.00 MiB (255 extents) to <2.00 GiB (511 extents).
      Logical volume myvg/stripe1 successfully resized.

Verification

  • Verify the new size of the extended striped LV:

    # lvs
      LV          VG       Attr     LSize      Pool       Origin Data%  Move Log Copy%  Convert
      stripe1    myvg     wi-ao---- 542.00 GB

Chapter 7. Customizing the LVM report

LVM provides a wide range of configuration and command line options to produce customized reports and to filter the report’s output. You can sort the output, specify units, use selection criteria, and update the lvm.conf file to customize the LVM report.

7.1. Controlling format of the LVM display

Whether you use pvs, lvs, or vgs, these commands determine the default set of fields displayed and the sort order. You can control the output of these commands by executing the following commands.

Procedure

  • Change the default fields in the LVM display using the -o option:

    # pvs -o pv_name,pv_size,pv_free
      PV         PSize  PFree
      /dev/vdb1  17.14G 17.14G
      /dev/vdb2  17.14G 17.09G
      /dev/vdb3  17.14G 17.14G
  • Sort LVM display by using the -O option:

    # pvs -o pv_name,pv_size,pv_free -O pv_free
      PV         PSize  PFree
      /dev/vdb2 17.14G 17.09G
      /dev/vdb1  17.14G 17.14G
      /dev/vdb3  17.14G 17.14G
  • Display a reverse sort by using the -O argument along with the - character:

    # pvs -o pv_name,pv_size,pv_free -O -pv_free
      PV         PSize  PFree
      /dev/vdb1  17.14G 17.14G
      /dev/vdb3  17.14G 17.14G
      /dev/vdb2  17.14G 17.09G

Additional resources

7.2. Specifying the units for an LVM report display

You can view the size of the LVM devices in base 2 or base 10 units by specifying the --units argument of the report command.

Base 2 units

The default units are displayed in powers of 2, which is multiples of 1024. You can specify it using human-readable (r) with < and > rounding indicator, bytes (b), sectors (s), kilobytes (k), megabytes (m), gigabytes (g), terabytes (t), petabytes (p), exabytes (e), and human-readable (h).

The default display is r, when --units is not specified. You can override the default by setting the units parameter in the global section of the /etc/lvm/lvm.conf file.

Base 10 units
You can specify the units to be displayed in multiples of 1000 by capitalizing the unit specification (R, B, S, K, M, G, T, P, E, H).

Procedure

  • Specify the units for the LVM for base 2 gigabytes units:

    # pvs --units g /dev/vdb
      PV        VG    Fmt  Attr PSize   PFree
      /dev/vdb  myvg  lvm2 a--  931.00g 930.00g
    
    # vgs --units g myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 931.00g 931.00g
    
    # lvs --units g myvg
      LV    VG   Attr     LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      mylv myvg  wi-a---- 1.OOg
  • Indicate the actual size of LVM by using the r option with the < or > prefix in the output:

    # vgs --units g myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 931.00g 930.00g
    
    # vgs --units r myvg
      VG   #PV #LV #SN Attr VSize    VFree
      myvg   1   1   0 wz-n <931.00g <930.00
    
    # vgs myvg
      VG   #PV #LV #SN Attr VSize    VFree
      myvg   1   1   0 wz-n <931.00g <930.00g

    The r unit works similarly to h (human-readable), but in addition, the reported value gets a prefix of < or > to indicate that the actual size is slightly more or less than the displayed size. LVM rounds the decimal value, causing non-exact sizes to be reported.

    It also shows how --units g or other --units do not always display exactly correct sizes. It also shows the primary purpose of r, which is the < to indicate that the displayed size is not exact. In this example, the value is not exact because the VG size is not an exact multiple of gigabytes, and .01 is also not an exact representation of the fraction.

  • Specify the units for the LVM for base 10 gigabytes units:

    # pvs --units G /dev/vdb
      PV        VG   Fmt  Attr  PSize   PFree
      /dev/vdb  myvg lvm2 a--   999.65G 998.58G
    
    # vgs --units G myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 999.65G 998.58G
    
    # lvs --units G myvg
      LV    VG   Attr     LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      mylv myvg wi-a---- 1.07G
  • Specify sectors (s), defined as 512 bytes, or custom units. The following example displays the output of the pvs command as several sectors:

    # pvs --units s
      PV         VG     Fmt  Attr PSize       PFree
      /dev/vdb   myvg   lvm2 a--  1952440320S 1950343168S
  • Specify megabytes (m). The following example displays the output of the pvs command in units of 4 MB:

    # pvs --units 4m
      PV         VG     Fmt  Attr PSize      PFree
      /dev/vdb   myvg   lvm2 a--  238335.00U 238079.00U

7.3. Customizing the LVM configuration file

By editing the lvm.conf file, you can customize the LVM according to your specific storage and system requirements. For example, you can use lvm.conf to modify filter settings, configure volume group auto activation, manage thin pool, or automatically extend a snapshot.

Procedure:

  1. Display the default lvm.conf file:

    # lvmconfig --typeconfig default --withcomments

    By default, the lvm.conf file contains only comments to display possible settings.

  2. Customize the lvm.conf file according to your requirements by uncommenting the setting in lvm.conf. The following setting focuses on changing the default display of certain commands:

    • In the lvm.conf file, adjust the lvs_cols parameter to only print the specified fields:

      {
        ...
        lvs_cols="lv_name,vg_name,lv_attr"
        ...
      }

      Use this option instead of the lvs -o lv_name,vg_name,lv_attr command to avoid unnecessary frequent use of the -o option.

    • In the lvm.conf file, use the compact_output=1 setting to avoid printing empty fields for the pvs, vgs, and lvs commands:

      {
        ...
        compact_output = 1
        ...
      }
  3. View the default values after modifying the lvm.conf file:

    # lvmconfig --typeconfig diff

Additional resources

  • lvm.conf(5) man page

7.4. Defining LVM selection criteria

Selection criteria are a set of statements in the form of <field> <operator> <value>, which use comparison operators to define values for specific fields. Objects that match the selection criteria are then processed or displayed. Statements are combined by logical and grouping operators. To define selection criteria use the -S or --select option followed by one or multiple statements.

Some LVM commands support the -S option to select which objects to process based on certain attributes. These objects can be physical volumes (PVs), volume groups (VGs), or logical volumes (LVs).

The -S option works by describing the objects to process, rather than naming each object. This is helpful when processing many objects and it would be difficult to find and name each object separately or when searching objects that have a complex set of characteristics. The select option can also be used as a shortcut to avoid typing many names.

Use the lvs -S help command to see full sets of fields and possible operators. Replace lvs with any reporting or processing command to see the details of that command.

Use selection criteria with LVM reporting and processing commands to only display or process the objects that satisfy chosen criteria:

  • Reporting commands include pvs, vgs, lvs, pvdisplay, vgdisplay, lvdisplay, and dmsetup info -c.
  • Processing commands include pvchange, vgchange, lvchange, vgimport, vgexport, vgremove, and lvremove.

Procedure

  • Examples of selection criteria using the pvs command:

    # pvs
      PV           VG   Fmt  Attr PSize    PFree
      /dev/nvme2n1       lvm2 ---     1.00g   1.00g
      /dev/vdb1     myvg lvm2 a--  1020.00m 396.00m
      /dev/vdb2     myvg lvm2 a--  1020.00m 896.00m
    # pvs -S name=~nvme
      PV           Fmt  Attr PSize PFree
      /dev/nvme2n1 lvm2 ---  1.00g 1.00g
    # pvs -S vg_name=myvg
      PV         VG   Fmt  Attr PSize    PFree
      /dev/vdb1   myvg lvm2 a--  1020.00m 396.00m
      /dev/vdb2   myvg lvm2 a--  1020.00m 896.00m
  • Examples of selection criteria using the lvs commands:

    # lvs
      LV    VG   Attr       LSize   Cpy%Sync
      mylv  myvg -wi-a----- 200.00m
      lvol0 myvg -wi-a----- 100.00m
      lvol1 myvg -wi-a----- 100.00m
      lvol2 myvg -wi------- 100.00m
      rr    myvg rwi-a-r--- 120.00m 100.00
    # lvs -S 'size > 100m && size < 200m'
      LV   VG   Attr       LSize   Cpy%Sync
      rr   myvg rwi-a-r--- 120.00m 100.00
    # lvs -S name=~lvol[02]
      LV    VG   Attr       LSize
      lvol0 myvg -wi-a----- 100.00m
      lvol2 myvg -wi------- 100.00m
    # lvs -S segtype=raid1
      LV   VG   Attr       LSize   Cpy%Sync
      rr   myvg rwi-a-r--- 120.00m 100.00
  • More advanced examples:

    # lvchange --addtag mytag -S active=1
      Logical volume myvg/mylv changed.
      Logical volume myvg/lvol0 changed.
      Logical volume myvg/lvol1 changed.
      Logical volume myvg/rr changed.
    # lvs -a -o lv_name,vg_name,attr,size,pool_lv,origin,role -S 'name!~_pmspare'
      LV         VG      Attr       LSize Pool Origin Role
      thin1      example Vwi-a-tz-- 2.00g tp          public,origin,thinorigin
      thin1s     example Vwi---tz-- 2.00g tp   thin1  public,snapshot,thinsnapshot
      thin2      example Vwi-a-tz-- 3.00g tp          public
      tp         example twi-aotz-- 1.00g             private
      [tp_tdata] example Twi-ao---- 1.00g             private,thin,pool,data
      [tp_tmeta] example ewi-ao---- 4.00m             private,thin,pool,metadata
    # lvchange --setactivationskip n -S 'role=thinsnapshot && origin=thin1'
      Logical volume myvg/thin1s changed.
    # lvs -a -S 'name=~_tmeta && role=metadata && size <= 4m'
      LV         VG      Attr       LSize
      [tp_tmeta] myvg   ewi-ao---- 4.00m

Additional resources

  • lvmreport(7) man page

Chapter 8. Configuring LVM on shared storage

Shared storage is storage that can be accessed by multiple nodes at the same time. You can use LVM to manage shared storage. Shared storage is commonly used in cluster and high-availability setups and there are two common scenarios for how shared storage appears on the system:

  • LVM devices are attached to a host and passed to a guest VM to use. In this case, the device is never intended to be used by the host, only by the guest VM.
  • Machines are attached to a storage area network (SAN), for example using Fiber Channel, and the SAN LUNs are visible to multiple machines:

8.1. Configuring LVM for VM disks

To prevent VM storage from being exposed to the host, you can configure LVM device access and LVM system ID. You can do this by excluding the devices in question from the host, which ensures that the LVM on the host doesn’t see or use the devices passed to the guest VM. You can protect against accidental usage of the VM’s VG on the host by setting the LVM system ID in the VG to match the guest VM.

Procedure

  1. In the lvm.conf file, check if the system.devices file is enabled:

    use_devicesfile=1
  2. Exclude the devices in question from the host’s devices file:

    $ lvmdevices --deldev <device>
  3. Optional: You can further protect LVM devices:

    1. Set the LVM system ID feature in both the host and the VM in the lvm.conf file:

      system_id_source = "uname"
    2. Set the VG’s system ID to match the VM system ID. This ensures that only the guest VM is capable of activating the VG:

      $ vgchange --systemid <VM_system_id> <VM_vg_name>

8.2. Configuring LVM to use SAN disks on one machine

To prevent the SAN LUNs from being used by the wrong machine, exclude the LUNs from the devices file on all machines except the one machine which is meant to use them.

You can also protect the VG from being used by the wrong machine by configuring a system ID on all machines, and setting the system ID in the VG to match the machine using it.

Procedure

  1. In the lvm.conf file, check if the system.devices file is enabled:

    use_devicesfile=1
  2. Exclude the devices in question from the host’s devices file:

    $ lvmdevices --deldev <device>
  3. Set the LVM system ID feature in the lvm.conf file:

    system_id_source = "uname"
  4. Set the VG’s system ID to match the system ID of the machine using this VG:

    $ vgchange --systemid <system_id> <vg_name>

8.3. Configuring LVM to use SAN disks for failover

You can configure LUNs to be moved between machines, for example for failover purposes. You can set up the LVM by configuring the LVM devices file and including the LUNs in the devices file on all machines that may use the devices and by configuring the LVM system ID on each machine.

The following procedure describes the initial LVM configuration, to finish setting up LVM for failover and move the VG between machines, you need to configure pacemaker and LVM-activate resource agent that will automatically modify the VG’s system ID to match the system ID of the machine where the VG can be used. For more information see Configuring and managing high availability clusters.

Procedure

  1. In the lvm.conf file, check if the system.devices file is enabled:

    use_devicesfile=1
  2. Include the devices in question in the host’s devices file:

    $ lvmdevices --adddev <device>
  3. Set the LVM system ID feature in all machines in the lvm.conf file:

    system_id_source = "uname"

8.4. Configuring LVM to share SAN disks among multiple machines

Using the lvmlockd daemon and a lock manager such as dlm or sanlock, you can enable access to a shared VG on the SAN disks from multiple machines. The specific commands may differ based on the lock manager and operating system used. The following procedure describes the overview of the required steps to configure LVM to share SAN disks among multiple machines.

Warning

When using pacemaker, the system must be configured and started using the pacemaker steps shown in Configuring and managing high availability clusters instead.

Procedure

  1. In the lvm.conf file, check if the system.devices file is enabled:

    use_devicesfile=1
  2. For each machine that will use the shared LUN, add the LUN in the machines devices file:

    $ lvmdevices --adddev <device>
  3. Configure the lvm.conf file to use the lvmlockd daemon on all machines:

    use_lvmlockd=1
  4. Start the lvmlockd daemon file on all machines.
  5. Start a lock manager to use with lvmlockd, such as dlm or sanlock on all machines.
  6. Create a new shared VG using the command vgcreate --shared.
  7. Start and stop access to existing shared VGs using the commands vgchange --lockstart and vgchange --lockstop on all machines.

Additional resources

  • lvmlockd(8) man page

Chapter 9. Configuring RAID logical volumes

You can create and manage Redundant Array of Independent Disks (RAID) volumes by using logical volume manager (LVM).

9.1. RAID logical volumes

Logical volume manager (LVM) supports Redundant Array of Independent Disks (RAID) levels 0, 1, 4, 5, 6, and 10. An LVM RAID volume has the following characteristics:

  • LVM creates and manages RAID logical volumes that leverage the Multiple Devices (MD) kernel drivers.
  • You can temporarily split RAID1 images from the array and merge them back into the array later.
  • LVM RAID volumes support snapshots.

Other characteristics include:

Clusters

RAID logical volumes are not cluster-aware.

Although you can create and activate RAID logical volumes exclusively on one machine, you cannot activate them simultaneously on more than one machine.

Subvolumes

When you create a RAID logical volume (LV), LVM creates a metadata subvolume that is one extent in size for every data or parity subvolume in the array.

For example, creating a 2-way RAID1 array results in two metadata subvolumes (lv_rmeta_0 and lv_rmeta_1) and two data subvolumes (lv_rimage_0 and lv_rimage_1). Similarly, creating a 3-way stripe and one implicit parity device, RAID4 results in four metadata subvolumes (lv_rmeta_0, lv_rmeta_1, lv_rmeta_2, and lv_rmeta_3) and four data subvolumes (lv_rimage_0, lv_rimage_1, lv_rimage_2, and lv_rimage_3).

Integrity
You can lose data when a RAID device fails or when soft corruption occurs. Soft corruption in data storage implies that the data retrieved from a storage device is different from the data written to that device. Adding integrity to a RAID LV reduces or prevent soft corruption. For more information, see Creating a RAID LV with DM integrity.

9.2. RAID levels and linear support

The following are the supported configurations by RAID, including levels 0, 1, 4, 5, 6, 10, and linear:

Level 0

RAID level 0, often called striping, is a performance-oriented striped data mapping technique. This means the data being written to the array is broken down into stripes and written across the member disks of the array, allowing high I/O performance at low inherent cost but provides no redundancy.

RAID level 0 implementations only stripe the data across the member devices up to the size of the smallest device in the array. This means that if you have multiple devices with slightly different sizes, each device gets treated as though it was the same size as the smallest drive. Therefore, the common storage capacity of a level 0 array is the total capacity of all disks. If the member disks have a different size, then the RAID0 uses all the space of those disks using the available zones.

Level 1

RAID level 1, or mirroring, provides redundancy by writing identical data to each member disk of the array, leaving a mirrored copy on each disk. Mirroring remains popular due to its simplicity and high level of data availability. Level 1 operates with two or more disks, and provides very good data reliability and improves performance for read-intensive applications but at relatively high costs.

RAID level 1 is costly because you write the same information to all of the disks in the array, which provides data reliability, but in a much less space-efficient manner than parity based RAID levels such as level 5. However, this space inefficiency comes with a performance benefit, which is parity-based RAID levels that consume considerably more CPU power in order to generate the parity while RAID level 1 simply writes the same data more than once to the multiple RAID members with very little CPU overhead. As such, RAID level 1 can outperform the parity-based RAID levels on machines where software RAID is employed and CPU resources on the machine are consistently taxed with operations other than RAID activities.

The storage capacity of the level 1 array is equal to the capacity of the smallest mirrored hard disk in a hardware RAID or the smallest mirrored partition in a software RAID. Level 1 redundancy is the highest possible among all RAID types, with the array being able to operate with only a single disk present.

Level 4

Level 4 uses parity concentrated on a single disk drive to protect data. Parity information is calculated based on the content of the rest of the member disks in the array. This information can then be used to reconstruct data when one disk in the array fails. The reconstructed data can then be used to satisfy I/O requests to the failed disk before it is replaced and to repopulate the failed disk after it has been replaced.

Since the dedicated parity disk represents an inherent bottleneck on all write transactions to the RAID array, level 4 is seldom used without accompanying technologies such as write-back caching. Or it is used in specific circumstances where the system administrator is intentionally designing the software RAID device with this bottleneck in mind such as an array that has little to no write transactions once the array is populated with data. RAID level 4 is so rarely used that it is not available as an option in Anaconda. However, it could be created manually by the user if needed.

The storage capacity of hardware RAID level 4 is equal to the capacity of the smallest member partition multiplied by the number of partitions minus one. The performance of a RAID level 4 array is always asymmetrical, which means reads outperform writes. This is because write operations consume extra CPU resources and main memory bandwidth when generating parity, and then also consume extra bus bandwidth when writing the actual data to disks because you are not only writing the data, but also the parity. Read operations need only read the data and not the parity unless the array is in a degraded state. As a result, read operations generate less traffic to the drives and across the buses of the computer for the same amount of data transfer under normal operating conditions.

Level 5

This is the most common type of RAID. By distributing parity across all the member disk drives of an array, RAID level 5 eliminates the write bottleneck inherent in level 4. The only performance bottleneck is the parity calculation process itself. Modern CPUs can calculate parity very fast. However, if you have a large number of disks in a RAID 5 array such that the combined aggregate data transfer speed across all devices is high enough, parity calculation can be a bottleneck.

Level 5 has asymmetrical performance, and reads substantially outperforming writes. The storage capacity of RAID level 5 is calculated the same way as with level 4.

Level 6

This is a common level of RAID when data redundancy and preservation, and not performance, are the paramount concerns, but where the space inefficiency of level 1 is not acceptable. Level 6 uses a complex parity scheme to be able to recover from the loss of any two drives in the array. This complex parity scheme creates a significantly higher CPU burden on software RAID devices and also imposes an increased burden during write transactions. As such, level 6 is considerably more asymmetrical in performance than levels 4 and 5.

The total capacity of a RAID level 6 array is calculated similarly to RAID level 5 and 4, except that you must subtract two devices instead of one from the device count for the extra parity storage space.

Level 10

This RAID level attempts to combine the performance advantages of level 0 with the redundancy of level 1. It also reduces some of the space wasted in level 1 arrays with more than two devices. With level 10, it is possible, for example, to create a 3-drive array configured to store only two copies of each piece of data, which then allows the overall array size to be 1.5 times the size of the smallest devices instead of only equal to the smallest device, similar to a 3-device, level 1 array. This avoids CPU process usage to calculate parity similar to RAID level 6, but it is less space efficient.

The creation of RAID level 10 is not supported during installation. It is possible to create one manually after installation.

Linear RAID

Linear RAID is a grouping of drives to create a larger virtual drive.

In linear RAID, the chunks are allocated sequentially from one member drive, going to the next drive only when the first is completely filled. This grouping provides no performance benefit, as it is unlikely that any I/O operations split between member drives. Linear RAID also offers no redundancy and decreases reliability. If any one member drive fails, the entire array cannot be used and data can be lost. The capacity is the total of all member disks.

9.3. LVM RAID segment types

To create a RAID logical volume, you can specify a RAID type by using the --type argument of the lvcreate command. For most users, specifying one of the five available primary types, which are raid1, raid4, raid5, raid6, and raid10, should be sufficient.

The following table describes the possible RAID segment types.

Table 9.1. LVM RAID segment types

Segment typeDescription

raid1

RAID1 mirroring. This is the default value for the --type argument of the lvcreate command, when you specify the -m argument without specifying striping.

raid4

RAID4 dedicated parity disk.

raid5_la

  • RAID5 left asymmetric.
  • Rotating parity 0 with data continuation.

raid5_ra

  • RAID5 right asymmetric.
  • Rotating parity N with data continuation.

raid5_ls

  • RAID5 left symmetric.
  • It is same as raid5.
  • Rotating parity 0 with data restart.

raid5_rs

  • RAID5 right symmetric.
  • Rotating parity N with data restart.

raid6_zr

  • RAID6 zero restart.
  • It is same as raid6.
  • Rotating parity zero (left-to-right) with data restart.

raid6_nr

  • RAID6 N restart.
  • Rotating parity N (left-to-right) with data restart.

raid6_nc

  • RAID6 N continue.
  • Rotating parity N (left-to-right) with data continuation.

raid10

  • Striped mirrors. This is the default value for the --type argument of the lvcreate command if you specify the -m argument along with the number of stripes that is greater than 1.
  • Striping of mirror sets.

raid0/raid0_meta

Striping. RAID0 spreads logical volume data across multiple data subvolumes in units of stripe size. This is used to increase performance. Logical volume data is lost if any of the data subvolumes fail.

9.4. Creating RAID logical volumes

You can create RAID1 arrays with multiple numbers of copies, according to the value you specify for the -m argument. Similarly, you can specify the number of stripes for a RAID 0, 4, 5, 6, and 10 logical volume with the -i argument. You can also specify the stripe size with the -I argument. The following procedure describes different ways to create different types of RAID logical volume.

Procedure

  • Create a 2-way RAID. The following command creates a 2-way RAID1 array, named my_lv, in the volume group my_vg, that is 1G in size:

    # lvcreate --type raid1 -m 1 -L 1G -n my_lv my_vg
    Logical volume "my_lv" created.
  • Create a RAID5 array with stripes. The following command creates a RAID5 array with three stripes and one implicit parity drive, named my_lv, in the volume group my_vg, that is 1G in size. Note that you can specify the number of stripes similar to an LVM striped volume. The correct number of parity drives is added automatically.

    # lvcreate --type raid5 -i 3 -L 1G -n my_lv my_vg
  • Create a RAID6 array with stripes. The following command creates a RAID6 array with three 3 stripes and two implicit parity drives, named my_lv, in the volume group my_vg, that is 1G one gigabyte in size:

    # lvcreate --type raid6 -i 3 -L 1G -n my_lv my_vg

Verification

  • Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:
# lvs -a -o name,copy_percent,devices _my_vg_
  LV                Copy%  Devices
  my_lv             6.25    my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]         /dev/sde1(0)
  [my_lv_rimage_1]         /dev/sdf1(1)
  [my_lv_rmeta_0]          /dev/sde1(256)
  [my_lv_rmeta_1]          /dev/sdf1(0)

Additional resources

  • lvcreate(8) and lvmraid(7) man pages

9.5. Creating a RAID0 striped logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size. The following procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks.

Prerequisites

  1. You have created three or more physical volumes. For more information about creating physical volumes, see Creating LVM physical volume.
  2. You have created the volume group. For more information, see Creating LVM volume group.

Procedure

  1. Create a RAID0 logical volume from the existing volume group. The following command creates the RAID0 volume mylv from the volume group myvg, which is 2G in size, with three stripes and a stripe size of 4kB:

    # lvcreate --type raid0 -L 2G --stripes 3 --stripesize 4 -n mylv my_vg
      Rounding size 2.00 GiB (512 extents) up to stripe boundary size 2.00 GiB(513 extents).
      Logical volume "mylv" created.
  2. Create a file system on the RAID0 logical volume. The following command creates an ext4 file system on the logical volume:

    # mkfs.ext4 /dev/my_vg/mylv
  3. Mount the logical volume and report the file system disk space usage:

    # mount /dev/my_vg/mylv /mnt
    
    # df
    Filesystem             1K-blocks     Used  Available  Use% Mounted on
    /dev/mapper/my_vg-mylv   2002684     6168  1875072    1%   /mnt

Verification

  • View the created RAID0 stripped logical volume:

    # lvs -a -o +devices,segtype my_vg
      LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices Type
      mylv my_vg rwi-a-r--- 2.00g mylv_rimage_0(0),mylv_rimage_1(0),mylv_rimage_2(0) raid0
      [mylv_rimage_0] my_vg iwi-aor--- 684.00m /dev/sdf1(0) linear
      [mylv_rimage_1] my_vg iwi-aor--- 684.00m /dev/sdg1(0) linear
      [mylv_rimage_2] my_vg iwi-aor--- 684.00m /dev/sdh1(0) linear

9.6. Configuring a stripe size for RAID LVM volumes using the storage RHEL System Role

With the storage System Role, you can configure a stripe size for RAID LVM volumes on RHEL using Red Hat Ansible Automation Platform. You can set up an Ansible playbook with the available parameters to configure an LVM pool with RAID.

Prerequisites

  • The Ansible Core package is installed on the control machine.
  • You have the rhel-system-roles package installed on the system from which you want to run the playbook.
  • You have an inventory file detailing the systems on which you want to configure an LVM pool with RAID using the storage System Role.

Procedure

  1. Create a new playbook.yml file with the following content:

    hosts: all
      vars:
        storage_safe_mode: false
        storage_pools:
          - name: my_pool
            type: lvm
            disks: [sdh, sdi]
            volumes:
              - name: my_volume
                size: "1 GiB"
                mount_point: "/mnt/app/shared"
                fs_type: xfs
                raid_level: raid1
                raid_stripe_size: "256 KiB"
                state: present
      roles:
        - name: rhel-system-roles.storage
  2. Optional. Verify playbook syntax:

    # ansible-playbook --syntax-check playbook.yml
  3. Run the playbook on your inventory file:

    # ansible-playbook -i inventory.file /path/to/file/playbook.yml

Additional resources

  • Managing RAID
  • The /usr/share/ansible/roles/rhel-system-roles.storage/README.md file.

9.7. Parameters for creating a RAID0

You can create a RAID0 striped logical volume using the lvcreate --type raid0[meta] --stripes _Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath] command.

The following table describes different parameters, which you can use while creating a RAID0 striped logical volume.

Table 9.2. Parameters for creating a RAID0 striped logical volume

ParameterDescription

--type raid0[_meta]

Specifying raid0 creates a RAID0 volume without metadata volumes. Specifying raid0_meta creates a RAID0 volume with metadata volumes. Since RAID0 is non-resilient, it does not store any mirrored data blocks as RAID1/10 or calculate and store any parity blocks as RAID4/5/6 do. Hence, it does not need metadata volumes to keep state about resynchronization progress of mirrored or parity blocks. Metadata volumes become mandatory on a conversion from RAID0 to RAID4/5/6/10. Specifying raid0_meta preallocates those metadata volumes to prevent a respective allocation failure.

--stripes Stripes

Specifies the number of devices to spread the logical volume across.

--stripesize StripeSize

Specifies the size of each stripe in kilobytes. This is the amount of data that is written to one device before moving to the next device.

VolumeGroup

Specifies the volume group to use.

PhysicalVolumePath

Specifies the devices to use. If this is not specified, LVM will choose the number of devices specified by the Stripes option, one for each stripe.

9.8. Soft data corruption

Soft corruption in data storage implies that the data retrieved from a storage device is different from the data written to that device. The corrupted data can exist indefinitely on storage devices. You might not discover this corrupted data until you retrieve and attempt to use this data.

Depending on the type of configuration, a Redundant Array of Independent Disks (RAID) logical volume(LV) prevents data loss when a device fails. If a device consisting of a RAID array fails, the data can be recovered from other devices that are part of that RAID LV. However, a RAID configuration does not ensure the integrity of the data itself. Soft corruption, silent corruption, soft errors, and silent errors are terms that describe data that has become corrupted, even if the system design and software continues to function as expected.

Device mapper (DM) integrity is used with RAID levels 1, 4, 5, 6, and 10 to mitigate or prevent data loss due to soft corruption. The RAID layer ensures that a non-corrupted copy of the data can fix the soft corruption errors. The integrity layer sits above each RAID image while an extra sub LV stores the integrity metadata or data checksums for each RAID image. When you retrieve data from an RAID LV with integrity, the integrity data checksums analyze the data for corruption. If corruption is detected, the integrity layer returns an error message, and the RAID layer retrieves a non-corrupted copy of the data from another RAID image. The RAID layer automatically rewrites non-corrupted data over the corrupted data to repair the soft corruption.

When creating a new RAID LV with DM integrity or adding integrity to an existing RAID LV, consider the following points:

  • The integrity metadata requires additional storage space. For each RAID image, every 500MB data requires 4MB of additional storage space because of the checksums that get added to the data.
  • While some RAID configurations are impacted more than others, adding DM integrity impacts performance due to latency when accessing the data. A RAID1 configuration typically offers better performance than RAID5 or its variants.
  • The RAID integrity block size also impacts performance. Configuring a larger RAID integrity block size offers better performance. However, a smaller RAID integrity block size offers greater backward compatibility.
  • There are two integrity modes available: bitmap or journal. The bitmap integrity mode typically offers better performance than journal mode.
Tip

If you experience performance issues, either use RAID1 with integrity or test the performance of a particular RAID configuration to ensure that it meets your requirements.

9.9. Creating a RAID LV with DM integrity

When you create a RAID LV with device mapper (DM) integrity or add integrity to an existing RAID LV, it mitigates the risk of losing data due to soft corruption. Wait for the integrity synchronization and the RAID metadata to complete before using the LV. Otherwise, the background initialization might impact the LV’s performance.

Procedure

  1. Create a RAID LV with DM integrity. The following example creates a new RAID LV with integrity named test-lv in the my_vg volume group, with a usable size of 256M and RAID level 1:

    # lvcreate --type raid1 --raidintegrity y -L 256M -n test-lv my_vg
    Creating integrity metadata LV test-lv_rimage_0_imeta with size 8.00 MiB.
    Logical volume "test-lv_rimage_0_imeta" created.
    Creating integrity metadata LV test-lv_rimage_1_imeta with size 8.00 MiB.
    Logical volume "test-lv_rimage_1_imeta" created.
    Logical volume "test-lv" created.
    Note

    To add DM integrity to an existing RAID LV, use the following command:

    # lvconvert --raidintegrity y my_vg/test-lv

    Adding integrity to a RAID LV limits the number of operations that you can perform on that RAID LV.

  2. Optional: Remove the integrity before performing certain operations.

    # lvconvert --raidintegrity n my_vg/test-lv
    Logical volume my_vg/test-lv has removed integrity.

Verification

  • View information about the added DM integrity:

    • View information about the test-lv RAID LV that was created in the my_vg volume group:

      # lvs -a my_vg
        LV                        VG      Attr       LSize   Origin                 Cpy%Sync
        test-lv                   my_vg rwi-a-r--- 256.00m                          2.10
        [test-lv_rimage_0]        my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig] 93.75
        [test-lv_rimage_0_imeta]  my_vg ewi-ao----   8.00m
        [test-lv_rimage_0_iorig]  my_vg -wi-ao---- 256.00m
        [test-lv_rimage_1]        my_vg gwi-aor--- 256.00m [test-lv_rimage_1_iorig] 85.94
       [...]

      The following describes different options from this output:

      g attribute
      It is the list of attributes under the Attr column indicates that the RAID image is using integrity. The integrity stores the checksums in the _imeta RAID LV.
      Cpy%Sync column
      It indicates the synchronization progress for both the top level RAID LV and for each RAID image.
      RAID image
      It is is indicated in the LV column by raid_image_N.
      LV column
      It ensures that the synchronization progress displays 100% for the top level RAID LV and for each RAID image.
    • Display the type for each RAID LV:

      # lvs -a my-vg -o+segtype
        LV                       VG      Attr       LSize   Origin                 Cpy%Sync Type
        test-lv                  my_vg rwi-a-r--- 256.00m                          87.96    raid1
        [test-lv_rimage_0]       my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig] 100.00   integrity
        [test-lv_rimage_0_imeta] my_vg ewi-ao----   8.00m                                   linear
        [test-lv_rimage_0_iorig] my_vg -wi-ao---- 256.00m                                   linear
        [test-lv_rimage_1]       my_vg gwi-aor--- 256.00m [test-lv_rimage_1_iorig] 100.00   integrity
       [...]
    • There is an incremental counter that counts the number of mismatches detected on each RAID image. View the data mismatches detected by integrity from rimage_0 under my_vg/test-lv:

      # lvs -o+integritymismatches my_vg/test-lv_rimage_0
        LV                 VG      Attr       LSize   Origin                    Cpy%Sync IntegMismatches
        [test-lv_rimage_0] my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig]    100.00                 0

      In this example, the integrity has not detected any data mismatches and thus the IntegMismatches counter shows zero (0).

    • View the data integrity information in the /var/log/messages log files, as shown in the following examples:

      Example 9.1. Example of dm-integrity mismatches from the kernel message logs

      device-mapper: integrity: dm-12: Checksum failed at sector 0x24e7

      Example 9.2. Example of dm-integrity data corrections from the kernel message logs

      md/raid1:mdX: read error corrected (8 sectors at 9448 on dm-16)

Additional resources

  • lvcreate(8) and lvmraid(7) man pages

9.10. Minimum and maximum I/O rate options

When you create a RAID logical volumes, the background I/O required to initialize the logical volumes with the sync operation can expel other I/O operations to LVM devices, such as updates to volume group metadata, particularly when you are creating many RAID logical volumes. This can cause the other LVM operations to slow down.

You can control the rate at which a RAID logical volume is initialized by implementing recovery throttling. To control the rate at which sync operations are performed, set the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvcreate command.

You can specify these options as follows:

--maxrecoveryrate Rate[bBsSkKmMgG]
Sets the maximum recovery rate for a RAID logical volume so that it will not expel nominal I/O operations. Specify the Rate as an amount per second for each device in the array. If you do not provide a suffix, then it assumes kiB/sec/device. Setting the recovery rate to 0 means it will be unbounded.
--minrecoveryrate Rate[bBsSkKmMgG]
Sets the minimum recovery rate for a RAID logical volume to ensure that I/O for sync operations achieves a minimum throughput, even when heavy nominal I/O is present. Specify the Rate as an amount per second for each device in the array. If you do not give a suffix, then it assumes kiB/sec/device.

For example, use the lvcreate --type raid10 -i 2 -m 1 -L 10G --maxrecoveryrate 128 -n my_lv my_vg command to create a 2-way RAID10 array my_lv, which is in the volume group my_vg with 3 stripes that is 10G in size with a maximum recovery rate of 128 kiB/sec/device. You can also specify minimum and maximum recovery rates for a RAID scrubbing operation.

9.11. Converting a linear device to a RAID logical volume

You can convert an existing linear logical volume to a RAID logical volume. To perform this operation, use the --type argument of the lvconvert command.

RAID logical volumes are composed of metadata and data subvolume pairs. When you convert a linear device to a RAID1 array, it creates a new metadata subvolume and associates it with the original logical volume on one of the same physical volumes that the linear volume is on. The additional images are added in a metadata/data subvolume pair. If the metadata image that pairs with the original logical volume cannot be placed on the same physical volume, the lvconvert fails.

Procedure

  1. View the logical volume device that needs to be converted:

    # lvs -a -o name,copy_percent,devices my_vg
      LV     Copy%  Devices
      my_lv         /dev/sde1(0)
  2. Convert the linear logical volume to a RAID device. The following command converts the linear logical volume my_lv in volume group __my_vg, to a 2-way RAID1 array:

    # lvconvert --type raid1 -m 1 my_vg/my_lv
      Are you sure you want to convert linear LV my_vg/my_lv to raid1 with 2 images enhancing resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.

Verification

  • Ensure if the logical volume is converted to a RAID device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            6.25   my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(0)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(256)
      [my_lv_rmeta_1]         /dev/sdf1(0)

Additional resources

  • The lvconvert(8) man page

9.12. Converting an LVM RAID1 logical volume to an LVM linear logical volume

You can convert an existing RAID1 LVM logical volume to an LVM linear logical volume. To perform this operation, use the lvconvert command and specify the -m0 argument. This removes all the RAID data subvolumes and all the RAID metadata subvolumes that make up the RAID array, leaving the top-level RAID1 image as the linear logical volume.

Procedure

  1. Display an existing LVM RAID1 logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(1)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(0)
      [my_lv_rmeta_1]         /dev/sdf1(0)
  2. Convert an existing RAID1 LVM logical volume to an LVM linear logical volume. The following command converts the LVM RAID1 logical volume my_vg/my_lv to an LVM linear device:

    # lvconvert -m0 my_vg/my_lv
      Are you sure you want to convert raid1 LV my_vg/my_lv to type linear losing all resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.

    When you convert an LVM RAID1 logical volume to an LVM linear volume, you can also specify which physical volumes to remove. In the following example, the lvconvert command specifies that you want to remove /dev/sde1, leaving /dev/sdf1 as the physical volume that makes up the linear device:

    # lvconvert -m0 my_vg/my_lv /dev/sde1

Verification

  • Verify if the RAID1 logical volume was converted to an LVM linear device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV    Copy%  Devices
      my_lv        /dev/sdf1(1)

Additional resources

  • The lvconvert(8) man page

9.13. Converting a mirrored LVM device to a RAID1 logical volume

You can convert an existing mirrored LVM device with a segment type mirror to a RAID1 LVM device. To perform this operation, use the lvconvert command with the --type raid1 argument. This renames the mirror subvolumes named mimage to RAID subvolumes named rimage.

In addition, it also removes the mirror log and creates metadata subvolumes named rmeta for the data subvolumes on the same physical volumes as the corresponding data subvolumes.

Procedure

  1. View the layout of a mirrored logical volume my_vg/my_lv:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             15.20 my_lv_mimage_0(0),my_lv_mimage_1(0)
      [my_lv_mimage_0]        /dev/sde1(0)
      [my_lv_mimage_1]        /dev/sdf1(0)
      [my_lv_mlog]            /dev/sdd1(0)
  2. Convert the mirrored logical volume my_vg/my_lv to a RAID1 logical volume:

    # lvconvert --type raid1 my_vg/my_lv
    Are you sure you want to convert mirror LV my_vg/my_lv to raid1 type? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.

Verification

  • Verify if the mirrored logical volume is converted to a RAID1 logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(0)
      [my_lv_rimage_1]        /dev/sdf1(0)
      [my_lv_rmeta_0]         /dev/sde1(125)
      [my_lv_rmeta_1]         /dev/sdf1(125)

Additional resources

  • The lvconvert(8) man page

9.14. Commands to resize a RAID logical volume

You can resize a RAID logical volume in the following ways:

  • You can increase the size of a RAID logical volume of any type with the lvresize or lvextend command. This does not change the number of RAID images. For striped RAID logical volumes, the same stripe rounding constraints apply when you create a striped RAID logical volume.
  • You can reduce the size of a RAID logical volume of any type with the lvresize or lvreduce command. This does not change the number of RAID images. As with the lvextend command, the same stripe rounding constraints apply when you create a striped RAID logical volume.
  • You can change the number of stripes on a striped RAID logical volume such as RAID4, RAID5, RAID6, or RAID10 with the --stripes N parameter of the lvconvert command. This increases or reduces the size of the RAID logical volume by the capacity of the stripes added or removed. Note that raid10 volumes are capable only of adding stripes. This capability is part of the RAID reshaping feature and with this feature, you can change attributes of a RAID logical volume while keeping the same RAID level.

9.15. Changing the number of images in an existing RAID1 device

You can change the number of images in an existing RAID1 array, similar to the way you can change the number of images in the implementation of LVM mirroring.

When you add images to a RAID1 logical volume with the lvconvert command, you can perform the following operations:

  • specify the total number of images for the resulting device,
  • how many images to add to the device, and
  • can optionally specify on which physical volumes the new metadata/data image pairs reside.

Procedure

  1. Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV                Copy%  Devices
      my_lv             6.25    my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]         /dev/sde1(0)
      [my_lv_rimage_1]         /dev/sdf1(1)
      [my_lv_rmeta_0]          /dev/sde1(256)
      [my_lv_rmeta_1]          /dev/sdf1(0)

    Metadata subvolumes named rmeta always exist on the same physical devices as their data subvolume counterparts rimage. The metadata/data subvolume pairs will not be created on the same physical volumes as those from another metadata/data subvolume pair in the RAID array unless you specify --alloc anywhere.

  2. Convert the 2-way RAID1 logical volume my_vg/my_lv to a 3-way RAID1 logical volume:

    # lvconvert -m 2 my_vg/my_lv
    Are you sure you want to convert raid1 LV my_vg/my_lv to 3 images enhancing resilience? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.

    The following are a few examples of changing the number of images in an existing RAID1 device:

    • You can also specify which physical volumes to use while adding an image to RAID. The following command converts the 2-way RAID1 logical volume my_vg/my_lv to a 3-way RAID1 logical volume by specifying the physical volume /dev/sdd1 to use for the array:

      # lvconvert -m 2 my_vg/my_lv /dev/sdd1
    • Convert the 3-way RAID1 logical volume into a 2-way RAID1 logical volume:

      # lvconvert -m1 my_vg/my_lv
      Are you sure you want to convert raid1 LV my_vg/my_lv to 2 images reducing resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.
    • Convert the 3-way RAID1 logical volume into a 2-way RAID1 logical volume by specifying the physical volume /dev/sde1, which contains the image to remove:

      # lvconvert -m1 my_vg/my_lv /dev/sde1

      Additionally, when you remove an image and its associated metadata subvolume volume, any higher-numbered images will be shifted down to fill the slot. Removing lv_rimage_1 from a 3-way RAID1 array that consists of lv_rimage_0, lv_rimage_1, and lv_rimage_2 results in a RAID1 array that consists of lv_rimage_0 and lv_rimage_1. The subvolume lv_rimage_2 will be renamed and take over the empty slot, becoming lv_rimage_1.

Verification

  • View the RAID1 device after changing the number of images in an existing RAID1 device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV Cpy%Sync Devices
      my_lv 100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0] /dev/sdd1(1)
      [my_lv_rimage_1] /dev/sde1(1)
      [my_lv_rimage_2] /dev/sdf1(1)
      [my_lv_rmeta_0] /dev/sdd1(0)
      [my_lv_rmeta_1] /dev/sde1(0)
      [my_lv_rmeta_2] /dev/sdf1(0)

Additional resources

  • The lvconvert(8) man page

9.16. Splitting off a RAID image as a separate logical volume

You can split off an image of a RAID logical volume to form a new logical volume. When you are removing a RAID image from an existing RAID1 logical volume or removing a RAID data subvolume and its associated metadata subvolume from the middle of the device, any higher numbered images will be shifted down to fill the slot. The index numbers on the logical volumes that make up a RAID array will thus be an unbroken sequence of integers.

Note

You cannot split off a RAID image if the RAID1 array is not yet in sync.

Procedure

  1. Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             12.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(1)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(0)
      [my_lv_rmeta_1]         /dev/sdf1(0)
  2. Split the RAID image into a separate logical volume:

    • The following example splits a 2-way RAID1 logical volume, my_lv, into two linear logical volumes, my_lv and new:

      # lvconvert --splitmirror 1 -n new my_vg/my_lv
      Are you sure you want to split raid1 LV my_vg/my_lv losing all resilience? [y/n]: y
    • The following example splits a 3-way RAID1 logical volume, my_lv, into a 2-way RAID1 logical volume, my_lv, and a linear logical volume, new:

      # lvconvert --splitmirror 1 -n new my_vg/my_lv

Verification

  • View the logical volume after you split off an image of a RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV      Copy%  Devices
      my_lv          /dev/sde1(1)
      new            /dev/sdf1(1)

Additional resources

  • The lvconvert(8) man page

9.17. Splitting and merging a RAID Image

You can temporarily split off an image of a RAID1 array for read-only use while tracking any changes by using the --trackchanges argument with the --splitmirrors argument of the lvconvert command. Using this feature, you can merge the image into an array at a later time while resyncing only those portions of the array that have changed since the image was split.

When you split off a RAID image with the --trackchanges argument, you can specify which image to split but you cannot change the name of the volume being split. In addition, the resulting volumes have the following constraints:

  • The new volume you create is read-only.
  • You cannot resize the new volume.
  • You cannot rename the remaining array.
  • You cannot resize the remaining array.
  • You can activate the new volume and the remaining array independently.

You can merge an image that was split off. When you merge the image, only the portions of the array that have changed since the image was split are resynced.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
      Logical volume "my_lv" created
  2. Optional: View the created RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv          100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  3. Split an image from the created RAID logical volume and track the changes to the remaining array:

    # lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
      my_lv_rimage_2 split from my_lv for read-only purposes.
      Use 'lvconvert --merge my_vg/my_lv_rimage_2' to merge back into my_lv
  4. Optional: View the logical volume after splitting the image:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sdc1(1)
      [my_lv_rimage_1]          /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdc1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
  5. Merge the volume back into the array:

    # lvconvert --merge my_vg/my_lv_rimage_1
      my_vg/my_lv_rimage_1 successfully merged back into my_vg/my_lv

Verification

  • View the merged logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sdc1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdc1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)

Additional resources

  • The lvconvert(8) man page

9.18. Setting a RAID fault policy

Based on the raid_fault_policy field preferences in the /etc/lvm/lvm.conf file, LVM RAID automatically handles device failures. You can set raid_fault_policy field to any one of the following parameter depending on the requirement:

warn

You can this parameter to manually repair the failed device and display warnings by using system logs.

By default, the value of the raid_fault_policy field is warn in lvm.conf. If enough devices are operational, the RAID logical volume continues to operate.

allocate
You can use this parameter to automatically replace the failed device.

9.18.1. Setting the RAID fault policy to allocate

You can set the raid_fault_policy field to the allocate parameter in the /etc/lvm/lvm.conf file. With this preference, the system attempts to replace the failed device with a spare device from the volume group. If there is no spare device, the system log includes this information.

Procedure

  1. View the RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
    
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  2. View the RAID logical volume if the /dev/sdb device fails:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdb: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        [unknown](1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [...]

    You can also view the system log for the error messages if the /dev/sdb device fails.

  3. Set the raid_fault_policy field to allocate in the lvm.conf file:

     # vi /etc/lvm/lvm.conf
     raid_fault_policy = "allocate"
    Note

    If you set raid_fault_policy to allocate but there are no spare devices, the allocation fails, leaving the logical volume as it is. If the allocation fails, you can fix and replace the failed device by using the lvconvert --repair command. For more information, see Replacing a failed RAID device in a logical volume.

Verification

  • Verify if the failed device is now replaced with a new device from the volume group:

    # lvs -a -o name,copy_percent,devices my_vg
      Couldn't find device with uuid 3lugiV-3eSP-AFAR-sdrP-H20O-wM2M-qdMANy.
      LV            Copy%  Devices
      lv            100.00 lv_rimage_0(0),lv_rimage_1(0),lv_rimage_2(0)
      [lv_rimage_0]        /dev/sdh1(1)
      [lv_rimage_1]        /dev/sdc1(1)
      [lv_rimage_2]        /dev/sdd1(1)
      [lv_rmeta_0]         /dev/sdh1(0)
      [lv_rmeta_1]         /dev/sdc1(0)
      [lv_rmeta_2]         /dev/sdd1(0)
    Note

    Even though the failed device is now replaced, the display still indicates that LVM could not find the failed device because the device is not yet removed from the volume group. You can remove the failed device from the volume group by executing the vgreduce --removemissing my_vg command.

Additional resources

  • lvm.conf(5) man page

9.18.2. Setting the RAID fault policy to warn

You can set the raid_fault_policy field to the warn parameter in the lvm.conf file. With this preference, the system adds a warning to the system log that indicates a failed device. Based on the warning, you can determine the further steps.

Procedure

  1. View the RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  2. Set the raid_fault_policy field to warn in the lvm.conf file:

    # vi /etc/lvm/lvm.conf
     # This configuration option has an automatic default value.
     raid_fault_policy = "warn"
  3. View the system log to display error messages if the /dev/sdb device fails:

    # grep lvm /var/log/messages
    
    Apr 14 18:48:59 virt-506 kernel: sd 25:0:0:0: rejecting I/O to offline device
    Apr 14 18:48:59 virt-506 kernel: I/O error, dev sdb, sector 8200 op 0x1:(WRITE) flags 0x20800 phys_seg 0 prio class 2
    [...]
    Apr 14 18:48:59 virt-506 dmeventd[91060]: WARNING: VG my_vg is missing PV 9R2TVV-bwfn-Bdyj-Gucu-1p4F-qJ2Q-82kCAF (last written to /dev/sdb).
    Apr 14 18:48:59 virt-506 dmeventd[91060]: WARNING: Couldn't find device with uuid 9R2TVV-bwfn-Bdyj-Gucu-1p4F-qJ2Q-82kCAF.
    Apr 14 18:48:59 virt-506 dmeventd[91060]: Use 'lvconvert --repair my_vg/ly_lv' to replace failed device.

    If the /dev/sdb device fails, the system log displays error messages. In this case, however, LVM will not automatically attempt to repair the RAID device by replacing one of the images. Instead, if the device has failed you can replace the device with the --repair argument of the lvconvert command. For more information, see Replacing a failed RAID device in a logical volume.

Additional resources

  • lvm.conf(5) man page

9.19. Replacing a RAID device in a logical volume

You can replace a RAID device in a logical volume depending on the following scenarios:

  • Replacing a working RAID device.
  • Replacing a failed RAID device in a logical volume.

9.19.1. Replacing a working RAID device

You can replace a working RAID device in a logical volume by using the --replace argument of the lvconvert command.

Warning

In the case of RAID device failure, the following commands do not work.

Prerequisites

  • The RAID device has not failed.

Procedure

  1. Create a RAID1 array:

    # lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
      Logical volume "my_lv" created
  2. Examine the created RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdb2(1)
      [my_lv_rimage_2]        /dev/sdc1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdb2(0)
      [my_lv_rmeta_2]         /dev/sdc1(0)
  3. Replace the RAID device with any of the following methods depending on your requirements:

    1. Replace a RAID1 device by specifying the physical volume that you want to replace:

      # lvconvert --replace /dev/sdb2 my_vg/my_lv
    2. Replace a RAID1 device by specifying the physical volume to use for the replacement:

      # lvconvert --replace /dev/sdb1 my_vg/my_lv /dev/sdd1
    3. Replace multiple RAID devices at a time by specifying multiple replace arguments:

      # lvconvert --replace /dev/sdb1 --replace /dev/sdc1 my_vg/my_lv

Verification

  1. Examine the RAID1 array after specifying the physical volume that you wanted to replace:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             37.50 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc2(1)
      [my_lv_rimage_2]        /dev/sdc1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc2(0)
      [my_lv_rmeta_2]         /dev/sdc1(0)
  2. Examine the RAID1 array after specifying the physical volume to use for the replacement:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             28.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sda1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sda1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
  3. Examine the RAID1 array after replacing multiple RAID devices at a time:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             60.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sda1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rimage_2]        /dev/sde1(1)
      [my_lv_rmeta_0]         /dev/sda1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
      [my_lv_rmeta_2]         /dev/sde1(0)

Additional resources

  • lvconvert(8) man page

9.19.2. Replacing a failed RAID device in a logical volume

RAID is not similar to traditional LVM mirroring. In case of LVM mirroring, remove the failed devices. Otherwise, the mirrored logical volume would hang while RAID arrays continue running with failed devices. For RAID levels other than RAID1, removing a device would mean converting to a lower RAID level, for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0.

Instead of removing a failed device and allocating a replacement, with LVM, you can replace a failed device that serves as a physical volume in a RAID logical volume by using the --repair argument of the lvconvert command.

Prerequisites

  • The volume group includes a physical volume that provides enough free capacity to replace the failed device.

    If no physical volume with enough free extents is available on the volume group, add a new, sufficiently large physical volume by using the vgextend utility.

Procedure

  1. View the RAID logical volume:

    # lvs --all --options name,copy_percent,devices my_vg
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdc1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdc1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  2. View the RAID logical volume after the /dev/sdc device fails:

    # lvs --all --options name,copy_percent,devices my_vg
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          [unknown](1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           [unknown](0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  3. Replace the failed device:

    # lvconvert --repair my_vg/my_lv
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
    Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
    Faulty devices in my_vg/my_lv successfully replaced.
  4. Optional: Manually specify the physical volume that replaces the failed device:

    # lvconvert --repair my_vg/my_lv replacement_pv
  5. Examine the logical volume with the replacement:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdc: open failed: No such device or address
      /dev/sdc1: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      LV               Cpy%Sync Devices
      my_lv            43.79    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

    Until you remove the failed device from the volume group, LVM utilities still indicate that LVM cannot find the failed device.

  6. Remove the failed device from the volume group:

    # vgreduce --removemissing my_vg

Verification

  1. View the available physical volumes after removing the failed device:

    # pvscan
    PV /dev/sde1 VG rhel_virt-506 lvm2 [<7.00 GiB / 0 free]
    PV /dev/sdb1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
  2. Examine the logical volume after the replacing the failed device:

    # lvs --all --options name,copy_percent,devices my_vg
    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

Additional resources

  • lvconvert(8) and vgreduce(8) man pages

9.20. Checking data coherency in a RAID logical volume

LVM provides scrubbing support for RAID logical volumes. RAID scrubbing is the process of reading all the data and parity blocks in an array and checking to see whether they are coherent. The lvchange --syncaction repair command initiates a background synchronization action on the array. The following attributes provide details about data coherency:

  • The raid_sync_action field displays the current synchronization action that the RAID logical volume is performing. It can be one of the following values:

    idle
    Completed all sync actions (doing nothing).
    resync
    Initializing or resynchronizing an array after an unclean machine shutdown.
    recover
    Replacing a device in the array.
    check
    Looking for array inconsistencies.
    repair
    Looking for and repairing inconsistencies.
  • The raid_mismatch_count field displays the number of discrepancies found during a check action.
  • The Cpy%Sync field displays the progress of the sync actions.
  • The lv_attr field provides additional indicators. Bit 9 of this field displays the health of the logical volume, and it supports the following indicators:

    m or mismatches
    Indicates that there are discrepancies in a RAID logical volume. You can see this character after the scrubbing operation detects the portions of the RAID, which are not coherent.
    r or refresh
    Indicates a failed device in a RAID array, even though LVM can read the device label and considers the device to be operational. Refresh the logical volume to notify the kernel that the device is now available, or replace the device if you suspect that it failed.

Procedure

  1. Optional: Limit the I/O bandwidth that the scrubbing process uses. When you perform a RAID scrubbing operation, the background I/O required by the sync actions can crowd out other I/O to LVM devices, such as updates to volume group metadata. This might cause the other LVM operations to slow down.

    You can control the rate of the scrubbing operation by implementing recovery throttling. You can set the recovery rate using --maxrecoveryrate Rate[bBsSkKmMgG] or --minrecoveryrate Rate[bBsSkKmMgG] with the lvchange --syncaction commands. For more information, see Minimum and maximum I/O rate options.

    Specify the Rate value as an amount per second for each device in the array. If you provide no suffix, the options assume kiB per second per device.

  2. Display the number of discrepancies in the array, without repairing them:

    # lvchange --syncaction check my_vg/my_lv

    This command initiates a background synchronization action on the array.

  3. Optional: View the var/log/syslog file for the kernel messages.
  4. Correct the discrepancies in the array:

    # lvchange --syncaction repair my_vg/my_lv

    This command repairs or replaces failed devices in a RAID logical volume. You can view the var/log/syslog file for the kernel messages after executing this command.

Verification

  1. Display information about the scrubbing operation:

    # lvs -o +raid_sync_action,raid_mismatch_count my_vg/my_lv
    LV    VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert SyncAction Mismatches
    my_lv my_vg rwi-a-r--- 500.00m                                    100.00           idle        0

Additional resources

9.21. Converting a RAID logical volume to another RAID level

LVM supports RAID takeover, which means converting a RAID logical volume from one RAID level to another, for example, from RAID 5 to RAID 6. You can change the RAID level to increase or decrease resilience to device failures.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid5 -i 3 -L 500M -n my_lv my_vg
    Using default stripesize 64.00 KiB.
    Rounding size 500.00 MiB (125 extents) up to stripe boundary size 504.00 MiB (126 extents).
    Logical volume "my_lv" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices,segtype
      LV               VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices                                                                 Type
      my_lv            my_vg         rwi-a-r--- 504.00m                                    100.00           my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0),my_lv_rimage_3(0) raid5
      [my_lv_rimage_0] my_vg         iwi-aor--- 168.00m                                                     /dev/sda(1)                                                             linear
  3. Convert the RAID logical volume to another RAID level:

    # lvconvert --type raid6 my_vg/my_lv
    Using default stripesize 64.00 KiB.
    Replaced LV type raid6 (same as raid6_zr) with possible type raid6_ls_6.
    Repeat this command to convert to raid6 after an interim conversion has finished.
    Are you sure you want to convert raid5 LV my_vg/my_lv to raid6_ls_6 type? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.
  4. Optional: If this command prompts to repeat the conversion, run:

    # lvconvert --type raid6 my_vg/my_lv

Verification

  1. View the RAID logical volume with the converted RAID level:

    # lvs -a -o +devices,segtype
      LV               VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices                                                                                   Type
      my_lv            my_vg         rwi-a-r--- 504.00m                                    100.00           my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0),my_lv_rimage_3(0),my_lv_rimage_4(0) raid6
      [my_lv_rimage_0] my_vg         iwi-aor--- 172.00m                                                     /dev/sda(1)                                                                               linear

Additional resources

  • lvconvert(8) and lvmraid(8) man pages

9.22. I/O Operations on a RAID1 logical volume

You can control the I/O operations for a device in a RAID1 logical volume by using the --writemostly and --writebehind parameters of the lvchange command. The following is the format for using these parameters:

--[raid]writemostly PhysicalVolume[:{t|y|n}]

Marks a device in a RAID1 logical volume as write-mostly and avoids all read actions to these drives unless necessary. Setting this parameter keeps the number of I/O operations to the drive to a minimum. Use the lvchange --writemostly /dev/sdb my_vg/ly_lv command to set this parameter.

You can set the writemostly attribute in the following ways:

:y
By default, the value of the writemostly attribute is yes for the specified physical volume in the logical volume.
:n
To remove the writemostly flag, append :n to the physical volume.
:t
To toggle the value of the writemostly attribute, specify the --writemostly argument. You can use this argument more than one time in a single command, to toggle the writemostly attributes for all the physical volumes in a logical volume at once.
--[raid]writebehind IOCount

Specifies the maximum number of pending writes marked as writemostly. These are the number of write operations applicable to devices in a RAID1 logical volume. After the value of this parameter exceeds, all write actions to the constituent devices complete synchronously before the RAID array notifies for completion of all write actions.

You can set this parameter by using the lvchange --writebehind 100 my_vg/ly_lv command. Setting the writemostly attribute’s value to zero clears the preference. With this setting, the system chooses the value arbitrarily.

9.23. Reshaping a RAID volume

RAID reshaping means changing attributes of a RAID logical volume without changing the RAID level. Some attributes that you can change include RAID layout, stripe size, and number of stripes.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid5 -i 2 -L 500M -n my_lv my_vg
    
    Using default stripesize 64.00 KiB.
    Rounding size 500.00 MiB (125 extents) up to stripe boundary size 504.00 MiB (126 extents).
    Logical volume "my_lv" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices
    
    LV               VG    Attr       LSize   Pool   Origin Data% Meta% Move Log Cpy%Sync Convert Devices
    my_lv            my_vg rwi-a-r--- 504.00m                                    100.00            my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
    [my_lv_rimage_0] my_vg iwi-aor--- 252.00m                                                      /dev/sda(1)
    [my_lv_rimage_1] my_vg iwi-aor--- 252.00m                                                      /dev/sdb(1)
    [my_lv_rimage_2] my_vg iwi-aor--- 252.00m                                                      /dev/sdc(1)
    [my_lv_rmeta_0]  my_vg ewi-aor---   4.00m                                                      /dev/sda(0)
    [my_lv_rmeta_1]  my_vg ewi-aor---   4.00m                                                      /dev/sdb(0)
    [my_lv_rmeta_2]  my_vg ewi-aor---   4.00m                                                      /dev/sdc(0)
  3. Optional: View the stripes images and stripesize of the RAID logical volume:

    # lvs -o stripes my_vg/my_lv
      #Str
         3
    # lvs -o stripesize my_vg/my_lv
      Stripe
      64.00k
  4. Modify the attributes of the RAID logical volume by using the following ways depending on your requirement:

    1. Modify the stripes images of the RAID logical volume:

      # lvconvert --stripes 3 my_vg/my_lv
      Using default stripesize 64.00 KiB.
      WARNING: Adding stripes to active logical volume my_vg/my_lv will grow it from 126 to 189 extents!
      Run "lvresize -l126 my_vg/my_lv" to shrink it or use the additional capacity.
      Are you sure you want to add 1 images to raid5 LV my_vg/my_lv? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.
    2. Modify the stripesize of the RAID logical volume:

      # lvconvert --stripesize 128k my_vg/my_lv
        Converting stripesize 64.00 KiB of raid5 LV my_vg/my_lv to 128.00 KiB.
      Are you sure you want to convert raid5 LV my_vg/my_lv? [y/n]: y
        Logical volume my_vg/my_lv successfully converted.
    3. Modify the maxrecoveryrate and minrecoveryrate attributes:

      # lvchange --maxrecoveryrate 4M my_vg/my_lv
        Logical volume my_vg/my_lv changed.
      # lvchange --minrecoveryrate 1M my_vg/my_lv
        Logical volume my_vg/my_lv changed.
    4. Modify the syncaction attribute:

      # lvchange --syncaction check my_vg/my_lv
    5. Modify the writemostly and writebehind attributes:

      # lvchange --writemostly /dev/sdb my_vg/my_lv
        Logical volume my_vg/my_lv changed.
      # lvchange --writebehind 100 my_vg/my_lv
        Logical volume my_vg/my_lv changed.

Verification

  1. View the stripes images and stripesize of the RAID logical volume:

    # lvs -o stripes my_vg/my_lv
      #Str
         4
    # lvs -o stripesize my_vg/my_lv
      Stripe
      128.00k
  2. View the RAID logical volume after modifying the maxrecoveryrate attribute:

    # lvs -a -o +raid_max_recovery_rate
      LV               VG       Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert MaxSync
      my_lv            my_vg    rwi-a-r---  10.00g                                     100.00           4096
      [my_lv_rimage_0] my_vg    iwi-aor---  10.00g
     [...]
  3. View the RAID logical volume after modifying the minrecoveryrate attribute:

    # lvs -a -o +raid_min_recovery_rate
      LV               VG     Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert MinSync
      my_lv            my_vg  rwi-a-r---  10.00g                                     100.00           1024
      [my_lv_rimage_0] my_vg  iwi-aor---  10.00g
      [...]
  4. View the RAID logical volume after modifying the syncaction attribute:

    # lvs -a
      LV               VG      Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      my_lv            my_vg   rwi-a-r---  10.00g                                     2.66
      [my_lv_rimage_0] my_vg   iwi-aor---  10.00g
      [...]

Additional resources

  • lvconvert(8) and lvmraid(8) man pages

9.24. Changing the region size on a RAID logical volume

When you create a RAID logical volume, the raid_region_size parameter from the /etc/lvm/lvm.conf file represents the region size for the RAID logical volume. After you created a RAID logical volume, you can change the region size of the volume. This parameter defines the granularity to keep track of the dirty or clean state. Dirty bits in the bitmap define the work set to synchronize after a dirty shutdown of a RAID volume, for example, a system failure.

If you set raid_region_size to a higher value, it reduces the size of bitmap as well as the congestion. But it impacts the write operation during resynchronizing the region because writes to RAID are postponed until synchronizing the region finishes.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid1 -m 1 -L 10G test
      Logical volume "lvol0" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices,region_size
    
    LV                VG      Attr     LSize Pool Origin Data% Meta% Move Log   Cpy%Sync Convert Devices                              Region
    lvol0             test rwi-a-r--- 10.00g                                    100.00           lvol0_rimage_0(0),lvol0_rimage_1(0)  2.00m
    [lvol0_rimage_0]  test iwi-aor--- 10.00g                                                     /dev/sde1(1)                            0
    [lvol0_rimage_1]  test iwi-aor--- 10.00g                                                     /dev/sdf1(1)                            0
    [lvol0_rmeta_0]   test ewi-aor---  4.00m                                                     /dev/sde1(0)                            0
    [lvol0_rmeta_1]   test ewi-aor---  4.00m

    The Region column indicates the raid_region_size parameter’s value.

  3. Optional: View the raid_region_size parameter’s value:

    # cat /etc/lvm/lvm.conf | grep raid_region_size
    
    # Configuration option activation/raid_region_size.
    	# raid_region_size = 2048
  4. Change the region size of a RAID logical volume:

    # lvconvert -R 4096K my_vg/my_lv
    
    Do you really want to change the region_size 512.00 KiB of LV my_vg/my_lv to 4.00 MiB? [y/n]: y
      Changed region size on RAID LV my_vg/my_lv to 4.00 MiB.
  5. Resynchronize the RAID logical volume:

    # lvchange --resync my_vg/my_lv
    
    Do you really want to deactivate logical volume my_vg/my_lv to resync it? [y/n]: y

Verification

  1. View the RAID logical volume:

    # lvs -a -o +devices,region_size
    
    LV               VG   Attr        LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices                              Region
    lvol0            test rwi-a-r--- 10.00g                                    6.25           lvol0_rimage_0(0),lvol0_rimage_1(0)  4.00m
    [lvol0_rimage_0] test iwi-aor--- 10.00g                                                   /dev/sde1(1)                            0
    [lvol0_rimage_1] test iwi-aor--- 10.00g                                                   /dev/sdf1(1)                            0
    [lvol0_rmeta_0]  test ewi-aor---  4.00m                                                   /dev/sde1(0)                            0

    The Region column indicates the changed value of the raid_region_size parameter.

  2. View the raid_region_size parameter’s value in the lvm.conf file:

    # cat /etc/lvm/lvm.conf | grep raid_region_size
    
    # Configuration option activation/raid_region_size.
    	# raid_region_size = 4096

Additional resources

  • lvconvert(8) man page

Chapter 10. Snapshot of logical volumes

Using the LVM snapshot feature, you can create virtual images of a volume, for example, /dev/sda, at a particular instant without causing a service interruption.

10.1. Overview of snapshot volumes

When you modify the original volume (the origin) after you take a snapshot, the snapshot feature makes a copy of the modified data area as it was prior to the change so that it can reconstruct the state of the volume. When you create a snapshot, full read and write access to the origin stays possible.

Since a snapshot copies only the data areas that change after the snapshot is created, the snapshot feature requires a minimal amount of storage. For example, with a rarely updated origin, 3-5 % of the origin’s capacity is sufficient to maintain the snapshot. It does not provide a substitute for a backup procedure. Snapshot copies are virtual copies and are not an actual media backup.

The size of the snapshot controls the amount of space set aside for storing the changes to the origin volume. For example, if you create a snapshot and then completely overwrite the origin, the snapshot should be at least as big as the origin volume to hold the changes. You should regularly monitor the size of the snapshot. For example, a short-lived snapshot of a read-mostly volume, such as /usr, would need less space than a long-lived snapshot of a volume because it contains many writes, such as /home.

If a snapshot is full, the snapshot becomes invalid because it can no longer track changes on the origin volume. But you can configure LVM to automatically extend a snapshot whenever its usage exceeds the snapshot_autoextend_threshold value to avoid snapshot becoming invalid. Snapshots are fully resizable and you can perform the following operations:

  • If you have the storage capacity, you can increase the size of the snapshot volume to prevent it from getting dropped.
  • If the snapshot volume is larger than you need, you can reduce the size of the volume to free up space that is needed by other logical volumes.

The snapshot volume provide the following benefits:

  • Most typically, you take a snapshot when you need to perform a backup on a logical volume without halting the live system that is continuously updating the data.
  • You can execute the fsck command on a snapshot file system to check the file system integrity and determine if the original file system requires file system repair.
  • Since the snapshot is read/write, you can test applications against production data by taking a snapshot and running tests against the snapshot without touching the real data.
  • You can create LVM volumes for use with Red Hat Virtualization. You can use LVM snapshots to create snapshots of virtual guest images. These snapshots can provide a convenient way to modify existing guests or create new guests with minimal additional storage.

10.2. Creating a snapshot of the original volume

Use the lvcreate command to create a snapshot of the original volume (the origin). A snapshot of a volume is writable. By default, a snapshot volume is activated with the origin during normal activation commands as compared to the thinly-provisioned snapshots. LVM does not support creating a snapshot volume that is larger than the sum of the origin volume’s size and the required metadata size for the volume. If you specify a snapshot volume that is larger than this, LVM creates a snapshot volume that is required for the size of the origin.

Note

The nodes in a cluster do not support LVM snapshots. You cannot create a snapshot volume in a shared volume group. However, if you need to create a consistent backup of data on a shared logical volume you can activate the volume exclusively and then create the snapshot.

The following procedure creates an origin logical volume named origin and a snapshot volume of this original volume named snap.

Prerequisites

Procedure

  1. Create a logical volume named origin from the volume group vg001:

    # lvcreate -L 1G -n origin vg001
    Logical volume "origin" created.
  2. Create a snapshot logical volume named snap of /dev/vg001/origin that is 100 MB in size:

    # lvcreate --size 100M --name snap --snapshot /dev/vg001/origin
      Logical volume "snap" created.

    You can also use the -L argument instead of using --size, -n instead of using --name, and -s instead of using --snapshot to create a snapshot.

    If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in to access the contents of the file system to run a backup while the original file system continues to get updated.

  3. Display the origin volume and the current percentage of the snapshot volume being used:

    # lvs -a -o +devices
      LV      VG    Attr       LSize  Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices
     origin vg001  owi-a-s---  1.00g                                                  /dev/sde1(0)
      snap vg001  swi-a-s--- 100.00m     origin 0.00                                 /dev/sde1(256)

    You can also display the status of logical volume /dev/vg001/origin with all the snapshot logical volumes and their status, such as active or inactive by using the lvdisplay /dev/vg001/origin command.

    Warning

    Space in the snapshot LV is consumed after the origin LV is written to. The lvs command reports the current snapshot space usage in the Data% data_percent field value. If the snapshot space reaches 100%, the snapshot becomes invalid and unusable.

    An invalid snapshot is reported with I in the fifth position of the Attr column, or the lv_snapshot_invalid reporting field in lvs. You can remove the invalid snapshot by using the lvremove command.

  4. Optional: Extend the snapshot before its space becomes 100% full and becomes invalid by using any one of the following options:

    • Configure LVM to automatically extend the snapshot by using the following parameters in the /etc/lvm.conf file:

      snapshot_autoextend_threshold
      Extends the snapshot after its usage exceeds the value set for this parameter. By default, it is set to 100, which disables automatic extension. The minimum value of this parameter is 50.
      snapshot_autoextend_percent
      Adds an additional space to the snapshot, which is the percent of its current size. By default, it is set to 20.

      In the following example, after setting the following parameters, the created 1G snapshot extends to 1.2G when its usage exceeds 700M:

      Example 10.1. Automatically extend the snapshot

      # vi /etc/lvm.conf
      snapshot_autoextend_threshold = 70
      snapshot_autoextend_percent = 20
      Note

      This feature requires unallocated space in the volume group. An automatic extension of a snapshot does not increase the size of a snapshot volume beyond the maximum calculated size that is necessary for the snapshot. Once a snapshot has grown large enough to cover the origin, it is no longer monitored for automatic extension.

    • Extend this snapshot manually by using the lvextend command:

      # lvextend -L+100M /dev/vg001/snap

Additional resources

  • lvcreate(8), lvextend(8), and lvs(8) man pages
  • /etc/lvm/lvm.conf file

10.3. Merging snapshot to its original volume

Use the lvconvert command with the --merge option to merge a snapshot into its original (the origin) volume. You can perform a system rollback if you have lost data or files, or otherwise you have to restore your system to a previous state. After you merge the snapshot volume, the resulting logical volume has the origin volume’s name, minor number, and UUID. While the merge is in progress, reads or writes to the origin appear as they were directed to the snapshot being merged. When the merge finishes, the merged snapshot is removed.

If both the origin and snapshot volume are not open and active, the merge starts immediately. Otherwise, the merge starts after either the origin or snapshot are activated and both are closed. You can merge a snapshot into an origin that cannot be closed, for example a root file system, after the origin volume is activated.

Procedure

  1. Merge the snapshot volume. The following command merges snapshot volume vg001/snap into its origin:

    # lvconvert --merge vg001/snap
    Merging of volume vg001/snap started.
      vg001/origin: Merged: 100.00%
  2. View the origin volume:

    # lvs -a -o +devices
      LV      VG    Attr       LSize  Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices
      origin vg001  owi-a-s---  1.00g                                                  /dev/sde1(0)

Additional resources

  • lvconvert(8) man page

Chapter 11. Creating and managing thin provisioned volumes (thin volumes)

Red Hat Enterprise Linux supports thin provisioned snapshot volumes and logical volumes.

Logical volumes and snapshot volumes can be thinly provisioned:

  • Using thin-provisioned logical volumes, you can create logical volumes that are larger than the available physical storage.
  • Using thin-provisioned snapshot volumes, you can store more virtual devices on the same data volume.

11.1. Overview of thin provisioning

Many modern storage stacks now provide the ability to choose between thick provisioning and thin provisioning:

  • Thick provisioning provides the traditional behavior of block storage where blocks are allocated regardless of their actual usage.
  • Thin provisioning grants the ability to provision a larger pool of block storage that may be larger in size than the physical device storing the data, resulting in over-provisioning. Over-provisioning is possible because individual blocks are not allocated until they are actually used. If you have multiple thin-provisioned devices that share the same pool, then these devices can be over-provisioned.

By using thin provisioning, you can over-commit the physical storage, and instead can manage a pool of free space known as a thin pool. You can allocate this thin pool to an arbitrary number of devices when needed by applications. You can expand the thin pool dynamically when needed for cost-effective allocation of storage space.

For example, if ten users each request a 100GB file system for their application, then you can create what appears to be a 100GB file system for each user but which is backed by less actual storage that is used only when needed.

Note

When using thin provisioning, it is important that you monitor the storage pool and add more capacity as the available physical space runs out.

The following are a few advantages of using thin-provisioned devices:

  • You can create logical volumes that are larger than the available physical storage.
  • You can have more virtual devices to be stored on the same data volume.
  • You can create file systems that can grow logically and automatically to support the data requirements and the unused blocks are returned to the pool for use by any file system in the pool

The following are the potential drawbacks of using thin-provisioned devices:

  • Thin-provisioned volumes have an inherent risk of running out of available physical storage. If you have over-provisioned your underlying storage, it could possibly result in an outage due to the lack of available physical storage. For example, if you create 10T of thinly provisioned storage with only 1T physical storage for backing, the volumes will become unavailable or unwritable after the 1T is exhausted.
  • If volumes are not sending discards to the layers after thin-provisioned devices, then the accounting for usage will not be accurate. For example, placing a file system without the -o discard mount option and not running fstrim periodically on top of thin-provisioned devices will never unallocate previously used storage. In such cases, you end up using the full provisioned amount over time even if you are not really using it.
  • You must monitor the logical and physical usage so as to not run out of available physical space.
  • Copy on Write (CoW) operation can be slower on file systems with snapshots.
  • Data blocks can be intermixed between multiple file systems leading to random access limitations of the underlying storage even when it does not appear that way to the end user.

11.2. Creating thinly-provisioned logical volumes

Using thin-provisioned logical volumes, you can create logical volumes that are larger than the available physical storage. Creating a thinly provisioned set of volumes allows the system to allocate what you use instead of allocating the full amount of storage that is requested.

Using the -T or --thin option of the lvcreate command, you can create either a thin pool or a thin volume. You can also use the -T option of the lvcreate command to create both a thin pool and a thin volume at the same time with a single command. This procedure describes how to create and grow thinly-provisioned logical volumes.

Prerequisites

Procedure

  1. Create a thin pool:

    # lvcreate -L 100M -T vg001/mythinpool
      Thin pool volume with chunk size 64.00 KiB can address at most 15.81 TiB of data.
      Logical volume "mythinpool" created.

    Note that since you are creating a pool of physical space, you must specify the size of the pool. The -T option of the lvcreate command does not take an argument; it determines what type of device is to be created from the other options that are added with the command. You can also create thin pool using additional parameters as shown in the following examples:

    • You can also create a thin pool using the --thinpool parameter of the lvcreate command. Unlike the -T option, the --thinpool parameter requires that you specify the name of the thin pool logical volume you are creating. The following example uses the --thinpool parameter to create a thin pool named mythinpool in the volume group vg001 that is 100M in size:

      # lvcreate -L 100M --thinpool mythinpool vg001
        Thin pool volume with chunk size 64.00 KiB can address at most 15.81 TiB of data.
        Logical volume "mythinpool" created.
    • As striping is supported for pool creation, you can use the -i and -I options to create stripes. The following command creates a 100M thin pool named as thinpool in volume group vg001 with two 64 kB stripes and a chunk size of 256 kB. It also creates a 1T thin volume named vg001/thinvolume.

      Note

      Ensure that there are two physical volumes with sufficient free space in the volume group or you cannot create the thin pool.

      # lvcreate -i 2 -I 64 -c 256 -L 100M -T vg001/thinpool -V 1T --name thinvolume
  2. Create a thin volume:

    # lvcreate -V 1G -T vg001/mythinpool -n thinvolume
      WARNING: Sum of all thin volume sizes (1.00 GiB) exceeds the size of thin pool vg001/mythinpool (100.00 MiB).
      WARNING: You have not turned on protection against thin pools running out of space.
      WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
      Logical volume "thinvolume" created.

    In this case, you are specifying virtual size for the volume that is greater than the pool that contains it. You can also create thin volumes using additional parameters as shown in the following examples:

    • To create both a thin volume and a thin pool, use the -T option of the lvcreate command and specify both the size and virtual size argument:

      # lvcreate -L 100M -T vg001/mythinpool -V 1G -n thinvolume
        Thin pool volume with chunk size 64.00 KiB can address at most 15.81 TiB of data.
        WARNING: Sum of all thin volume sizes (1.00 GiB) exceeds the size of thin pool vg001/mythinpool (100.00 MiB).
        WARNING: You have not turned on protection against thin pools running out of space.
        WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
        Logical volume "thinvolume" created.
    • To use the remaining free space to create a thin volume and thin pool, use the 100%FREE option:

      # lvcreate -V 1G -l 100%FREE -T vg001/mythinpool -n thinvolume
      Thin pool volume with chunk size 64.00 KiB can address at most <15.88 TiB of data.
        Logical volume "thinvolume" created.
    • To convert an existing logical volume to a thin pool volume, use the --thinpool parameter of the lvconvert command. You must also use the --poolmetadata parameter in conjunction with the --thinpool parameter to convert an existing logical volume to a thin pool volume’s metadata volume.

      The following example converts the existing logical volume lv1 in volume group vg001 to a thin pool volume and converts the existing logical volume lv2 in volume group vg001 to the metadata volume for that thin pool volume:

      # lvconvert --thinpool vg001/lv1 --poolmetadata vg001/lv2
        Converted vg001/lv1 to thin pool.
      Note

      Converting a logical volume to a thin pool volume or a thin pool metadata volume destroys the content of the logical volume, as lvconvert does not preserve the content of the devices but instead overwrites the content.

    • By default, the lvcreate command approximately sets the size of the thin pool metadata logical volume by using the following formula:

      Pool_LV_size / Pool_LV_chunk_size * 64

      If you have large numbers of snapshots or if you have have small chunk sizes for your thin pool and therefore expect significant growth of the size of the thin pool at a later time, you may need to increase the default value of the thin pool’s metadata volume using the --poolmetadatasize parameter of the lvcreate command. The supported value for the thin pool’s metadata logical volume is in the range between 2MiB and 16GiB.

      The following example illustrates how to increase the default value of the thin pools’ metadata volume:

      # lvcreate -V 1G -l 100%FREE -T vg001/mythinpool --poolmetadatasize 16M -n thinvolume
      Thin pool volume with chunk size 64.00 KiB can address at most 15.81 TiB of data.
         Logical volume "thinvolume" created.
  3. View the created thin pool and thin volume:

    # lvs -a -o +devices
      LV                 VG    Attr       LSize   Pool       Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
      [lvol0_pmspare]    vg001 ewi-------   4.00m                                                           /dev/sda(0)
      mythinpool         vg001 twi-aotz-- 100.00m                   0.00   10.94                            mythinpool_tdata(0)
      [mythinpool_tdata] vg001 Twi-ao---- 100.00m                                                           /dev/sda(1)
      [mythinpool_tmeta] vg001 ewi-ao----   4.00m                                                           /dev/sda(26)
      thinvolume         vg001 Vwi-a-tz--   1.00g mythinpool        0.00
  4. Optional: Extend the size of a thin pool with the lvextend command. You cannot, however, reduce the size of a thin pool.

    Note

    This command fails if you use -l 100%FREE argument while creating a thin pool and thin volume.

    The following command resizes an existing thin pool that is 100M in size by extending it another 100M:

    # lvextend -L+100M vg001/mythinpool
      Size of logical volume vg001/mythinpool_tdata changed from 100.00 MiB (25 extents) to 200.00 MiB (50 extents).
      WARNING: Sum of all thin volume sizes (1.00 GiB) exceeds the size of thin pool vg001/mythinpool (200.00 MiB).
      WARNING: You have not turned on protection against thin pools running out of space.
      WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
    
      Logical volume vg001/mythinpool successfully resized
    # lvs -a -o +devices
      LV                 VG    Attr       LSize   Pool       Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
      [lvol0_pmspare]    vg001 ewi-------   4.00m                                                           /dev/sda(0)
      mythinpool         vg001 twi-aotz-- 200.00m                   0.00   10.94                            mythinpool_tdata(0)
      [mythinpool_tdata] vg001 Twi-ao---- 200.00m                                                           /dev/sda(1)
      [mythinpool_tdata] vg001 Twi-ao---- 200.00m                                                           /dev/sda(27)
      [mythinpool_tmeta] vg001 ewi-ao----   4.00m                                                           /dev/sda(26)
      thinvolume         vg001 Vwi-a-tz--   1.00g mythinpool        0.00
  5. Optional: To rename the thin pool and thin volume, use the following command:

    # lvrename vg001/mythinpool vg001/mythinpool1
      Renamed "mythinpool" to "mythinpool1" in volume group "vg001"
    
    # lvrename vg001/thinvolume vg001/thinvolume1
      Renamed "thinvolume" to "thinvolume1" in volume group "vg001"

    View the thin pool and thin volume after renaming:

    # lvs
      LV          VG       Attr     LSize   Pool       Origin Data%  Move Log Copy%  Convert
    mythinpool1 vg001   twi-a-tz 100.00m                     0.00
    thinvolume1 vg001   Vwi-a-tz   1.00g mythinpool1         0.00
  6. Optional: To remove the thin pool, use the following command:

    # lvremove -f vg001/mythinpool1
      Logical volume "thinvolume1" successfully removed.
      Logical volume "mythinpool1" successfully removed.

Additional resources

  • lvcreate(8), lvrename(8), lvs(8), and lvconvert(8) man pages

11.3. Overview of chunk size

A chunk is the largest unit of physical disk dedicated to snapshot storage.

Use the following criteria for using the chunk size:

  • A smaller chunk size requires more metadata and hinders performance, but provides better space utilization with snapshots.
  • A bigger chunk size requires less metadata manipulation, but makes the snapshot less space efficient.

Be default, lvm2 starts with a 64KiB chunk size and estimates good metadata size for such chunk size. The minimal metadata size lvm2 can create and use is 2 MiB. If the metadata size needs to be larger than 128 MiB it begins to increase the chunk size, so the metadata size stays compact. However, this may result in some big chunk size values, which are less space efficient for snapshot usage. In such cases, a smaller chunk size and bigger metadata size is a better option.

To specify the chunk size according to your requirement, use the -c or --chunksize parameter to overrule lvm2 estimated chunk size. Be aware that you cannot change the chunk size once the thinpool is created.

If the volume data size is in the range of TiB, use ~15.8GiB as the metadata size, which is the maximum supported size, and set the chunk size according to your requirement. But, note that it is not possible to increase the metadata size if you need to extend the volume’s data size and have a small chunk size.

Note

Using the inappropriate combination of chunk size and metadata size may result in potentially problematic situation, when user runs out of space in metadata or they may not further grow their thin-pool size because of limited maximum addressable thin-pool data size.

Additional resources

  • lvmthin(7) man page

11.4. Thinly-provisioned snapshot volumes

Red Hat Enterprise Linux supports thinly-provisioned snapshot volumes. A snapshot of a thin logical volume also creates a thin logical volume (LV). A thin snapshot volume has the same characteristics as any other thin volume. You can independently activate the volume, extend the volume, rename the volume, remove the volume, and even snapshot the volume.

Note

Similarly to all LVM snapshot volumes, and all thin volumes, thin snapshot volumes are not supported across the nodes in a cluster. The snapshot volume must be exclusively activated on only one cluster node.

Traditional snapshots must allocate new space for each snapshot created, where data is preserved as changes are made to the origin. But thin-provisioning snapshots share the same space with the origin. Snapshots of thin LVs are efficient because the data blocks common to a thin LV and any of its snapshots are shared. You can create snapshots of thin LVs or from the other thin snapshots. Blocks common to recursive snapshots are also shared in the thin pool.

Thin snapshot volumes provide the following benefits:

  • Increasing the number of snapshots of the origin has a negligible impact on performance.
  • A thin snapshot volume can reduce disk usage because only the new data is written and is not copied to each snapshot.
  • There is no need to simultaneously activate the thin snapshot volume with the origin, which is a requirement of traditional snapshots.
  • When restoring an origin from a snapshot, it is not required to merge the thin snapshot. You can remove the origin and instead use the snapshot. Traditional snapshots have a separate volume where they store changes that must be copied back, that is, merged to the origin to reset it.
  • There is a significantly higher limit on the number of allowed snapshots as compared to the traditional snapshots.

Although there are many advantages for using thin snapshot volumes, there are some use cases for which the traditional LVM snapshot volume feature might be more appropriate to your needs. You can use traditional snapshots with all types of volumes. However, to use thin-snapshots requires you to use thin-provisioning.

Note

You cannot limit the size of a thin snapshot volume; the snapshot uses all of the space in the thin pool, if necessary. In general, you should consider the specific requirements of your site when deciding which snapshot format to use.

By default, a thin snapshot volume is skipped during normal activation commands.

11.5. Creating thinly-provisioned snapshot volumes

Using thin-provisioned snapshot volumes, you can have more virtual devices stored on the same data volume.

Important

When creating a thin snapshot volume, do not specify the size of the volume. If you specify a size parameter, the snapshot that will be created will not be a thin snapshot volume and will not use the thin pool for storing data. For example, the command lvcreate -s vg/thinvolume -L10M will not create a thin snapshot, even though the origin volume is a thin volume.

Thin snapshots can be created for thinly-provisioned origin volumes, or for origin volumes that are not thinly-provisioned. The following procedure describes different ways to create a thinly-provisioned snapshot volume.

Prerequisites

Procedure

  • Create a thinly-provisioned snapshot volume. The following command creates a thinly-provisioned snapshot volume named as mysnapshot1 of the thinly-provisioned logical volume vg001/thinvolume:

    # lvcreate -s --name mysnapshot1 vg001/thinvolume
      Logical volume "mysnapshot1" created
    # lvs
      LV          VG       Attr     LSize   Pool       Origin     Data%  Move Log Copy%  Convert
      mysnapshot1 vg001    Vwi-a-tz   1.00g mythinpool thinvolume   0.00
      mythinpool  vg001    twi-a-tz 100.00m                         0.00
      thinvolume  vg001    Vwi-a-tz   1.00g mythinpool              0.00
    Note

    When using thin provisioning, it is important that the storage administrator monitor the storage pool and add more capacity if it starts to become full. For information about extending the size of a thin volume, see Creating thinly-provisioned logical volumes .

  • You can also create a thinly-provisioned snapshot of a non-thinly-provisioned logical volume. Since the non-thinly-provisioned logical volume is not contained within a thin pool, it is referred to as an external origin. External origin volumes can be used and shared by many thinly-provisioned snapshot volumes, even from different thin pools. The external origin must be inactive and read-only at the time the thinly-provisioned snapshot is created.

    The following example creates a thin snapshot volume of the read-only, inactive logical volume named origin_volume. The thin snapshot volume is named mythinsnap. The logical volume origin_volume then becomes the thin external origin for the thin snapshot volume mythinsnap in volume group vg001 that uses the existing thin pool vg001/pool. The origin volume must be in the same volume group as the snapshot volume. Do not specify the volume group when specifying the origin logical volume.

    # lvcreate -s --thinpool vg001/pool origin_volume --name mythinsnap
  • You can create a second thinly-provisioned snapshot volume of the first snapshot volume by executing the following command.

    # lvcreate -s vg001/mysnapshot1 --name mysnapshot2
    Logical volume "mysnapshot2" created.

    To create a third thinly-provisioned snapshot volume, use the following command:

    # lvcreate -s vg001/mysnapshot2 --name mysnapshot3
    Logical volume "mysnapshot3" created.

Verification

  • Display a list of all ancestors and descendants of a thin snapshot logical volume:

    $ lvs -o name,lv_ancestors,lv_descendants vg001
      LV           Ancestors                           Descendants
      mysnapshot2  mysnapshot1,thinvolume              mysnapshot3
      mysnapshot1  thinvolume              mysnapshot2,mysnapshot3
      mysnapshot3  mysnapshot2,mysnapshot1,thinvolume
      mythinpool
      thinvolume           	           	     mysnapshot1,mysnapshot2,mysnapshot3

    Here,

  • thinvolume is an origin volume in volume group vg001.
  • mysnapshot1 is a snapshot of thinvolume
  • mysnapshot2 is a snapshot of mysnapshot1
  • mysnapshot3 is a snapshot of mysnapshot2

    Note

    The lv_ancestors and lv_descendants fields display existing dependencies. However, they do not track removed entries which can break a dependency chain if the entry was removed from the middle of the chain.

Additional resources

  • lvcreate(8) man page

11.6. Historical logical volumes

You can configure the system to track thin snapshot and thin logical volumes that have been removed by enabling the record_lvs_history metadata option in the lvm.conf configuration file. This allows you to display a full thin snapshot dependency chain that includes logical volumes that have been removed from the original dependency chain and have become historical logical volumes.

You can configure the system to retain historical volumes for a defined period of time by specifying the retention time, in seconds, with the lvs_history_retention_time metadata option in the lvm.conf configuration file.

A historical logical volume retains a simplified representation of the already removed logical volume, including the following reporting fields for the volume:

  • lv_time_removed: the removal time of the logical volume
  • lv_time: the creation time of the logical volume
  • lv_name: the name of the logical volume
  • lv_uuid: the UUID of the logical volume
  • vg_name: the volume group that contains the logical volume.
Important

A historical logical volume cannot be reactivated.

When you remove a volume, the historical logical volume name acquires a hyphen as a prefix. For example, if you remove the logical volume lvol1, the name of the historical volume becomes -lvol1.

Logical Volume Manager (LVM) does not keep historical logical volumes if the volume has no live descendant. This means that if you remove a logical volume at the end of a snapshot chain, the logical volume is not retained as a historical logical volume.

To include historical logical volumes in volume display, you specify the -H|--history option of an LVM display command. You can display a full thin snapshot dependency chain that includes historical volumes by specifying the lv_full_ancestors and lv_full_descendants reporting fields along with the -H option.

11.7. Tracking and displaying removed thin snapshot volumes

This procedure describes how to display and manage deleted historical logical volumes.

Procedure

  1. Ensure that historical logical volumes are retained by setting record_lvs_history=1 in the lvm.conf file. This metadata option is not enabled by default.
  2. Optional: Set the value of the lvs_history_retention_time option in seconds. This is the time interval after which a record about individual historical logical volume is automatically destroyed. The automatic default value is 0 and it disables this feature. If you do not set this option, you can also delete individual historical volumes manually.
  3. To display a thin-provisioned snapshot chain:

    In this example:

    • lvol1 is an origin volume, the first volume in the chain.
    • lvol2 is a snapshot of lvol1.
    • lvol3 is a snapshot of lvol2.
    • lvol4 is a snapshot of lvol3.
    • lvol5 is also a snapshot of lvol3.

      # lvs -H -o name,full_ancestors,full_descendants
        LV    FAncestors        FDescendants
        lvol1                   lvol2,lvol3,lvol4,lvol5
        lvol2 lvol1             lvol3,lvol4,lvol5
        lvol3 lvol2,lvol1       lvol4,lvol5
        lvol4 lvol3,lvol2,lvol1
        lvol5 lvol3,lvol2,lvol1
        pool

      Note that, despite using lvs utility with the -H option, no thin snapshot volume is removed and there are no historical logical volumes to display.

  4. Remove logical volume lvol3 from the snapshot chain:

    # lvremove -f vg/lvol3
      Logical volume "lvol3" successfully removed
  5. Run the lvs utility to see the details of historical logical volumes, along with their ancestors and descendants:

    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors         FDescendants
      lvol1                     lvol2,-lvol3,lvol4,lvol5
      lvol2  lvol1              -lvol3,lvol4,lvol5
      -lvol3 lvol2,lvol1        lvol4,lvol5
      lvol4  -lvol3,lvol2,lvol1
      lvol5  -lvol3,lvol2,lvol1
      pool
  6. Additionally, display the removal timestamp of a historical volume:

    # lvs -H -o name,full_ancestors,full_descendants,time_removed
      LV     FAncestors         FDescendants              RTime
      lvol1                     lvol2,-lvol3,lvol4,lvol5
      lvol2  lvol1              -lvol3,lvol4,lvol5
      -lvol3 lvol2,lvol1        lvol4,lvol5               2016-03-14 14:14:32 +0100
      lvol4  -lvol3,lvol2,lvol1
      lvol5  -lvol3,lvol2,lvol1
      pool
  7. You can reference historical logical volumes individually in a display command by specifying the vgname/lvname format:

    # lvs -H vg/-lvol3
      LV     VG   Attr       LSize
      -lvol3 vg   ----h-----    0

    Note that the fifth bit in the lv_attr field is set to h to indicate the volume is a historical one.

  8. LVM does not keep historical logical volumes if the volume has no live descendant. This means that if you remove a logical volume at the end of a snapshot chain, the logical volume is not retained as a historical logical volume.

    # lvremove -f vg/lvol5
      Automatically removing historical logical volume vg/-lvol5.
      Logical volume "lvol5" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors         FDescendants
      lvol1                     lvol2,-lvol3,lvol4
      lvol2  lvol1              -lvol3,lvol4
      -lvol3 lvol2,lvol1        lvol4
      lvol4  -lvol3,lvol2,lvol1
      pool
  9. Remove the volume lvol1 and lvol2 and to see how the lvs command displays the volumes once they have been removed.

    # lvremove -f vg/lvol1 vg/lvol2
      Logical volume "lvol1" successfully removed
      Logical volume "lvol2" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors           FDescendants
      -lvol1                      -lvol2,-lvol3,lvol4
      -lvol2 -lvol1               -lvol3,lvol4
      -lvol3 -lvol2,-lvol1        lvol4
      lvol4  -lvol3,-lvol2,-lvol1
      pool
  10. Remove a historical logical volume completely by specifying the name of the historical volume that now includes the hyphen, as in the following example

    # lvremove -f vg/-lvol3
    Historical logical volume "lvol3" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors    FDescendants
      -lvol1               -lvol2,lvol4
      -lvol2 -lvol1        lvol4
      lvol4  -lvol2,-lvol1
      pool

Chapter 12. Enabling caching to improve logical volume performance

You can add caching to an LVM logical volume to improve performance. LVM then caches I/O operations to the logical volume using a fast device, such as an SSD.

The following procedures create a special LV from the fast device, and attach this special LV to the original LV to improve the performance.

12.1. Caching methods in LVM

LVM provides the following kinds of caching. Each one is suitable for different kinds of I/O patterns on the logical volume.

dm-cache

This method speeds up access to frequently used data by caching it on the faster volume. The method caches both read and write operations.

The dm-cache method creates logical volumes of the type cache.

dm-writecache

This method caches only write operations. The faster volume stores the write operations and then migrates them to the slower disk in the background. The faster volume is usually an SSD or a persistent memory (PMEM) disk.

The dm-writecache method creates logical volumes of the type writecache.

Additional resources

  • lvmcache(7) man page

12.2. LVM caching components

LVM provides support for adding a cache to LVM logical volumes. LVM caching uses the following LVM logical volume types:

Main LV
The larger, slower, and original volume.
Cache pool LV
A composite LV that you can use for caching data from the main LV. It has two sub-LVs: data for holding cache data and metadata for managing the cache data. You can configure specific disks for data and metadata. You can use the cache pool only with dm-cache.
Cachevol LV
A linear LV that you can use for caching data from the main LV. You cannot configure separate disks for data and metadata. cachevol can be only used with either dm-cache or dm-writecache.

All of these associated LVs must be in the same volume group.

You can combine a main logical volume (LV) with a faster, usually smaller, LV that holds the cached data. The fast LV is created from fast block devices, such as SSD drives. When you enable caching for a logical volume, LVM renames and hides the original volumes, and presents a new logical volume that is composed of the original logical volumes. The composition of the new logical volume depends on the caching method and whether you are using the cachevol or cachepool option.

The cachevol and cachepool options expose different levels of control over the placement of the caching components:

  • With the cachevol option, the faster device stores both the cached copies of data blocks and the metadata for managing the cache.
  • With the cachepool option, separate devices can store the cached copies of data blocks and the metadata for managing the cache.

    The dm-writecache method is not compatible with cachepool.

In all configurations, LVM exposes a single resulting device, which groups together all the caching components. The resulting device has the same name as the original slow logical volume.

Additional resources

12.3. Enabling dm-cache caching for a logical volume

This procedure enables caching of commonly used data on a logical volume using the dm-cache method.

Prerequisites

  • A slow logical volume that you want to speed up using dm-cache exists on your system.
  • The volume group that contains the slow logical volume also contains an unused physical volume on a fast block device.

Procedure

  1. Create a cachevol volume on the fast device:

    # lvcreate --size cachevol-size --name <fastvol> <vg> </dev/fast-pv>

    Replace the following values:

    cachevol-size
    The size of the cachevol volume, such as 5G
    fastvol
    A name for the cachevol volume
    vg
    The volume group name
    /dev/fast-pv

    The path to the fast block device, such as /dev/sdf

    Example 12.1. Creating a cachevol volume

    # lvcreate --size 5G --name fastvol vg /dev/sdf
    Logical volume "fastvol" created.
  2. Attach the cachevol volume to the main logical volume to begin caching:

    # lvconvert --type cache --cachevol <fastvol> <vg/main-lv>

    Replace the following values:

    fastvol
    The name of the cachevol volume
    vg
    The volume group name
    main-lv

    The name of the slow logical volume

    Example 12.2. Attaching the cachevol volume to the main LV

    # lvconvert --type cache --cachevol fastvol vg/main-lv
    Erase all existing data on vg/fastvol? [y/n]: y
    Logical volume vg/main-lv is now cached.

Verification steps

  • Verify if the newly created logical volume has dm-cache enabled:

    # lvs --all --options +devices <vg>
    
    LV              Pool           Type   Devices
    main-lv         [fastvol_cvol] cache  main-lv_corig(0)
    [fastvol_cvol]                 linear /dev/fast-pv
    [main-lv_corig]                linear /dev/slow-pv

Additional resources

  • lvmcache(7) man page

12.4. Enabling dm-cache caching with a cachepool for a logical volume

This procedure enables you to create the cache data and the cache metadata logical volumes individually and then combine the volumes into a cache pool.

Prerequisites

  • A slow logical volume that you want to speed up using dm-cache exists on your system.
  • The volume group that contains the slow logical volume also contains an unused physical volume on a fast block device.

Procedure

  1. Create a cachepool volume on the fast device:

    # lvcreate --type cache-pool --size <cachepool-size> --name <fastpool> <vg /dev/fast>

    Replace the following values:

    cachepool-size
    The size of the cachepool, such as 5G
    fastpool
    A name for the cachepool volume
    vg
    The volume group name
    /dev/fast

    The path to the fast block device, such as /dev/sdf1

    Note

    You can use --poolmetadata option to specify the location of the pool metadata when creating the cache-pool.

    Example 12.3. Creating a cachepool volume

    # lvcreate --type cache-pool --size 5G --name fastpool vg /dev/sde
    Logical volume "fastpool" created.
  2. Attach the cachepool to the main logical volume to begin caching:

    # lvconvert --type cache --cachepool <fastpool> <vg/main>

    Replace the following values:

    fastpool
    The name of the cachepool volume
    vg
    The volume group name
    main

    The name of the slow logical volume

    Example 12.4. Attaching the cachepool to the main LV

    # lvconvert --type cache --cachepool fastpool vg/main
    Do you want wipe existing metadata of cache pool vg/fastpool? [y/n]: y
    Logical volume vg/main is now cached.

Verification steps

  • Examine the newly created devicevolume with the cache-pool type:

    # lvs --all --options +devices <vg>
    
    LV                      Pool               Type        Devices
    [fastpool_cpool]                           cache-pool  fastpool_pool_cdata(0)
    [fastpool_cpool_cdata]                     linear      /dev/sdf1(4)
    [fastpool_cpool_cmeta]                     linear      /dev/sdf1(2)
    [lvol0_pmspare]                            linear      /dev/sdf1(0)
    main                    [fastpoool_cpool]  cache       main_corig(0)
    [main_corig]                               linear      /dev/sdf1(O)

Additional resources

  • lvcreate(8) man page
  • lvmcache(7) man page
  • lvconvert(8) man page

12.5. Enabling dm-writecache caching for a logical volume

This procedure enables caching of write I/O operations to a logical volume using the dm-writecache method.

Prerequisites

  • A slow logical volume that you want to speed up using dm-writecache exists on your system.
  • The volume group that contains the slow logical volume also contains an unused physical volume on a fast block device.
  • If the slow logical volume is active, deactivate it.

Procedure

  1. If the slow logical volume is active, deactivate it:

    # lvchange --activate n <vg>/<main-lv>

    Replace the following values:

    vg
    The volume group name
    main-lv
    The name of the slow logical volume
  2. Create a deactivated cachevol volume on the fast device:

    # lvcreate --activate n --size <cachevol-size> --name <fastvol> <vg> </dev/fast-pv>

    Replace the following values:

    cachevol-size
    The size of the cachevol volume, such as 5G
    fastvol
    A name for the cachevol volume
    vg
    The volume group name
    /dev/fast-pv

    The path to the fast block device, such as /dev/sdf

    Example 12.5. Creating a deactivated cachevol volume

    # lvcreate --activate n --size 5G --name fastvol vg /dev/sdf
    WARNING: Logical volume vg/fastvol not zeroed.
    Logical volume "fastvol" created.
  3. Attach the cachevol volume to the main logical volume to begin caching:

    # lvconvert --type writecache --cachevol <fastvol> <vg/main-lv>

    Replace the following values:

    fastvol
    The name of the cachevol volume
    vg
    The volume group name
    main-lv

    The name of the slow logical volume

    Example 12.6. Attaching the cachevol volume to the main LV

    # lvconvert --type writecache --cachevol fastvol vg/main-lv
    Erase all existing data on vg/fastvol? [y/n]?: y
    Using writecache block size 4096 for unknown file system block size, logical block size 512, physical block size 512.
    WARNING: unable to detect a file system block size on vg/main-lv
    WARNING: using a writecache block size larger than the file system block size may corrupt the file system.
    Use writecache block size 4096? [y/n]: y
    Logical volume vg/main-lv now has writecache.
  4. Activate the resulting logical volume:

    # lvchange --activate y <vg/main-lv>

    Replace the following values:

    vg
    The volume group name
    main-lv
    The name of the slow logical volume

Verification steps

  • Examine the newly created devices:

    # lvs --all --options +devices vg
    
    LV                VG Attr       LSize   Pool           Origin           Data%  Meta%  Move Log Cpy%Sync Convert Devices
     main-lv          vg Cwi-a-C--- 500.00m [fastvol_cvol] [main-lv_wcorig] 0.00                                    main-lv_wcorig(0)
     [fastvol_cvol]   vg Cwi-aoC--- 252.00m                                                                         /dev/sdc1(0)
     [main-lv_wcorig] vg owi-aoC--- 500.00m                                                                         /dev/sdb1(0)

Additional resources

  • lvmcache(7) man page

12.6. Disabling caching for a logical volume

This procedure disables dm-cache or dm-writecache caching that is currently enabled on a logical volume.

Prerequisites

  • Caching is enabled on a logical volume.

Procedure

  1. Deactivate the logical volume:

    # lvchange --activate n <vg>/<main-lv>

    Replace vg with the volume group name, and main-lv with the name of the logical volume where caching is enabled.

  2. Detach the cachevol or cachepool volume:

    # lvconvert --splitcache <vg>/<main-lv>

    Replace the following values:

    Replace vg with the volume group name, and main-lv with the name of the logical volume where caching is enabled.

    Example 12.7. Detaching the cachevol or cachepool volume

    # lvconvert --splitcache vg/main-lv
    Detaching writecache already clean.
    Logical volume vg/main-lv writecache has been detached.

Verification steps

  • Check that the logical volumes are no longer attached together:

    # lvs --all --options +devices <vg>
    
    LV      Attr       Type   Devices
    fastvol -wi------- linear /dev/fast-pv
    main-lv -wi------- linear /dev/slow-pv

Additional resources

  • The lvmcache(7) man page

Chapter 13. Logical volume activation

By default, when you create a logical volume, it is in an active state. A logical volume that is an active state can be used through a block device. An activated logical volume is accessible and is subject to change.

There are various circumstances, where you need to make an individual logical volume inactive and therefore unknown to the kernel. You can activate or deactivate individual logical volume with the -a option of the lvchange command.

The following is the format to deactivate an individual logical volume:

# lvchange -an vg/lv

The following is the format to activate an individual logical volume:

# lvchange -ay vg/lv

You can activate or deactivate all of the logical volumes in a volume group with the -a option of the vgchange command. This is the equivalent of running the lvchange -a command on each individual logical volume in the volume group.

The following is the format to deactivate all of the logical volumes in a volume group:

# vgchange -an vg

The following is the format to activate all of the logical volumes in a volume group:

# vgchange -ay vg
Note

During manual activation, the systemd automatically mounts LVM volumes with the corresponding mount point from the /etc/fstab file unless the systemd-mount unit is masked.

13.1. Controlling autoactivation of logical volumes and volume groups

Autoactivation of a logical volume refers to the event-based automatic activation of a logical volume during system startup.

You can set the autoactivation property on a VG or LV. When the autoactivation property is disabled, the VG or LV will not be activated by a command doing autoactivation, such as vgchange, lvchange, or pvscan using -aay option. If autoactivation is disabled on a VG, no LVs will be autoactivated in that VG, and the autoactivation property has no effect. If autoactivation is enabled on a VG, autoactivation can be disabled for individual LVs.

Procedure

  • You can update the autoactivation settings in one of the following ways:

    • Control autoactivation of a VG using the command line:

      # vgchange --setautoactivation <y|n>
    • Control autoactivation of a LV using the command line:

      # lvchange --setautoactivation <y|n>
    • You can use the activation/auto_activation_volume_list configuration option in the /etc/lvm/lvm.conf configuration file to control autoactivation of specific LVs and VGs:

      auto_activation_volume_list = [ "<VG_name>", "<VG_name>/<LV_name>", "<@tag1>", "<…​>" ]

      If you set the auto_activation_volume_list to [ ] (empty list), autoactivation is disabled entirely.

Additional resources

  • /etc/lvm/lvm.conf configuration file
  • lvmautoactivation(7) man page

13.2. Controlling logical volume activation

You can control the activation of logical volume in the following ways:

  • Through the activation/volume_list setting in the /etc/lvm/conf file. This allows you to specify which logical volumes are activated. For information about using this option, see the /etc/lvm/lvm.conf configuration file.
  • By means of the activation skip flag for a logical volume. When this flag is set for a logical volume, the volume is skipped during normal activation commands.

Alternatively, you can use the --setactivationskip y|n option with the lvcreate or the lvchange commands to enable or disable the activation skip flag.

Procedure

  • You can set the activation skip flag on a logical volume in the following ways:

    • To determine whether the activation skip flag is set for a logical volume run the lvs command, which displays the k attribute as in the following example:

      # lvs vg/thin1s1
        LV         VG  Attr       LSize Pool  Origin
        thin1s1    vg  Vwi---tz-k 1.00t pool0 thin1

      You can activate a logical volume with the k attribute set by using the -K or --ignoreactivationskip option in addition to the standard -ay or --activate y option.

      By default, thin snapshot volumes are flagged for activation skip when they are created. You can control the default activation skip setting on new thin snapshot volumes with the auto_set_activation_skip setting in the /etc/lvm/lvm.conf file.

    • The following command activates a thin snapshot logical volume that has the activation skip flag set:

      # lvchange -ay -K VG/SnapLV
    • The following command creates a thin snapshot without the activation skip flag:

      # lvcreate -n SnapLV -kn -s vg/ThinLV --thinpool vg/ThinPoolLV
    • The following command removes the activation skip flag from a snapshot logical volume:

      # lvchange -kn VG/SnapLV

Verification steps

  • Verify if a thin snapshot without the activation skip flag has been created:

    # lvs -a -o +devices,segtype
      LV                 VG            Attr       LSize    Pool       Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices             Type
      SnapLV             vg            Vwi-a-tz--  100.00m ThinPoolLV ThinLV 0.00                                                        thin
      ThinLV             vg            Vwi-a-tz--  100.00m ThinPoolLV        0.00                                                        thin
      ThinPoolLV         vg            twi-aotz--  100.00m                   0.00   10.94                            ThinPoolLV_tdata(0) thin-pool
      [ThinPoolLV_tdata] vg            Twi-ao----  100.00m                                                           /dev/sdc1(1)        linear
      [ThinPoolLV_tmeta] vg            ewi-ao----    4.00m                                                           /dev/sdd1(0)        linear
      [lvol0_pmspare]    vg            ewi-------    4.00m                                                           /dev/sdc1(0)        linear

13.3. Activating shared logical volumes

You can control logical volume activation of a shared logical volume with the -a option of the lvchange and vgchange commands, as follows:

CommandActivation

lvchange -ay|-aey

Activate the shared logical volume in exclusive mode, allowing only a single host to activate the logical volume. If the activation fails, as would happen if the logical volume is active on another host, an error is reported.

lvchange -asy

Activate the shared logical volume in shared mode, allowing multiple hosts to activate the logical volume concurrently. If the activation fails, as would happen if the logical volume is active exclusively on another host, an error is reported. If the logical type prohibits shared access, such as a snapshot, the command will report an error and fail. Logical volume types that cannot be used concurrently from multiple hosts include thin, cache, raid, and snapshot.

lvchange -an

Deactivate the logical volume.

13.4. Activating a logical volume with missing devices

You can control whether LVs that are missing devices can be activated by using the lvchange command with the --activationmode partial|degraded|complete option. The values are described below:

Activation ModeMeaning

complete

Allows only logical volumes with no missing physical volumes to be activated. This is the most restrictive mode.

degraded

Allows RAID logical volumes with missing physical volumes to be activated.

partial

Allows any logical volume with missing physical volumes to be activated. This option should be used for recovery or repair only.

The default value of activationmode is determined by the activationmode setting in the /etc/lvm/lvm.conf file. It is used if no command line option is given.

Additional resources

  • lvmraid(7) man page

Chapter 14. Limiting LVM device visibility and usage

You can limit the devices that are visible and usable to Logical Volume Manager (LVM) by controlling the devices that LVM can scan.

Use LVM commands to control LVM device scanning. LVM commands interact with a file called the system.devices file, which lists the visible and usable devices. This feature is enabled by default in Red Hat Enterprise Linux 9.

If you disable the devices file feature, the LVM device filter is enabled automatically.

To adjust the configuration of LVM device scanning, edit the LVM device filter settings in the /etc/lvm/lvm.conf file. The filters in the lvm.conf file consist of a series of simple regular expressions. The system applies these expressions to each device name in the /dev directory to decide whether to accept or reject each detected block device.

14.1. The LVM devices file

The Logical Volume Manager (LVM) system.devices file controls device visibility and usability to LVM. You can find the devices file in the /etc/lvm/devices/ directory. Use LVM commands to manage the devices file. Do not directly edit the system.devices file.

By default, the system.devices file feature is enabled in Red Hat Enterprise Linux 9. When active, it replaces the LVM device filter. To enable the LVM device filter, disable the system.devices file. For more information see Disabling the system.devices file.

14.1.1. Additional resources

  • lvmdevices(8) and lvm.conf(5) man pages

14.1.2. Adding devices to the system.devices file

To use devices with the Logical Volume Manager (LVM), the system.devices file must contain a list of the device IDs, otherwise LVM ignores them. The operating system (OS) installer adds devices to the system.devices file during installation. A newly installed system includes the root device into the devices file automatically. Any Physical Volumes (PV) attached to the system during OS installation are also included into the devices file. You can also specifically add devices to the devices file. LVM detects and uses only the list of devices stored in the devices file.

Procedure

Add devices to the system.devices file by using one of the following methods:

  • Add devices by including their names to the devices file:

    $ lvmdevices --adddev <device_name>
  • Add all devices in a Volume Group (VG) to the devices file:

    $ vgimportdevices <vg_name>
  • Add all devices in all visible VGs to the devices file:

    $ vgimportdevices --all

To implicitly include new devices into the system.devices file, use one of the following commands:

  • Use the pvcreate command to initialize a new device:

    $ pvcreate <device_name>
    • This action automatically adds the new Physical Volume (PV) to the system.devices file.
  • Initialize new devices and add the new device arguments to the devices file automatically:

    $ vgcreate <vg_name> <device_names>
    • Replace <vg_name> with the name of the VG, from which you want to add devices.
    • Replace <device_names> with a space-separated list of the devices you want to add.
  • Use the vgextend command to initialize new devices:

    $ vgextend <vg_name> <device_names>
    • Replace <vg_name> with the name of the VG, from which you want to add devices.
    • Replace <device_names> with the names of the devices you want to add.
    • This adds the new device arguments to the devices file automatically.

Verification

Use the following verification steps only in case you need to explicitly add new devices to the system.devices file.

  • Display the system.devices file, to check the list of devices:

    $ cat /etc/lvm/devices/system.devices
  • Update the system.devices file to match most recent device information:

    $ lvmdevices --update

Additional resources

  • lvmdevices(8), pvcreate(8), vgcreate(8) and vgextend(8) man pages

14.1.3. Removing devices from the system.devices file

Remove a device to prevent the Logical Volume Manager (LVM) from detecting or using that device.

Procedure

  • Remove a device by using one of the following methods depending on the information you have about that device:

    • Remove a device by name:

      $ lvmdevices --deldev <device_name>
    • Remove a device by the Physical Volume ID (PVID) of the device:

      $ lvmdevices --delpvid <PV_UUID>

Verification

Use the following verification steps only in case you need to explicitly remove a devices in the system.devices file.

  • Display the system.devices file to verify, that the deleted device no longer present:

    $ cat /etc/lvm/devices/system.devices
  • Update the system.devices file to match most recent device information:

    $ lvmdevices --update

Additional resources

  • lvmdevices(8) man page

14.1.4. Creating custom devices files

Logical Volume Manager (LVM) commands use the default system.devices file of the system. You can also create and use custom devices files by specifying the new file name in the LVM commands. Custom devices files are useful in cases when only certain applications need to use certain devices.

Procedure

  1. Create a custom devices file in the /etc/lvm/devices/ directory.
  2. Include the new devices file name in the LVM command:

    $ lvmdevices --devicesfile <devices_file_name>
  3. Optional: Display the new devices file to verify that the name of the new device is present:

    $ cat /etc/lvm/devices/<devices_file_name>

Additional resources

  • lvmdevices(8) man page

14.1.5. Accessing all devices on the system

You can enable Logical Volume Manager (LVM) to access and use all devices on the system, which overrides the restrictions caused by the devices listed in the system.devices file.

Procedure

  • Specify an empty devices file:

    $ lvmdevices --devicesfile ""

Additional resources

  • lvmdevices(8) man page

14.1.6. Disabling the system.devices file

You can disable the system.devices file functionality. This action automatically enables the Logical Volume Manager (LVM) device filter.

Procedure

  1. Open the lvm.conf file.
  2. Set the following value in the devices section:
use_devicesfile=0
Important

If you remove the system.devices file, this action effectively disables it. This applies even if you enable the system.devices file in the lvm.conf configuration file by setting use_devicesfile=1 in the devices section. Disabling the devices file automatically enables the lvm.conf device filter.

Additional resources

  • lvmdevices(8) and lvm.conf(5) man pages

14.2. Persistent identifiers for LVM filtering

Traditional Linux device names, such as /dev/sda, are subject to changes during system modifications and reboots. Persistent Naming Attributes (PNAs) like World Wide Identifier (WWID), Universally Unique Identifier (UUID), and path names are based on unique characteristics of the storage devices and are resilient to changes in hardware configurations. This makes them more stable and predictable across system reboots.

Implementation of persistent device identifiers in LVM filtering enhances the stability and reliability of LVM configurations. It also reduces the risk of system boot failures associated with the dynamic nature of device names.

14.3. The LVM device filter

The Logical Volume Manager (LVM) device filter is a list of device name patterns. You can use it to specify a set of mandatory criteria by which the system can evaluate devices and consider them as valid for use with LVM. The LVM device filter enables you control over which devices LVM uses. This can help to prevent accidental data loss or unauthorized access to storage devices.

14.3.1. LVM device filter pattern characteristics

The patterns of LVM device filter are in the form of regular expression. A regular expression delimits with a character and precedes with either a for acceptance, or r for rejection. The first regular expression in the list that matches a device determines if LVM accepts or rejects (ignores) a specific device. Then, LVM looks for the initial regular expression in the list that matches the path of a device. LVM uses this regular expression to determine whether the device should be approved with an a outcome or rejected with an r outcome.

If a single device has multiple path names, LVM accesses these path names according to their order of listing. Before any r pattern, if at least one path name matches an a pattern, LVM approves the device. However, if all path names are consistent with an r pattern before an a pattern is found, the device is rejected.

Path names that do not match the pattern do not affect the approval status of the device. If no path names correspond to a pattern for a device, LVM still approves the device.

For each device on the system, the udev rules generate multiple symlinks. Directories contain symlinks, such as /dev/disk/by-id/, /dev/disk/by-uuid/, /dev/disk/by-path/ to ensure that each device on the system is accessible through multiple path names.

To reject a device in the filter, all of the path names associated with that particular device must match the corresponding reject r expressions. However, identifying all possible path names to reject can be challenging. This is why it is better to create filters that specifically accept certain paths and reject all others, using a series of specific a expressions followed by a single r|.*| expression that rejects everything else.

While defining a specific device in the filter, use a symlink name for that device instead of the kernel name. The kernel name for a device can change, such as /dev/sda while certain symlink names do not change such as /dev/disk/by-id/wwn-*.

The default device filter accepts all devices connected to the system. An ideal user configured device filter accepts one or more patterns and rejects everything else. For example, the pattern list ending with r|.*|.

You can find the LVM devices filter configuration in the devices/filter and devices/global_filter configuration fields in the lvm.conf file. The devices/filter and devices/global_filter configuration fields are equivalent.

Important

In Red Hat Enterprise Linux 9, the /etc/lvm/devices/system.devices file is enabled by default. The system automatically enables the LVM devices filter, when the system.devices file is disabled.

Additional resources

  • lvm.conf(5) man page

14.3.2. Examples of LVM device filter configurations

The following examples display the filter configurations to control the devices that LVM scans and uses later. To configure the device filter in the lvm.conf file, see

Note

You may encountered duplicate Physical Volume (PV) warnings when dealing with copied or cloned PVs. You can set up filters to resolve this. See the example filter configurations in link::https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html/configuring_and_managing_logical_volumes/troubleshooting-lvm_configuring-and-managing-logical-volumes#troubleshooting-duplicate-physical-volume-warnings-for-multipathed-lvm-devices_troubleshooting-lvm[Troubleshooting duplicate physical volume warnings for multipathed LVM devices].

  • To scan all the devices, enter:

    filter = [ "a|.*|" ]
  • To remove the cdrom device to avoid delays if the drive contains no media, enter:

    filter = [ "r|^/dev/cdrom$|" ]
  • To add all loop devices and remove all other devices, enter:

    filter = [ "a|loop|", "r|.*|" ]
  • To add all loop and SCSI devices and remove all other block devices, enter:

    filter = [ "a|loop|", "a|/dev/sd.*|", "r|.*|" ]
  • To add only partition 8 on the first SCSI drive and remove all other block devices, enter:

    filter = [ "a|^/dev/sda8$|", "r|.*|" ]
  • To add all partitions from a specific device identified by WWID along with all multipath devices, enter:

    filter = [ "a|/dev/disk/by-id/<disk-id>.|", "a|/dev/mapper/mpath.|", "r|.*|" ]

    The command also removes any other block devices.

Additional resources

  • lvm.conf(5) man page

14.3.3. Applying an LVM device filter configuration

You can control which devices LVM scans by setting up filters in the lvm.conf configuration file.

Prerequisites

  • You have disabled the system.devices file feature.
  • You have prepared the device filter pattern that you want to use.

Procedure

  1. Use the following command to test the device filter pattern, without actually modifying the /etc/lvm/lvm.conf file. The following includes an example filter configuration.

    # lvs --config 'devices{ filter = [ "a|/dev/emcpower.*|", "r|*.|" ] }'
  2. Add the device filter pattern in the configuration section devices of the /etc/lvm/lvm.conf file:

    filter = [ "a|/dev/emcpower.*|", "r|*.|" ]
  3. Scan only necessary devices on reboot:

    # dracut --force --verbose

    This command rebuilds the initramfs file system so that LVM scans only the necessary devices at the time of reboot.

Chapter 15. Controlling LVM allocation

By default, a volume group uses the normal allocation policy. This allocates physical extents according to common-sense rules such as not placing parallel stripes on the same physical volume. You can specify a different allocation policy (contiguous, anywhere, or cling) by using the --alloc argument of the vgcreate command. In general, allocation policies other than normal are required only in special cases where you need to specify unusual or nonstandard extent allocation.

15.1. Allocating extents from specified devices

You can restrict the allocation from specific devices by using the device arguments at the end of the command line with the lvcreate and the lvconvert commands. You can specify the actual extent ranges for each device for more control. The command only allocates extents for the new logical volume (LV) by using the specified physical volume (PV) as arguments. It takes available extents from each PV until they run out and then takes extents from the next PV listed. If there is not enough space on all the listed PVs for the requested LV size, then command fails. Note that the command only allocates from the named PVs. Raid LVs use sequential PVs for separate raid images or separate stripes. If the PVs are not large enough for an entire raid image, then the resulting device use is not entirely predictable.

Procedure

  1. Create a volume group (VG):

    # vgcreate <vg_name> <PV> ...

    Where:

    • <vg_name> is the name of the VG.
    • <PV> are the PVs.
  2. You can allocate PV to create different volume types, such as linear or raid:

    1. Allocate extents to create a linear volume:

      # lvcreate -n <lv_name> -L <lv_size> <vg_name> [ <PV> ... ]

      Where:

      • <lv_name> is the name of the LV.
      • <lv_size> is the size of the LV. Default unit is megabytes.
      • <vg_name> is the name of the VG.
      • [ <PV …​> ] are the PVs.

        You can specify one of the PVs, all of them, or none on the command line:

        • If you specify one PV, extents for that LV will be allocated from it.

          Note

          If the PV does not have sufficient free extents for the entire LV, then the lvcreate fails.

        • If you specify two PVs, extents for that LV will be allocated from one of them, or a combination of both.
        • If you do not specify any PV, extents will be allocated from one of the PVs in the VG, or any combination of all PVs in the VG.

          Note

          In these cases, LVM might not use all of the named or available PVs. If the first PV has sufficient free extents for the entire LV, then the other PV will probably not be used. However, if the first PV does not have a set allocation size of free extents, then LV might be allocated partly from the first PV and partly from the second PV.

          Example 15.1. Allocating extents from one PV

          In this example, lv1 extents will be allocated from sda.

          # lvcreate -n lv1 -L1G vg /dev/sda

          Example 15.2. Allocating extents from two PVs

          In this example, lv2 extents will be allocated from either sda, or sdb, or a combination of both.

          # lvcreate -n lv2 L1G vg /dev/sda /dev/sdb

          Example 15.3. Allocating extents without specifying PV

          In this example, lv3 extents will be allocated from one of the PVs in the VG, or any combination of all PVs in the VG.

          # lvcreate -n lv3 -L1G vg

          or

    2. Allocate extents to create a raid volume:

      # lvcreate --type <segment_type> -m <mirror_images> -n <lv_name> -L <lv_size> <vg_name> [ <PV> ... ]

      Where:

      • <segment_type> is the specified segment type (for example raid5, mirror, snapshot).
      • <mirror_images> creates a raid1 or a mirrored LV with the specified number of images. For example, -m 1 would result in a raid1 LV with two images.
      • <lv_name> is the name of the LV.
      • <lv_size> is the size of the LV. Default unit is megabytes.
      • <vg_name> is the name of the VG.
      • <[PV …​]> are the PVs.

        The first raid image will be allocated from the first PV, the second raid image from the second PV, and so on.

        Example 15.4. Allocating raid images from two PVs

        In this example, lv4 first raid image will be allocated from sda and second image will be allocated from sdb.

        # lvcreate --type raid1 -m 1 -n lv4 -L1G vg /dev/sda /dev/sdb

        Example 15.5. Allocating raid images from three PVs

        In this example, lv5 first raid image will be allocated from sda, second image will be allocated from sdb, and third image will be allocated from sdc.

        # lvcreate --type raid1 -m 2 -n lv5 -L1G vg /dev/sda /dev/sdb /dev/sdc

Additional resources

  • lvcreate(8) man page
  • lvconvert(8) man page
  • lvmraid(7) man page

15.2. LVM allocation policies

When an LVM operation must allocate physical extents for one or more logical volumes (LVs), the allocation proceeds as follows:

  • The complete set of unallocated physical extents in the volume group is generated for consideration. If you supply any ranges of physical extents at the end of the command line, only unallocated physical extents within those ranges on the specified physical volumes (PVs) are considered.
  • Each allocation policy is tried in turn, starting with the strictest policy (contiguous) and ending with the allocation policy specified using the --alloc option or set as the default for the particular LV or volume group (VG). For each policy, working from the lowest-numbered logical extent of the empty LV space that needs to be filled, as much space as possible is allocated, according to the restrictions imposed by the allocation policy. If more space is needed, LVM moves on to the next policy.

The allocation policy restrictions are as follows:

  • The contiguous policy requires that the physical location of any logical extent is adjacent to the physical location of the immediately preceding logical extent, with the exception of the first logical extent of a LV.

    When a LV is striped or mirrored, the contiguous allocation restriction is applied independently to each stripe or raid image that needs space.

  • The cling allocation policy requires that the PV used for any logical extent be added to an existing LV that is already in use by at least one logical extent earlier in that LV.
  • An allocation policy of normal will not choose a physical extent that shares the same PV as a logical extent already allocated to a parallel LV (that is, a different stripe or raid image) at the same offset within that parallel LV.
  • If there are sufficient free extents to satisfy an allocation request but a normal allocation policy would not use them, the anywhere allocation policy will, even if that reduces performance by placing two stripes on the same PV.

You can change the allocation policy by using the vgchange command.

Note

Future updates can bring code changes in layout behavior according to the defined allocation policies. For example, if you supply on the command line two empty physical volumes that have an identical number of free physical extents available for allocation, LVM currently considers using each of them in the order they are listed; there is no guarantee that future releases will maintain that property. If you need a specific layout for a particular LV, build it up through a sequence of lvcreate and lvconvert steps such that the allocation policies applied to each step leave LVM no discretion over the layout.

15.3. Preventing allocation on a physical volume

You can prevent allocation of physical extents on the free space of one or more physical volumes with the pvchange command. This might be necessary if there are disk errors, or if you will be removing the physical volume.

Procedure

  • Use the following command to disallow the allocation of physical extents on device_name:

    # pvchange -x n /dev/sdk1

    You can also allow allocation where it had previously been disallowed by using the -xy arguments of the pvchange command.

Additional resources

  • pvchange(8) man page

Chapter 16. Grouping LVM objects with tags

You can assign tags to logical volume management (LVM) objects to group them. With this feature, you can automate the control of LVM behavior, such as activation, by a group. You can also use tags on LVM objects as a command.

16.1. LVM object tags

A logical volume management (LVM) tag is a word that is used to group LVM2 objects of the same type. You can attach tags to objects such as physical volumes, volume groups, and logical volumes .

To avoid ambiguity, prefix each tag with @. Each tag is expanded by replacing it with all the objects that possess that tag and that are of the type expected by its position on the command line.

LVM tags are strings of up to 1024 characters. LVM tags cannot start with a hyphen.

A valid tag consists of a limited range of characters only. The allowed characters are A-Z a-z 0-9 _ + . - / = ! : # &.

Only objects in a volume group can be tagged. Physical volumes lose their tags if they are removed from a volume group; this is because tags are stored as part of the volume group metadata and that is deleted when a physical volume is removed.

You can apply some commands to all volume groups (VG), logical volumes (LV), or physical volumes (PV) that have the same tag. The man page of the given command shows the syntax, such as VG|Tag, LV|Tag, or PV|Tag when you can substitute a tag name for a VG, LV, or PV name.

16.2. Adding tags to LVM objects

You can add tags to LVM objects to group them by using the --addtag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.

Procedure

  • To add a tag to an existing PV, use:

    # pvchange --addtag <@tag> <PV>
  • To add a tag to an existing VG, use:

    # vgchange --addtag <@tag> <VG>
  • To add a tag to a VG during creation, use:

    # vgcreate --addtag <@tag> <VG>
  • To add a tag to an existing LV, use:

    # lvchange --addtag <@tag> <LV>
  • To add a tag to a LV during creation, use:

    # lvcreate --addtag <@tag> ...

16.3. Removing tags from LVM objects

If you no longer want to keep your LVM objects grouped, you can remove tags from the objects by using the --deltag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.
  • You have created tags on physical volumes (PV), volume groups (VG), or logical volumes (LV).

Procedure

  • To remove a tag from an existing PV, use:

    # pvchange --deltag @tag PV
  • To remove a tag from an existing VG, use:

    # vgchange --deltag @tag VG
  • To remove a tag from an existing LV, use:

    # lvchange --deltag @tag LV

16.4. Displaying tags on LVM objects

You can display tags on your LVM objects with the following commands.

Prerequisites

  • The lvm2 package is installed.
  • You have created tags on physical volumes (PV), volume groups (VG), or logical volumes (LV).

Procedure

  • To display all tags on an existing PV, use:

    # pvs -o tags <PV>
  • To display all tags on an existing VG, use:

    # vgs -o tags <VG>
  • To display all tags on an existing LV, use:

    # lvs -o tags <LV>

16.5. Controlling logical volume activation with tags

This procedure describes how to specify in the configuration file that only certain logical volumes should be activated on that host.

Procedure

For example, the following entry acts as a filter for activation requests (such as vgchange -ay) and only activates vg1/lvol0 and any logical volumes or volume groups with the database tag in the metadata on that host:

activation { volume_list = ["vg1/lvol0", "@database" ] }

The special match @* that causes a match only if any metadata tag matches any host tag on that machine.

As another example, consider a situation where every machine in the cluster has the following entry in the configuration file:

tags { hosttags = 1 }

If you want to activate vg1/lvol2 only on host db2, do the following:

  1. Run lvchange --addtag @db2 vg1/lvol2 from any host in the cluster.
  2. Run lvchange -ay vg1/lvol2.

This solution involves storing host names inside the volume group metadata.

Chapter 17. Troubleshooting LVM

You can use Logical Volume Manager (LVM) tools to troubleshoot a variety of issues in LVM volumes and groups.

17.1. Gathering diagnostic data on LVM

If an LVM command is not working as expected, you can gather diagnostics in the following ways.

Procedure

  • Use the following methods to gather different kinds of diagnostic data:

    • Add the -v argument to any LVM command to increase the verbosity level of the command output. Verbosity can be further increased by adding additional v’s. A maximum of four such v’s is allowed, for example, -vvvv.
    • In the log section of the /etc/lvm/lvm.conf configuration file, increase the value of the level option. This causes LVM to provide more details in the system log.
    • If the problem is related to the logical volume activation, enable LVM to log messages during the activation:

      1. Set the activation = 1 option in the log section of the /etc/lvm/lvm.conf configuration file.
      2. Execute the LVM command with the -vvvv option.
      3. Examine the command output.
      4. Reset the activation option to 0.

        If you do not reset the option to 0, the system might become unresponsive during low memory situations.

    • Display an information dump for diagnostic purposes:

      # lvmdump
    • Display additional system information:

      # lvs -v
      # pvs --all
      # dmsetup info --columns
    • Examine the last backup of the LVM metadata in the /etc/lvm/backup/ directory and archived versions in the /etc/lvm/archive/ directory.
    • Check the current configuration information:

      # lvmconfig
    • Check the /run/lvm/hints cache file for a record of which devices have physical volumes on them.

Additional resources

  • lvmdump(8) man page

17.2. Displaying information about failed LVM devices

Troubleshooting information about a failed Logical Volume Manager (LVM) volume can help you determine the reason of the failure. You can check the following examples of the most common LVM volume failures.

Example 17.1. Failed volume groups

In this example, one of the devices that made up the volume group myvg failed. The volume group usability then depends on the type of failure. For example, the volume group is still usable if RAID volumes are also involved. You can also see information about the failed device.

# vgs --options +devices
 /dev/vdb1: open failed: No such device or address
 /dev/vdb1: open failed: No such device or address
  WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
  WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/sdb1).
  WARNING: Couldn't find all devices for LV myvg/mylv while checking used and assumed devices.

VG    #PV #LV #SN Attr   VSize  VFree  Devices
myvg   2   2   0 wz-pn- <3.64t <3.60t [unknown](0)
myvg   2   2   0 wz-pn- <3.64t <3.60t [unknown](5120),/dev/vdb1(0)

Example 17.2. Failed logical volume

In this example, one of the devices failed. This can be a reason for the logical volume in the volume group to fail. The command output shows the failed logical volumes.

# lvs --all --options +devices

  /dev/vdb1: open failed: No such device or address
  /dev/vdb1: open failed: No such device or address
  WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
  WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/sdb1).
  WARNING: Couldn't find all devices for LV myvg/mylv while checking used and assumed devices.

  LV    VG  Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
  mylv myvg -wi-a---p- 20.00g                                                     [unknown](0)                                                 [unknown](5120),/dev/sdc1(0)

Example 17.3. Failed image of a RAID logical volume

The following examples show the command output from the pvs and lvs utilities when an image of a RAID logical volume has failed. The logical volume is still usable.

# pvs

  Error reading device /dev/sdc1 at 0 length 4.

  Error reading device /dev/sdc1 at 4096 length 4.

  Couldn't find device with uuid b2J8oD-vdjw-tGCA-ema3-iXob-Jc6M-TC07Rn.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rimage_1 while checking used and assumed devices.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rmeta_1 while checking used and assumed devices.

  PV           VG         Fmt  Attr PSize    PFree
  /dev/sda2    rhel_bp-01 lvm2 a--  <464.76g    4.00m
  /dev/sdb1    myvg       lvm2 a--  <836.69g  736.68g
  /dev/sdd1    myvg       lvm2 a--  <836.69g <836.69g
  /dev/sde1    myvg       lvm2 a--  <836.69g <836.69g
  [unknown]    myvg       lvm2 a-m  <836.69g  736.68g
# lvs -a --options name,vgname,attr,size,devices myvg

  Couldn't find device with uuid b2J8oD-vdjw-tGCA-ema3-iXob-Jc6M-TC07Rn.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rimage_1 while checking used and assumed devices.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rmeta_1 while checking used and assumed devices.

  LV                  VG   Attr       LSize   Devices
  my_raid1            myvg rwi-a-r-p- 100.00g my_raid1_rimage_0(0),my_raid1_rimage_1(0)
  [my_raid1_rimage_0] myvg iwi-aor--- 100.00g /dev/sdb1(1)
  [my_raid1_rimage_1] myvg Iwi-aor-p- 100.00g [unknown](1)
  [my_raid1_rmeta_0]  myvg ewi-aor---   4.00m /dev/sdb1(0)
  [my_raid1_rmeta_1]  myvg ewi-aor-p-   4.00m [unknown](0)

17.3. Removing lost LVM physical volumes from a volume group

If a physical volume fails, you can activate the remaining physical volumes in the volume group and remove all the logical volumes that used that physical volume from the volume group.

Procedure

  1. Activate the remaining physical volumes in the volume group:

    # vgchange --activate y --partial myvg
  2. Check which logical volumes will be removed:

    # vgreduce --removemissing --test myvg
  3. Remove all the logical volumes that used the lost physical volume from the volume group:

    # vgreduce --removemissing --force myvg
  4. Optional: If you accidentally removed logical volumes that you wanted to keep, you can reverse the vgreduce operation:

    # vgcfgrestore myvg
    Warning

    If you remove a thin pool, LVM cannot reverse the operation.

17.4. Finding the metadata of a missing LVM physical volume

If the volume group’s metadata area of a physical volume is accidentally overwritten or otherwise destroyed, you get an error message indicating that the metadata area is incorrect, or that the system was unable to find a physical volume with a particular UUID.

This procedure finds the latest archived metadata of a physical volume that is missing or corrupted.

Procedure

  1. Find the archived metadata file of the volume group that contains the physical volume. The archived metadata files are located at the /etc/lvm/archive/volume-group-name_backup-number.vg path:

    # cat /etc/lvm/archive/myvg_00000-1248998876.vg

    Replace 00000-1248998876 with the backup-number. Select the last known valid metadata file, which has the highest number for the volume group.

  2. Find the UUID of the physical volume. Use one of the following methods.

    • List the logical volumes:

      # lvs --all --options +devices
      
        Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
    • Examine the archived metadata file. Find the UUID as the value labeled id = in the physical_volumes section of the volume group configuration.
    • Deactivate the volume group using the --partial option:

      # vgchange --activate n --partial myvg
      
        PARTIAL MODE. Incomplete logical volumes will be processed.
        WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
        WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/vdb1).
        0 logical volume(s) in volume group "myvg" now active

17.5. Restoring metadata on an LVM physical volume

This procedure restores metadata on a physical volume that is either corrupted or replaced with a new device. You might be able to recover the data from the physical volume by rewriting the metadata area on the physical volume.

Warning

Do not attempt this procedure on a working LVM logical volume. You will lose your data if you specify the incorrect UUID.

Prerequisites

Procedure

  1. Restore the metadata on the physical volume:

    # pvcreate --uuid physical-volume-uuid \
               --restorefile /etc/lvm/archive/volume-group-name_backup-number.vg \
               block-device
    Note

    The command overwrites only the LVM metadata areas and does not affect the existing data areas.

    Example 17.4. Restoring a physical volume on /dev/vdb1

    The following example labels the /dev/vdb1 device as a physical volume with the following properties:

    • The UUID of FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk
    • The metadata information contained in VG_00050.vg, which is the most recent good archived metadata for the volume group
    # pvcreate --uuid "FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk" \
               --restorefile /etc/lvm/archive/VG_00050.vg \
               /dev/vdb1
    
      ...
      Physical volume "/dev/vdb1" successfully created
  2. Restore the metadata of the volume group:

    # vgcfgrestore myvg
    
      Restored volume group myvg
  3. Display the logical volumes on the volume group:

    # lvs --all --options +devices myvg

    The logical volumes are currently inactive. For example:

      LV     VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
      mylv myvg   -wi--- 300.00G                               /dev/vdb1 (0),/dev/vdb1(0)
      mylv myvg   -wi--- 300.00G                               /dev/vdb1 (34728),/dev/vdb1(0)
  4. If the segment type of the logical volumes is RAID, resynchronize the logical volumes:

    # lvchange --resync myvg/mylv
  5. Activate the logical volumes:

    # lvchange --activate y myvg/mylv
  6. If the on-disk LVM metadata takes at least as much space as what overrode it, this procedure can recover the physical volume. If what overrode the metadata went past the metadata area, the data on the volume may have been affected. You might be able to use the fsck command to recover that data.

Verification steps

  • Display the active logical volumes:

    # lvs --all --options +devices
    
      LV     VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
     mylv myvg   -wi--- 300.00G                               /dev/vdb1 (0),/dev/vdb1(0)
     mylv myvg   -wi--- 300.00G                               /dev/vdb1 (34728),/dev/vdb1(0)

17.6. Rounding errors in LVM output

LVM commands that report the space usage in volume groups round the reported number to 2 decimal places to provide human-readable output. This includes the vgdisplay and vgs utilities.

As a result of the rounding, the reported value of free space might be larger than what the physical extents on the volume group provide. If you attempt to create a logical volume the size of the reported free space, you might get the following error:

Insufficient free extents

To work around the error, you must examine the number of free physical extents on the volume group, which is the accurate value of free space. You can then use the number of extents to create the logical volume successfully.

17.7. Preventing the rounding error when creating an LVM volume

When creating an LVM logical volume, you can specify the number of logical extents of the logical volume to avoid rounding error.

Procedure

  1. Find the number of free physical extents in the volume group:

    # vgdisplay myvg

    Example 17.5. Free extents in a volume group

    For example, the following volume group has 8780 free physical extents:

    --- Volume group ---
     VG Name               myvg
     System ID
     Format                lvm2
     Metadata Areas        4
     Metadata Sequence No  6
     VG Access             read/write
    [...]
    Free  PE / Size       8780 / 34.30 GB
  2. Create the logical volume. Enter the volume size in extents rather than bytes.

    Example 17.6. Creating a logical volume by specifying the number of extents

    # lvcreate --extents 8780 --name mylv myvg

    Example 17.7. Creating a logical volume to occupy all the remaining space

    Alternatively, you can extend the logical volume to use a percentage of the remaining free space in the volume group. For example:

    # lvcreate --extents 100%FREE --name mylv myvg

Verification steps

  • Check the number of extents that the volume group now uses:

    # vgs --options +vg_free_count,vg_extent_count
    
      VG     #PV #LV #SN  Attr   VSize   VFree  Free  #Ext
      myvg   2   1   0   wz--n- 34.30G    0    0     8780

17.8. LVM metadata and their location on disk

LVM headers and metadata areas are available in different offsets and sizes.

The default LVM disk header:

  • Is found in label_header and pv_header structures.
  • Is in the second 512-byte sector of the disk. Note that if a non-default location was specified when creating the physical volume (PV), the header can also be in the first or third sector.

The standard LVM metadata area:

  • Begins 4096 bytes from the start of the disk.
  • Ends 1 MiB from the start of the disk.
  • Begins with a 512 byte sector containing the mda_header structure.

A metadata text area begins after the mda_header sector and goes to the end of the metadata area. LVM VG metadata text is written in a circular fashion into the metadata text area. The mda_header points to the location of the latest VG metadata within the text area.

You can print LVM headers from a disk by using the # pvck --dump headers /dev/sda command. This command prints label_header, pv_header, mda_header, and the location of metadata text if found. Bad fields are printed with the CHECK prefix.

The LVM metadata area offset will match the page size of the machine that created the PV, so the metadata area can also begin 8K, 16K or 64K from the start of the disk.

Larger or smaller metadata areas can be specified when creating the PV, in which case the metadata area may end at locations other than 1 MiB. The pv_header specifies the size of the metadata area.

When creating a PV, a second metadata area can be optionally enabled at the end of the disk. The pv_header contains the locations of the metadata areas.

17.9. Extracting VG metadata from a disk

Choose one of the following procedures to extract VG metadata from a disk, depending on your situation. For information about how to save extracted metadata, see Saving extracted metadata to a file.

Note

For repair, you can use backup files in /etc/lvm/backup/ without extracting metadata from disk.

Procedure

  • Print current metadata text as referenced from valid mda_header:

    # pvck --dump metadata <disk>

    Example 17.8. Metadata text from valid mda_header

    # pvck --dump metadata /dev/sdb
      metadata text at 172032 crc Oxc627522f # vgname test segno 59
      ---
      <raw metadata from disk>
      ---
  • Print the locations of all metadata copies found in the metadata area, based on finding a valid mda_header:

    # pvck --dump metadata_all <disk>

    Example 17.9. Locations of metadata copies in the metadata area

    # pvck --dump metadata_all /dev/sdb
      metadata at 4608 length 815 crc 29fcd7ab vg test seqno 1 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 7168 length 1450 crc 5652ea55 vg test seqno 3 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
  • Search for all copies of metadata in the metadata area without using an mda_header, for example, if headers are missing or damaged:

    # pvck --dump metadata_search <disk>

    Example 17.10. Copies of metadata in the metadata area without using an mda_header

    # pvck --dump metadata_search /dev/sdb
      Searching for metadata at offset 4096 size 1044480
      metadata at 4608 length 815 crc 29fcd7ab vg test seqno 1 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 7168 length 1450 crc 5652ea55 vg test seqno 3 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
  • Include the -v option in the dump command to show the description from each copy of metadata:

    # pvck --dump metadata -v <disk>

    Example 17.11. Showing description from each copy of metadata

    # pvck --dump metadata -v /dev/sdb
      metadata text at 199680 crc 0x628cf243 # vgname my_vg seqno 40
      ---
    my_vg {
    id = "dmEbPi-gsgx-VbvS-Uaia-HczM-iu32-Rb7iOf"
    seqno = 40
    format = "lvm2"
    status = ["RESIZEABLE", "READ", "WRITE"]
    flags = []
    extent_size = 8192
    max_lv = 0
    max_pv = 0
    metadata_copies = 0
    
    physical_volumes {
    
    pv0 {
    id = "8gn0is-Hj8p-njgs-NM19-wuL9-mcB3-kUDiOQ"
    device = "/dev/sda"
    
    device_id_type = "sys_wwid"
    device_id = "naa.6001405e635dbaab125476d88030a196"
    status = ["ALLOCATABLE"]
    flags = []
    dev_size = 125829120
    pe_start = 8192
    pe_count = 15359
    }
    
    pv1 {
    id = "E9qChJ-5ElL-HVEp-rc7d-U5Fg-fHxL-2QLyID"
    device = "/dev/sdb"
    
    device_id_type = "sys_wwid"
    device_id = "naa.6001405f3f9396fddcd4012a50029a90"
    status = ["ALLOCATABLE"]
    flags = []
    dev_size = 125829120
    pe_start = 8192
    pe_count = 15359
    }

This file can be used for repair. The first metadata area is used by default for dump metadata. If the disk has a second metadata area at the end of the disk, you can use the --settings "mda_num=2" option to use the second metadata area for dump metadata instead.

17.10. Saving extracted metadata to a file

If you need to use dumped metadata for repair, it is required to save extracted metadata to a file with the -f option and the --setings option.

Procedure

  • If -f <filename> is added to --dump metadata, the raw metadata is written to the named file. You can use this file for repair.
  • If -f <filename> is added to --dump metadata_all or --dump metadata_search, then raw metadata from all locations is written to the named file.
  • To save one instance of metadata text from --dump metadata_all|metadata_search add --settings "metadata_offset=<offset>" where <offset> is from the listing output "metadata at <offset>".

    Example 17.12. Output of the command

    # pvck --dump metadata_search --settings metadata_offset=5632 -f meta.txt /dev/sdb
      Searching for metadata at offset 4096 size 1044480
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
    # head -2 meta.txt
    test {
    id = "FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv"

17.11. Repairing a disk with damaged LVM headers and metadata using the pvcreate and the vgcfgrestore commands

You can restore metadata and headers on a physical volume that is either corrupted or replaced with a new device. You might be able to recover the data from the physical volume by rewriting the metadata area on the physical volume.

Warning

These instructions should be used with extreme caution, and only if you are familiar with the implications of each command, the current layout of the volumes, the layout that you need to achieve, and the contents of the backup metadata file. These commands have the potential to corrupt data, and as such, it is recommended that you contact Red Hat Global Support Services for assistance in troubleshooting.

Prerequisites

Procedure

  1. Collect the following information needed for the pvcreate and vgcfgrestore commands. You can collect the information about your disk and UUID by running the # pvs -o+uuid command.

    • metadata-file is the path to the most recent metadata backup file for the VG, for example, /etc/lvm/backup/<vg-name>
    • vg-name is the name of the VG that has the damaged or missing PV.
    • UUID of the PV that was damaged on this device is the value taken from the output of the # pvs -i+uuid command.
    • disk is the name of the disk where the PV is supposed to be, for example, /dev/sdb. Be certain this is the correct disk, or seek help, otherwise following these steps may lead to data loss.
  2. Recreate LVM headers on the disk:

    # pvcreate --restorefile <metadata-file> --uuid <UUID> <disk>

    Optionally, verify that the headers are valid:

    # pvck --dump headers <disk>
  3. Restore the VG metadata on the disk:

    # vgcfgrestore --file <metadata-file> <vg-name>

    Optionally, verify the metadata is restored:

    # pvck --dump metadata <disk>

If there is no metadata backup file for the VG, you can get one by using the procedure in Saving extracted metadata to a file.

Verification

  • To verify that the new physical volume is intact and the volume group is functioning correctly, check the output of the following command:
# vgs

17.12. Repairing a disk with damaged LVM headers and metadata using the pvck command

This is an alternative to the Repairing a disk with damaged LVM headers and metadata using the pvcreate and the vgcfgrestore commands. There may be cases where the pvcreate and the vgcfgrestore commands do not work. This method is more targeted at the damaged disk.

This method uses a metadata input file that was extracted by pvck --dump, or a backup file from /etc/lvm/backup. When possible, use metadata saved by pvck --dump from another PV in the same VG, or from a second metadata area on the PV. For more information, see Saving extracted metadata to a file.

Procedure

  • Repair the headers and metadata on the disk:

    # pvck --repair -f <metadata-file> <disk>

    where

    • <metadata-file> is a file containing the most recent metadata for the VG. This can be /etc/lvm/backup/vg-name, or it can be a file containing raw metadata text from the pvck --dump metadata_search command output.
    • <disk> is the name of the disk where the PV is supposed to be, for example, /dev/sdb. To prevent data loss, verify that is the correct disk. If you are not certain the disk is correct, contact Red Hat Support.
Note

If the metadata file is a backup file, the pvck --repair should be run on each PV that holds metadata in VG. If the metadata file is raw metadata that has been extracted from another PV, the pvck --repair needs to be run only on the damaged PV.

Verification

  • To check that the new physical volume is intact and the volume group is functioning correctly, check outputs of the following commands:

    # vgs <vgname>
    # pvs <pvname>
    # lvs <lvname>

17.13. Troubleshooting LVM RAID

You can troubleshoot various issues in LVM RAID devices to correct data errors, recover devices, or replace failed devices.

17.13.1. Checking data coherency in a RAID logical volume

LVM provides scrubbing support for RAID logical volumes. RAID scrubbing is the process of reading all the data and parity blocks in an array and checking to see whether they are coherent. The lvchange --syncaction repair command initiates a background synchronization action on the array. The following attributes provide details about data coherency:

  • The raid_sync_action field displays the current synchronization action that the RAID logical volume is performing. It can be one of the following values:

    idle
    Completed all sync actions (doing nothing).
    resync
    Initializing or resynchronizing an array after an unclean machine shutdown.
    recover
    Replacing a device in the array.
    check
    Looking for array inconsistencies.
    repair
    Looking for and repairing inconsistencies.
  • The raid_mismatch_count field displays the number of discrepancies found during a check action.
  • The Cpy%Sync field displays the progress of the sync actions.
  • The lv_attr field provides additional indicators. Bit 9 of this field displays the health of the logical volume, and it supports the following indicators:

    m or mismatches
    Indicates that there are discrepancies in a RAID logical volume. You can see this character after the scrubbing operation detects the portions of the RAID, which are not coherent.
    r or refresh
    Indicates a failed device in a RAID array, even though LVM can read the device label and considers the device to be operational. Refresh the logical volume to notify the kernel that the device is now available, or replace the device if you suspect that it failed.

Procedure

  1. Optional: Limit the I/O bandwidth that the scrubbing process uses. When you perform a RAID scrubbing operation, the background I/O required by the sync actions can crowd out other I/O to LVM devices, such as updates to volume group metadata. This might cause the other LVM operations to slow down.

    You can control the rate of the scrubbing operation by implementing recovery throttling. You can set the recovery rate using --maxrecoveryrate Rate[bBsSkKmMgG] or --minrecoveryrate Rate[bBsSkKmMgG] with the lvchange --syncaction commands. For more information, see Minimum and maximum I/O rate options.

    Specify the Rate value as an amount per second for each device in the array. If you provide no suffix, the options assume kiB per second per device.

  2. Display the number of discrepancies in the array, without repairing them:

    # lvchange --syncaction check my_vg/my_lv

    This command initiates a background synchronization action on the array.

  3. Optional: View the var/log/syslog file for the kernel messages.
  4. Correct the discrepancies in the array:

    # lvchange --syncaction repair my_vg/my_lv

    This command repairs or replaces failed devices in a RAID logical volume. You can view the var/log/syslog file for the kernel messages after executing this command.

Verification

  1. Display information about the scrubbing operation:

    # lvs -o +raid_sync_action,raid_mismatch_count my_vg/my_lv
    LV    VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert SyncAction Mismatches
    my_lv my_vg rwi-a-r--- 500.00m                                    100.00           idle        0

Additional resources

17.13.2. Replacing a failed RAID device in a logical volume

RAID is not similar to traditional LVM mirroring. In case of LVM mirroring, remove the failed devices. Otherwise, the mirrored logical volume would hang while RAID arrays continue running with failed devices. For RAID levels other than RAID1, removing a device would mean converting to a lower RAID level, for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0.

Instead of removing a failed device and allocating a replacement, with LVM, you can replace a failed device that serves as a physical volume in a RAID logical volume by using the --repair argument of the lvconvert command.

Prerequisites

  • The volume group includes a physical volume that provides enough free capacity to replace the failed device.

    If no physical volume with enough free extents is available on the volume group, add a new, sufficiently large physical volume by using the vgextend utility.

Procedure

  1. View the RAID logical volume:

    # lvs --all --options name,copy_percent,devices my_vg
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdc1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdc1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  2. View the RAID logical volume after the /dev/sdc device fails:

    # lvs --all --options name,copy_percent,devices my_vg
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          [unknown](1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           [unknown](0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  3. Replace the failed device:

    # lvconvert --repair my_vg/my_lv
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
    Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
    Faulty devices in my_vg/my_lv successfully replaced.
  4. Optional: Manually specify the physical volume that replaces the failed device:

    # lvconvert --repair my_vg/my_lv replacement_pv
  5. Examine the logical volume with the replacement:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdc: open failed: No such device or address
      /dev/sdc1: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      LV               Cpy%Sync Devices
      my_lv            43.79    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

    Until you remove the failed device from the volume group, LVM utilities still indicate that LVM cannot find the failed device.

  6. Remove the failed device from the volume group:

    # vgreduce --removemissing my_vg

Verification

  1. View the available physical volumes after removing the failed device:

    # pvscan
    PV /dev/sde1 VG rhel_virt-506 lvm2 [<7.00 GiB / 0 free]
    PV /dev/sdb1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
  2. Examine the logical volume after the replacing the failed device:

    # lvs --all --options name,copy_percent,devices my_vg
    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

Additional resources

  • lvconvert(8) and vgreduce(8) man pages

17.14. Troubleshooting duplicate physical volume warnings for multipathed LVM devices

When using LVM with multipathed storage, LVM commands that list a volume group or logical volume might display messages such as the following:

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/dm-5 not /dev/sdd
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/emcpowerb not /dev/sde
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/sddlmab not /dev/sdf

You can troubleshoot these warnings to understand why LVM displays them, or to hide the warnings.

17.14.1. Root cause of duplicate PV warnings

When a multipath software such as Device Mapper Multipath (DM Multipath), EMC PowerPath, or Hitachi Dynamic Link Manager (HDLM) manages storage devices on the system, each path to a particular logical unit (LUN) is registered as a different SCSI device.

The multipath software then creates a new device that maps to those individual paths. Because each LUN has multiple device nodes in the /dev directory that point to the same underlying data, all the device nodes contain the same LVM metadata.

Table 17.1. Example device mappings in different multipath software

Multipath softwareSCSI paths to a LUNMultipath device mapping to paths

DM Multipath

/dev/sdb and /dev/sdc

/dev/mapper/mpath1 or /dev/mapper/mpatha

EMC PowerPath

/dev/emcpowera

HDLM

/dev/sddlmab

As a result of the multiple device nodes, LVM tools find the same metadata multiple times and report them as duplicates.

17.14.2. Cases of duplicate PV warnings

LVM displays the duplicate PV warnings in either of the following cases:

Single paths to the same device

The two devices displayed in the output are both single paths to the same device.

The following example shows a duplicate PV warning in which the duplicate devices are both single paths to the same device.

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/sdd not /dev/sdf

If you list the current DM Multipath topology using the multipath -ll command, you can find both /dev/sdd and /dev/sdf under the same multipath map.

These duplicate messages are only warnings and do not mean that the LVM operation has failed. Rather, they are alerting you that LVM uses only one of the devices as a physical volume and ignores the others.

If the messages indicate that LVM chooses the incorrect device or if the warnings are disruptive to users, you can apply a filter. The filter configures LVM to search only the necessary devices for physical volumes, and to leave out any underlying paths to multipath devices. As a result, the warnings no longer appear.

Multipath maps

The two devices displayed in the output are both multipath maps.

The following examples show a duplicate PV warning for two devices that are both multipath maps. The duplicate physical volumes are located on two different devices rather than on two different paths to the same device.

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/mapper/mpatha not /dev/mapper/mpathc

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/emcpowera not /dev/emcpowerh

This situation is more serious than duplicate warnings for devices that are both single paths to the same device. These warnings often mean that the machine is accessing devices that it should not access: for example, LUN clones or mirrors.

Unless you clearly know which devices you should remove from the machine, this situation might be unrecoverable. Red Hat recommends that you contact Red Hat Technical Support to address this issue.

17.14.3. Example LVM device filters that prevent duplicate PV warnings

The following examples show LVM device filters that avoid the duplicate physical volume warnings that are caused by multiple storage paths to a single logical unit (LUN).

You can configure the filter for logical volume manager (LVM) to check metadata for all devices. Metadata includes local hard disk drive with the root volume group on it and any multipath devices. By rejecting the underlying paths to a multipath device (such as /dev/sdb, /dev/sdd), you can avoid these duplicate PV warnings, because LVM finds each unique metadata area once on the multipath device itself.

  • To accept the second partition on the first hard disk drive and any device mapper (DM) Multipath devices and reject everything else, enter:

    filter = [ "a|/dev/sda2$|", "a|/dev/mapper/mpath.*|", "r|.*|" ]
  • To accept all HP SmartArray controllers and any EMC PowerPath devices, enter:

    filter = [ "a|/dev/cciss/.*|", "a|/dev/emcpower.*|", "r|.*|" ]
  • To accept any partitions on the first IDE drive and any multipath devices, enter:

    filter = [ "a|/dev/hda.*|", "a|/dev/mapper/mpath.*|", "r|.*|" ]

17.14.4. Additional resources

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