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Configuring and managing logical volumes

Red Hat Enterprise Linux 8

A guide to the configuration and management of LVM logical volumes

Red Hat Customer Content Services

Abstract

This documentation collection provides instructions on how to manage LVM logical volumes on Red Hat Enterprise Linux 8.

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Chapter 1. Logical volumes

Volume management creates a layer of abstraction over physical storage, allowing 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.

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

  • Flexible capacity

    When using logical volumes, file systems can extend across multiple disks, since you can aggregate disks and partitions into a single logical volume.

  • Resizeable storage pools

    You can extend logical volumes or reduce logical volumes in size with simple software commands, without reformatting and repartitioning the underlying disk devices.

  • Online data relocation

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

  • Convenient device naming

    Logical storage volumes can be managed in user-defined and custom named groups.

  • Disk striping

    You can create a logical volume that stripes data across two or more disks. This can dramatically increase throughput.

  • Mirroring volumes

    Logical volumes provide a convenient way to configure a mirror for your data.

  • Volume snapshots

    Using logical volumes, you can take device snapshots for consistent backups or to test the effect of changes without affecting the real data.

  • Thin volumes

    Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available extents.

  • Cache volumes

    A cache logical volume uses a small logical volume consisting of fast block devices (such as SSD drives) to improve the performance of a larger and slower logical volume by storing the frequently used blocks on the smaller, faster logical volume.

1.1. LVM architecture overview

The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. This device is initialized as an LVM physical volume (PV).

To create an LVM logical volume, the physical volumes are combined into a volume group (VG). This creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated. This process is analogous to the way in which disks are divided into partitions. A logical volume is used by file systems and applications (such as databases).

Figure 1.1, “LVM logical volume components” shows the components of a simple LVM logical volume:

Figure 1.1. LVM logical volume components

LVM Logical Volume Components

1.2. Physical volumes

The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. To use the device for an LVM logical volume, the device must be initialized as a physical volume (PV). Initializing a block device as a physical volume places a label near the start of the device.

By default, the LVM label is placed in the second 512-byte sector. You can overwrite this default 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.

An LVM label provides correct identification and device ordering for a physical device, since devices can come up in any order when the system is booted. 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.

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.

1.2.1. LVM physical volume layout

Figure 1.2, “Physical volume layout” shows 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 1.2. Physical volume layout

LVM Physical Volume Layout

1.2.2. Multiple partitions on a disk

LVM allows you to create physical volumes out of disk partitions. 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. In addition, multiple physical volumes on a single disk may cause a kernel warning about unknown partition types at boot.

  • 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.

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 striped volumes.

1.3. Volume groups

Physical volumes are combined into volume groups (VGs). This creates a pool of disk space out of which logical volumes 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 thus the same for all logical volumes in the volume group. The volume group maps the logical extents to physical extents.

1.4. LVM logical volumes

In LVM, a volume group is divided up into logical volumes. The following sections describe the different types of logical volumes.

1.4.1. 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.

Creating a linear volume assigns a range of physical extents to an area of a logical volume in order. For example, as shown in Figure 1.3, “Extent Mapping” logical extents 1 to 99 could map to one physical volume and logical extents 100 to 198 could map to a second physical volume. From the point of view of the application, there is one device that is 198 extents in size.

Figure 1.3. Extent Mapping

Mapping extents in a linear volume

The physical volumes that make up a logical volume do not have to be the same size. Figure 1.4, “Linear volume with unequal physical volumes” shows volume group VG1 with a physical extent size of 4MB. This volume group includes 2 physical volumes named PV1 and PV2. The physical volumes are divided into 4MB units, since that is the extent size. In this example, PV1 is 200 extents in size (800MB) and PV2 is 100 extents in size (400MB). You can create a linear volume any size between 1 and 300 extents (4MB to 1200MB). In this example, the linear volume named LV1 is 300 extents in size.

Figure 1.4. Linear volume with unequal physical volumes

Linear volume with unequal physical volumes

You can configure more than one linear logical volume of whatever size you require from the pool of physical extents. Figure 1.5, “Multiple logical volumes” shows the same volume group as in Figure 1.4, “Linear volume with unequal physical volumes”, but in this case two logical volumes have been carved out of the volume group: LV1, which is 250 extents in size (1000MB) and LV2 which is 50 extents in size (200MB).

Figure 1.5. Multiple logical volumes

Multiple logical volumes

1.4.2. 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.

The following illustration shows data being striped across three physical volumes. In this figure:

  • the first stripe of data is written to the first physical volume
  • the second stripe of data is written to the second physical volume
  • the third stripe of data is written to the third physical volume
  • the fourth stripe of data is written to the first physical volume

In a striped logical volume, the size of the stripe cannot exceed the size of an extent.

Figure 1.6. Striping data across three PVs

Striping data across three physical volumes

Striped logical volumes can be extended by concatenating another set of devices onto the end of the first set. In order to extend a striped logical volume, however, there must be enough free space on the set of underlying physical volumes that make up the volume group to support the stripe. For example, if you have a two-way stripe that uses up an entire volume group, adding a single physical volume to the volume group will not enable you to extend the stripe. Instead, you must add at least two physical volumes to the volume group.

1.4.3. RAID logical volumes

LVM supports RAID0/1/4/5/6/10. An LVM RAID volume has the following characteristics:

  • RAID logical volumes created and managed by means of LVM leverage the MD kernel drivers.
  • RAID1 images can be temporarily split from the array and merged back into the array later.
  • LVM RAID volumes support snapshots.
Note

RAID logical volumes are not cluster-aware. While RAID logical volumes can be created and activated exclusively on one machine, they cannot be activated simultaneously on more than one machine.

1.4.4. Thinly-provisioned logical volumes (thin volumes)

Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available extents. Using thin provisioning, you can manage a storage pool of free space, known as a thin pool, which can be allocated to an arbitrary number of devices when needed by applications. You can then create devices that can be bound to the thin pool for later allocation when an application actually writes to the logical volume. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space.

Note

Thin volumes are not supported across the nodes in a cluster. The thin pool and all its thin volumes must be exclusively activated on only one cluster node.

By using thin provisioning, a storage administrator can overcommit the physical storage, often avoiding the need to purchase additional storage. For example, if ten users each request a 100GB file system for their application, the storage administrator 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. 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.

To make sure that all available space can be used, LVM supports data discard. This allows for re-use of the space that was formerly used by a discarded file or other block range.

Thin volumes provide support for a new implementation of copy-on-write (COW) snapshot logical volumes, which allow many virtual devices to share the same data in the thin pool.

1.4.5. 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.

Note

LVM supports thinly-provisioned snapshots.

Because 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.

Note

Snapshot copies of a file system are virtual copies, not an actual media backup for a file system. Snapshots do not provide a substitute for a backup procedure.

The size of the snapshot governs the amount of space set aside for storing the changes to the origin volume. For example, if you made a snapshot and then completely overwrote the origin the snapshot would have to be at least as big as the origin volume to hold the changes. You need to dimension a snapshot according to the expected level of change. So 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 that sees a greater number of writes, such as /home.

If a snapshot runs full, the snapshot becomes invalid, since it can no longer track changes on the origin volume. You should regularly monitor the size of the snapshot. Snapshots are fully resizable, however, so if you have the storage capacity you can increase the size of the snapshot volume to prevent it from getting dropped. Conversely, if you find that 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.

When you create a snapshot file system, full read and write access to the origin stays possible. If a chunk on a snapshot is changed, that chunk is marked and never gets copied from the original volume.

There are several uses for the snapshot feature:

  • Most typically, a snapshot is taken 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 whether the original file system requires file system repair.
  • Because the snapshot is read/write, you can test applications against production data by taking a snapshot and running tests against the snapshot, leaving the real data untouched.
  • You can create LVM volumes for use with Red Hat Virtualization. LVM snapshots can be used 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.

You can use the --merge option of the lvconvert command to merge a snapshot into its origin volume. One use for this feature is to perform system rollback if you have lost data or files or otherwise need to restore your system to a previous state. After you merge the snapshot volume, the resulting logical volume will have the origin volume’s name, minor number, and UUID and the merged snapshot is removed.

1.4.6. Thinly-provisioned snapshot volumes

Red Hat Enterprise Linux provides support for thinly-provisioned snapshot volumes. Thin snapshot volumes allow many virtual devices to be stored on the same data volume. This simplifies administration and allows for the sharing of data between snapshot volumes.

As for all LVM snapshot volumes, as well as 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.

Thin snapshot volumes provide the following benefits:

  • A thin snapshot volume can reduce disk usage when there are multiple snapshots of the same origin volume.
  • If there are multiple snapshots of the same origin, then a write to the origin will cause one COW operation to preserve the data. Increasing the number of snapshots of the origin should yield no major slowdown.
  • Thin snapshot volumes can be used as a logical volume origin for another snapshot. This allows for an arbitrary depth of recursive snapshots (snapshots of snapshots of snapshots…​).
  • A snapshot of a thin logical volume also creates a thin logical volume. This consumes no data space until a COW operation is required, or until the snapshot itself is written.
  • A thin snapshot volume does not need to be activated with its origin, so a user may have only the origin active while there are many inactive snapshot volumes of the origin.
  • When you delete the origin of a thinly-provisioned snapshot volume, each snapshot of that origin volume becomes an independent thinly-provisioned volume. This means that instead of merging a snapshot with its origin volume, you may choose to delete the origin volume and then create a new thinly-provisioned snapshot using that independent volume as the origin volume for the new snapshot.

Although there are many advantages to using thin snapshot volumes, there are some use cases for which the older LVM snapshot volume feature may be more appropriate to your needs:

  • You cannot change the chunk size of a thin pool. If the thin pool has a large chunk size (for example, 1MB) and you require a short-living snapshot for which a chunk size that large is not efficient, you may elect to use the older snapshot feature.
  • You cannot limit the size of a thin snapshot volume; the snapshot will use all of the space in the thin pool, if necessary. This may not be appropriate for your needs.

In general, you should consider the specific requirements of your site when deciding which snapshot format to use.

1.4.7. 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.

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 8.

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 on RHEL System Roles and how to apply them, see What RHEL System Roles are and which tasks they can be used for.

2.1. Introduction to the storage role

The storage role can manage:

  • File systems on whole disks
  • Complete LVM volume groups including their logical volumes and 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

2.2. Storage role parameters

This section lists input variables that the storage role accepts.

Table 2.1. Storage role parameters

ParameterDescriptionAdditional information

storage_volumes

List of file systems on whole disks to be managed.

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.

storage_safe_mode

Boolean option to enable and disable safe mode.

In safe mode, the storage role does not reformat disks containing data such as file systems or LVM physical volumes to create new file systems or other storage objects, but the role aborts with an error message instead. By default, safe mode is on to prevent accidentally overwriting existing data. Red Hat recommends to keep safe mode enabled, and disable it only for specific cases and after detailed testing.

The storage role only changes file systems, volumes and pools listed in the storage_pools and storage_volumes variables. File systems, volumes and pools not listed in storage_pools and storage_volumes are not changed.

Additional resources

For more information on the storage role, see System roles documentation.

Chapter 3. Configuring LVM logical volumes

The following procedures provide examples of basic LVM administration tasks.

3.1. Using CLI commands

The following sections describe some general operational features of LVM CLI commands.

Specifying units in a command line argument

When sizes are required in a command line argument, units can always be specified explicitly. If you do not specify a unit, then a default is assumed, usually KB or MB. LVM CLI commands do not accept fractions.

When specifying units in a command line argument, LVM is case-insensitive; specifying M or m is equivalent, for example, and powers of 2 (multiples of 1024) are used. However, when specifying the --units argument in a command, lower-case indicates that units are in multiples of 1024 while upper-case indicates that units are in multiples of 1000.

Specifying volume groups and logical volumes

Note the following when specifying volume groups or logical volumes in an LVM CLI command.

  • Where commands take volume group or logical volume names as arguments, the full path name is optional. A logical volume called lvol0 in a volume group called vg0 can be specified as vg0/lvol0.
  • Where a list of volume groups is required but is left empty, a list of all volume groups will be substituted.
  • Where a list of logical volumes is required but a volume group is given, a list of all the logical volumes in that volume group will be substituted. For example, the lvdisplay vg0 command will display all the logical volumes in volume group vg0.

Increasing output verbosity

All LVM commands accept a -v argument, which can be entered multiple times to increase the output verbosity. The following examples shows the default output of the lvcreate command.

# lvcreate -L 50MB new_vg
  Rounding up size to full physical extent 52.00 MB
  Logical volume "lvol0" created

The following command shows the output of the lvcreate command with the -v argument.

# lvcreate -v -L 50MB new_vg
  Rounding up size to full physical extent 52.00 MB
    Archiving volume group "new_vg" metadata (seqno 1).
    Creating logical volume lvol0
    Creating volume group backup "/etc/lvm/backup/new_vg" (seqno 2).
    Activating logical volume new_vg/lvol0.
    activation/volume_list configuration setting not defined: Checking only host tags for new_vg/lvol0.
    Creating new_vg-lvol0
    Loading table for new_vg-lvol0 (253:0).
    Resuming new_vg-lvol0 (253:0).
    Wiping known signatures on logical volume "new_vg/lvol0"
    Initializing 4.00 KiB of logical volume "new_vg/lvol0" with value 0.
  Logical volume "lvol0" created

The -vv, -vvv and the -vvvv arguments display increasingly more details about the command execution. The -vvvv argument provides the maximum amount of information at this time. The following example shows the first few lines of output for the lvcreate command with the -vvvv argument specified.

# lvcreate -vvvv -L 50MB new_vg
#lvmcmdline.c:913         Processing: lvcreate -vvvv -L 50MB new_vg
#lvmcmdline.c:916         O_DIRECT will be used
#config/config.c:864       Setting global/locking_type to 1
#locking/locking.c:138       File-based locking selected.
#config/config.c:841       Setting global/locking_dir to /var/lock/lvm
#activate/activate.c:358       Getting target version for linear
#ioctl/libdm-iface.c:1569         dm version   OF   [16384]
#ioctl/libdm-iface.c:1569         dm versions   OF   [16384]
#activate/activate.c:358       Getting target version for striped
#ioctl/libdm-iface.c:1569         dm versions   OF   [16384]
#config/config.c:864       Setting activation/mirror_region_size to 512
...

Displaying help for LVM CLI commands

You can display help for any of the LVM CLI commands with the --help argument of the command.

# commandname --help

To display the man page for a command, execute the man command:

# man commandname

The man lvm command provides general online information about LVM.

3.2. Creating an LVM logical volume on three disks

This example procedure creates an LVM logical volume called mylv that consists of the disks at /dev/sda1, /dev/sdb1, and /dev/sdc1.

  1. To use disks in a volume group, label them as LVM physical volumes with the pvcreate command.

    Warning

    This command destroys any data on /dev/sda1, /dev/sdb1, and /dev/sdc1.

    # pvcreate /dev/sda1 /dev/sdb1 /dev/sdc1
      Physical volume "/dev/sda1" successfully created
      Physical volume "/dev/sdb1" successfully created
      Physical volume "/dev/sdc1" successfully created
  2. Create the a volume group that consists of the LVM physical volumes you have created. The following command creates the volume group myvg.

    # vgcreate myvg /dev/sda1 /dev/sdb1 /dev/sdc1
      Volume group "myvg" successfully created

    You can use the vgs command to display the attributes of the new volume group.

    # vgs
      VG   #PV #LV #SN Attr   VSize  VFree
      myvg   3   0   0 wz--n- 51.45G 51.45G
  3. Create the logical volume from the volume group you have created. The following command creates the logical volume mylv from the volume group myvg. This example creates a logical volume that uses 2 gigabytes of the volume group.

    # lvcreate -L 2G -n mylv myvg
      Logical volume "mylv" created
  4. Create a file system on the logical volume. The following command creates an ext4 file system on the logical volume.

    # mkfs.ext4 /dev/myvg/mylv
    mke2fs 1.44.3 (10-July-2018)
    Creating filesystem with 524288 4k blocks and 131072 inodes
    Filesystem UUID: 616da032-8a48-4cd7-8705-bd94b7a1c8c4
    Superblock backups stored on blocks:
            32768, 98304, 163840, 229376, 294912
    
    Allocating group tables: done
    Writing inode tables: done
    Creating journal (16384 blocks): done
    Writing superblocks and filesystem accounting information: done

    The following commands mount the logical volume and report the file system disk space usage.

    # mount /dev/myvg/mylv /mnt
    # df
    Filesystem             1K-blocks     Used  Available Use% Mounted on
    /dev/mapper/myvg-mylv    1998672     6144    1871288   1% /mnt

3.3. Creating a RAID0 (striped) logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size.

The format for the command to create a RAID0 volume is as follows.

lvcreate --type raid0[_meta] --stripes Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath ...]

Table 3.1. RAID0 Command Creation parameters

ParameterDescription

--type raid0[_meta]

Specifying raid0 creates a RAID0 volume without metadata volumes. Specifying raid0_meta creates a RAID0 volume with metadata volumes. Because RAID0 is non-resilient, it does not have to 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, however, and 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.

This example procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks at /dev/sda1, /dev/sdb1, and /dev/sdc1.

  1. Label the disks you will use in the volume group as LVM physical volumes with the pvcreate command.

    Warning

    This command destroys any data on /dev/sda1, /dev/sdb1, and /dev/sdc1.

    # pvcreate /dev/sda1 /dev/sdb1 /dev/sdc1
      Physical volume "/dev/sda1" successfully created
      Physical volume "/dev/sdb1" successfully created
      Physical volume "/dev/sdc1" successfully created
  2. Create the volume group myvg. The following command creates the volume group myvg.

    # vgcreate myvg /dev/sda1 /dev/sdb1 /dev/sdc1
      Volume group "myvg" successfully created

    You can use the vgs command to display the attributes of the new volume group.

    # vgs
      VG   #PV #LV #SN Attr   VSize  VFree
      myvg   3   0   0 wz--n- 51.45G 51.45G
  3. Create a RAID0 logical volume from the volume group you have created. The following command creates the RAID0 volume mylv from the volume group myvg. This example creates a logical volume that is 2 gigabytes in size, with three stripes and a stripe size of 4 kilobytes.

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

    # mkfs.ext4 /dev/myvg/mylv
    mke2fs 1.44.3 (10-July-2018)
    Creating filesystem with 525312 4k blocks and 131376 inodes
    Filesystem UUID: 9d4c0704-6028-450a-8b0a-8875358c0511
    Superblock backups stored on blocks:
            32768, 98304, 163840, 229376, 294912
    
    Allocating group tables: done
    Writing inode tables: done
    Creating journal (16384 blocks): done
    Writing superblocks and filesystem accounting information: done

    The following commands mount the logical volume and report the file system disk space usage.

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

3.4. Renaming LVM logical volumes

This procedure renames an existing logical volume using the command-line LVM interface.

Procedure

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

    [root@node-n]# lvchange --activate n vg-name/lv-name
  3. Use the lvrename utility to rename an existing logical volume:

    # lvrename vg-name original-lv-name new-lv-name

    Optionally, you can specify the full paths to the devices:

    # lvrename /dev/vg-name/original-lv-name /dev/vg-name/new-lv-name

Additional resources

  • The lvrename(8) man page

3.5. Removing a disk from a logical volume

These example procedures show how you can 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.

3.5.1. Moving extents to existing physical volumes

In this example, the logical volume is distributed across four physical volumes in the volume group myvg.

# pvs -o+pv_used
  PV         VG   Fmt  Attr PSize  PFree  Used
  /dev/sda1  myvg lvm2 a-   17.15G 12.15G  5.00G
  /dev/sdb1  myvg lvm2 a-   17.15G 12.15G  5.00G
  /dev/sdc1  myvg lvm2 a-   17.15G 12.15G  5.00G
  /dev/sdd1  myvg lvm2 a-   17.15G  2.15G 15.00G

This examples moves the extents off of /dev/sdb1 so that it can be removed from the volume group.

  1. If there are enough free extents on the other physical volumes in the volume group, you can execute the pvmove command on the device you want to remove with no other options and the extents will be distributed to the other devices.

    In a cluster, the pvmove command can move only logical volume that are active exclusively on a single node.

    # pvmove /dev/sdb1
      /dev/sdb1: Moved: 2.0%
     ...
      /dev/sdb1: Moved: 79.2%
     ...
      /dev/sdb1: Moved: 100.0%

    After the pvmove command has finished executing, the distribution of extents is as follows:

    # pvs -o+pv_used
      PV         VG   Fmt  Attr PSize  PFree  Used
      /dev/sda1  myvg lvm2 a-   17.15G  7.15G 10.00G
      /dev/sdb1  myvg lvm2 a-   17.15G 17.15G     0
      /dev/sdc1  myvg lvm2 a-   17.15G 12.15G  5.00G
      /dev/sdd1  myvg lvm2 a-   17.15G  2.15G 15.00G
  2. Use the vgreduce command to remove the physical volume /dev/sdb1 from the volume group.

    # vgreduce myvg /dev/sdb1
      Removed "/dev/sdb1" from volume group "myvg"
    # pvs
      PV         VG   Fmt  Attr PSize  PFree
      /dev/sda1  myvg lvm2 a-   17.15G  7.15G
      /dev/sdb1       lvm2 --   17.15G 17.15G
      /dev/sdc1  myvg lvm2 a-   17.15G 12.15G
      /dev/sdd1  myvg lvm2 a-   17.15G  2.15G

The disk can now be physically removed or allocated to other users.

3.5.2. Moving Extents to a New Disk

In this example, the logical volume is distributed across three physical volumes in the volume group myvg as follows:

# pvs -o+pv_used
  PV         VG   Fmt  Attr PSize  PFree  Used
  /dev/sda1  myvg lvm2 a-   17.15G  7.15G 10.00G
  /dev/sdb1  myvg lvm2 a-   17.15G 15.15G  2.00G
  /dev/sdc1  myvg lvm2 a-   17.15G 15.15G  2.00G

This example procedure moves the extents of /dev/sdb1 to a new device, /dev/sdd1.

  1. Create a new physical volume from /dev/sdd1.

    # pvcreate /dev/sdd1
      Physical volume "/dev/sdd1" successfully created
  2. Add the new physical volume /dev/sdd1 to the existing volume group myvg.

    # vgextend myvg /dev/sdd1
      Volume group "myvg" successfully extended
    # pvs -o+pv_used
      PV         VG   Fmt  Attr PSize  PFree  Used
      /dev/sda1   myvg lvm2 a-   17.15G  7.15G 10.00G
      /dev/sdb1   myvg lvm2 a-   17.15G 15.15G  2.00G
      /dev/sdc1   myvg lvm2 a-   17.15G 15.15G  2.00G
      /dev/sdd1   myvg lvm2 a-   17.15G 17.15G     0
  3. Use the pvmove command to move the data from /dev/sdb1 to /dev/sdd1.

    # pvmove /dev/sdb1 /dev/sdd1
      /dev/sdb1: Moved: 10.0%
    ...
      /dev/sdb1: Moved: 79.7%
    ...
      /dev/sdb1: Moved: 100.0%
    
    # pvs -o+pv_used
      PV          VG   Fmt  Attr PSize  PFree  Used
      /dev/sda1   myvg lvm2 a-   17.15G  7.15G 10.00G
      /dev/sdb1   myvg lvm2 a-   17.15G 17.15G     0
      /dev/sdc1   myvg lvm2 a-   17.15G 15.15G  2.00G
      /dev/sdd1   myvg lvm2 a-   17.15G 15.15G  2.00G
  4. After you have moved the data off /dev/sdb1, you can remove it from the volume group.

    # vgreduce myvg /dev/sdb1
      Removed "/dev/sdb1" from volume group "myvg"

You can now reallocate the disk to another volume group or remove the disk from the system.

3.6. Configuring persistent device numbers

Major and minor device numbers are allocated dynamically at module load. Some applications work best if the block device is always activated with the same device (major and minor) number. You can specify these with the lvcreate and the lvchange commands by using the following arguments:

--persistent y --major major --minor minor

Use a large minor number to be sure that it has not already been allocated to another device dynamically.

If you are exporting a file system using NFS, specifying the fsid parameter in the exports file may avoid the need to set a persistent device number within LVM.

3.7. Specifying LVM extent size

When physical volumes are used to create a volume group, its disk space is divided into 4MB extents, by default. This extent is the minimum amount by which the logical volume may be increased or decreased in size. Large numbers of extents will have no impact on I/O performance of the logical volume.

You can specify the extent size with the -s option to the vgcreate command if the default extent size is not suitable. You can put limits on the number of physical or logical volumes the volume group can have by using the -p and -l arguments of the vgcreate command.

3.8. Managing LVM logical volumes using RHEL System Roles

This section describes how to apply the storage role to achieve the following:

  • 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 exists.

For information on how to apply such playbook, see Applying a role.

3.8.1. An example Ansible playbook to manage logical volumes

This section shows an example Ansible playbook applying the storage role to create a LVM logical volume called mylv in a volume group called myvg. The volume group consists of the following disks:

  • /dev/sda
  • /dev/sdb
  • /dev/sdc

The playbook creates an ext4 file system with a given label on the logical volume, and persistently mounts the file system.

- hosts: all
  vars:
    storage_pools:
      - name: myvg
        disks:
          - /dev/sda
          - /dev/sdb
          - /dev/sdc
        volumes:
          - name: mylv
            size: 2G
            fs_type: ext4
            mount_point: /mnt
  roles:
    - rhel-system-roles.storage
Note

If a volume group called myvg already exists, the logical volume is added to it.

If such volume group does not exist, it is created.

3.8.2. Additional resources

3.9. Removing LVM logical volumes

This procedure removes an existing logical volume using the command-line LVM interface.

The following commands remove the logical volume /dev/vg-name/lv-name from the volume group vg-name.

Procedure

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

    [root@node-n]# lvchange --activate n vg-name/lv-name
  3. Remove the logical volume using the lvremove utility:

    # lvremove /dev/vg-name/lv-name
    Do you really want to remove active logical volume "lv-name"? [y/n]: y
      Logical volume "lv-name" 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

  • The lvremove(8) man page

Chapter 4. Modifying the size of a logical volume

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

4.1. Growing logical volumes

To increase the size of a logical volume, use the lvextend command.

When you extend the logical volume, you can indicate how much you want to extend the volume, or how large you want it to be after you extend it.

The following command extends the logical volume /dev/myvg/homevol to 12 gigabytes.

# lvextend -L12G /dev/myvg/homevol
lvextend -- extending logical volume "/dev/myvg/homevol" to 12 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended

The following command adds another gigabyte to the logical volume /dev/myvg/homevol.

# lvextend -L+1G /dev/myvg/homevol
lvextend -- extending logical volume "/dev/myvg/homevol" to 13 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended

As with the lvcreate command, you can use the -l argument of the lvextend command to specify the number of extents by which to increase the size of the logical volume. You can also use this argument to specify a percentage of the volume group, or a percentage of the remaining free space in the volume group. The following command extends the logical volume called testlv to fill all of the unallocated space in the volume group myvg.

# lvextend -l +100%FREE /dev/myvg/testlv
  Extending logical volume testlv to 68.59 GB
  Logical volume testlv successfully resized

After you have extended the logical volume it is necessary to increase the file system size to match.

By default, most file system resizing tools will increase the size of the file system to be the size of the underlying logical volume so you do not need to worry about specifying the same size for each of the two commands.

4.2. Growing a file system on a logical volume

To grow a file system on a logical volume, perform the following steps:

  1. Determine whether there is sufficient unallocated space in the existing volume group to extend the logical volume. If not, perform the following procedure:

    1. Create a new physical volume with the pvcreate command.
    2. Use the vgextend command to extend the volume group that contains the logical volume with the file system you are growing to include the new physical volume.
  2. Once the volume group is large enough to include the larger file system, extend the logical volume with the lvresize command.
  3. Resize the file system on the logical volume.

Note that you can use the -r option of the lvresize command to extend the logical volume and resize the underlying file system with a single command

4.3. Shrinking logical volumes

You can reduce the size of a logical volume with the lvreduce command.

Note

Shrinking is not supported on a GFS2 or XFS file system, so you cannot reduce the size of a logical volume that contains a GFS2 or XFS file system.

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, it is recommended that you use the --resizefs option of the lvreduce command when the logical volume contains a file system. When you use this option, the lvreduce command attempts to reduce the file system before shrinking the logical volume. If shrinking the file system fails, as can occur if the file system is full or the file system does not support shrinking, then the lvreduce command will fail and not attempt to shrink the logical volume.

Warning

In most cases, the lvreduce command warns about possible data loss and asks for a 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 where the operation is safe, as this option does not check the file system or test the file system resize.

The following command shrinks the logical volume lvol1 in volume group vg00 to be 64 megabytes. In this example, lvol1 contains a file system, which this command resizes together with the logical volume. This example shows the output to the command.

# lvreduce --resizefs -L 64M vg00/lvol1
fsck from util-linux 2.23.2
/dev/mapper/vg00-lvol1: clean, 11/25688 files, 8896/102400 blocks
resize2fs 1.42.9 (28-Dec-2013)
Resizing the filesystem on /dev/mapper/vg00-lvol1 to 65536 (1k) blocks.
The filesystem on /dev/mapper/vg00-lvol1 is now 65536 blocks long.

  Size of logical volume vg00/lvol1 changed from 100.00 MiB (25 extents) to 64.00 MiB (16 extents).
  Logical volume vg00/lvol1 successfully resized.

Specifying the - sign before the resize value indicates that the value will be subtracted from the logical volume’s actual size. The following example shows the command you would use if, instead of shrinking a logical volume to an absolute size of 64 megabytes, you wanted to shrink the volume by a value 64 megabytes.

# lvreduce --resizefs -L -64M vg00/lvol1

4.4. Extending a striped logical volume

In order to increase the size of a striped logical volume, there must be enough free space on the underlying physical volumes that make up the volume group to support the stripe. For example, if you have a two-way stripe that that uses up an entire volume group, adding a single physical volume to the volume group will not enable you to extend the stripe. Instead, you must add at least two physical volumes to the volume group.

For example, consider a volume group vg that consists of two underlying physical volumes, as displayed with the following vgs command.

# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     2   0   0 wz--n- 271.31G 271.31G

You can create a stripe using the entire amount of space in the volume group.

# lvcreate -n stripe1 -L 271.31G -i 2 vg
  Using default stripesize 64.00 KB
  Rounding up size to full physical extent 271.31 GB
  Logical volume "stripe1" created
# lvs -a -o +devices
  LV      VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
  stripe1 vg   -wi-a- 271.31G                               /dev/sda1(0),/dev/sdb1(0)

Note that the volume group now has no more free space.

# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     2   1   0 wz--n- 271.31G    0

The following command adds another physical volume to the volume group, which then has 135 gigabytes of additional space.

# vgextend vg /dev/sdc1
  Volume group "vg" successfully extended
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     3   1   0 wz--n- 406.97G 135.66G

At this point you cannot extend the striped logical volume to the full size of the volume group, because two underlying devices are needed in order to stripe the data.

# lvextend vg/stripe1 -L 406G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 406.00 GB
  Insufficient suitable allocatable extents for logical volume stripe1: 34480
more required

To extend the striped logical volume, add another physical volume and then extend the logical volume. In this example, having added two physical volumes to the volume group we can extend the logical volume to the full size of the volume group.

# vgextend vg /dev/sdd1
  Volume group "vg" successfully extended
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     4   1   0 wz--n- 542.62G 271.31G
# lvextend vg/stripe1 -L 542G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 542.00 GB
  Logical volume stripe1 successfully resized

If you do not have enough underlying physical devices to extend the striped logical volume, it is possible to extend the volume anyway if it does not matter that the extension is not striped, which may result in uneven performance. When adding space to the logical volume, the default operation is to use the same striping parameters of the last segment of the existing logical volume, but you can override those parameters. The following example extends the existing striped logical volume to use the remaining free space after the initial lvextend command fails.

# lvextend vg/stripe1 -L 406G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 406.00 GB
  Insufficient suitable allocatable extents for logical volume stripe1: 34480
more required
# lvextend -i1 -l+100%FREE vg/stripe1

Chapter 5. Managing LVM volume groups

This section describes the commands that perform the various aspects of volume group administration.

5.1. Volume groups

Physical volumes are combined into volume groups (VGs). This creates a pool of disk space out of which logical volumes 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 thus the same for all logical volumes in the volume group. The volume group maps the logical extents to physical extents.

5.2. Displaying volume groups

There are two commands you can use to display properties of LVM volume groups: vgs and vgdisplay. The vgscan command, which scans all supported LVM block devices in the system for volume groups, can also be used to display the existing volume groups.

The vgs command provides volume group information in a configurable form, displaying one line per volume group. The vgs command provides a great deal of format control, and is useful for scripting.

The vgdisplay command displays volume group properties (such as size, extents, number of physical volumes, and so on) in a fixed form. The following example shows the output of the vgdisplay command for the volume group new_vg. If you do not specify a volume group, all existing volume groups are displayed.

# vgdisplay new_vg
  --- Volume group ---
  VG Name               new_vg
  System ID
  Format                lvm2
  Metadata Areas        3
  Metadata Sequence No  11
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               0
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size               51.42 GB
  PE Size               4.00 MB
  Total PE              13164
  Alloc PE / Size       13 / 52.00 MB
  Free  PE / Size       13151 / 51.37 GB
  VG UUID               jxQJ0a-ZKk0-OpMO-0118-nlwO-wwqd-fD5D32

The following example shows the output of the vgscan command.

# vgscan
Reading all physical volumes.  This may take a while...
Found volume group "new_vg" using metadata type lvm2
Found volume group "officevg" using metadata type lvm2

5.3. Combining 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.

The following command merges the inactive volume group my_vg into the active or inactive volume group databases giving verbose runtime information.

# vgmerge -v databases my_vg

5.4. Splitting a volume group

In this example procedure, an existing volume group consists of three physical volumes. If there is enough unused space on the physical volumes, a new volume group can be created without adding new disks.

In the initial set up, the logical volume mylv is carved from the volume group myvg, which in turn consists of the three physical volumes, /dev/sda1, /dev/sdb1, and /dev/sdc1.

After completing this procedure, the volume group myvg will consist of /dev/sda1 and /dev/sdb1. A second volume group, yourvg, will consist of /dev/sdc1.

  1. Use the pvscan command to determine how much free space is currently available in the volume group.

    # pvscan
      PV /dev/sda1  VG myvg   lvm2 [17.15 GB / 0    free]
      PV /dev/sdb1  VG myvg   lvm2 [17.15 GB / 12.15 GB free]
      PV /dev/sdc1  VG myvg   lvm2 [17.15 GB / 15.80 GB free]
      Total: 3 [51.45 GB] / in use: 3 [51.45 GB] / in no VG: 0 [0   ]
  2. Move all the used physical extents in /dev/sdc1 to /dev/sdb1 with the pvmove command. The pvmove command can take a long time to execute.

    In a cluster, the pvmove command can move only logical volume that are active exclusively on a single node.

    # pvmove /dev/sdc1 /dev/sdb1
      /dev/sdc1: Moved: 14.7%
      /dev/sdc1: Moved: 30.3%
      /dev/sdc1: Moved: 45.7%
      /dev/sdc1: Moved: 61.0%
      /dev/sdc1: Moved: 76.6%
      /dev/sdc1: Moved: 92.2%
      /dev/sdc1: Moved: 100.0%

    After moving the data, you can see that all of the space on /dev/sdc1 is free.

    # pvscan
      PV /dev/sda1   VG myvg   lvm2 [17.15 GB / 0    free]
      PV /dev/sdb1   VG myvg   lvm2 [17.15 GB / 10.80 GB free]
      PV /dev/sdc1   VG myvg   lvm2 [17.15 GB / 17.15 GB free]
      Total: 3 [51.45 GB] / in use: 3 [51.45 GB] / in no VG: 0 [0   ]
  3. To create the new volume group yourvg, use the vgsplit command to split the volume group myvg.

    The following command splits the volume group yourvg from the volume group myvg, moving the physical volume /dev/sdc1 into the new volume group yourvg.

    # lvchange -a n /dev/myvg/mylv
    # vgsplit myvg yourvg /dev/sdc1
      Volume group "yourvg" successfully split from "myvg"

    You can use the vgs command to see 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
  4. After creating the new volume group, create the new logical volume yourlv.

    # lvcreate -L 5G -n yourlv yourvg
      Logical volume "yourlv" created
  5. Create a file system on the new logical volume and mount it.

    # mkfs.ext4 /dev/yourvg/yourlv
    mke2fs 1.44.3 (10-July-2018)
    Creating filesystem with 524288 4k blocks and 131072 inodes
    Filesystem UUID: 616da032-8a48-4cd7-8705-bd94b7a1c8c4
    Superblock backups stored on blocks:
            32768, 98304, 163840, 229376, 294912
    
    Allocating group tables: done
    Writing inode tables: done
    Creating journal (16384 blocks): done
    Writing superblocks and filesystem accounting information: done
    
    # mount /dev/yourvg/yourlv /mnt

5.5. Renaming LVM volume groups

This procedure renames an existing volume group using the command-line LVM interface.

Procedure

  1. If the volume group exists in a clustered environment, deactivate the volume group on all nodes where it is active. Use the following command on each such node:

    [root@node-n]# vgchange --activate n vg-name
  2. Use the vgrename utility to rename an existing volume group:

    # vgrename original-vg-name new-vg-name

    Optionally, you can specify the full paths to the devices:

    # vgrename /dev/original-vg-name  /dev/new-vg-name

Additional resources

  • The vgrename(8) man page

5.6. Moving a volume group to another system

You can move an entire LVM volume group to another system. It is recommended that you use the vgexport and vgimport commands when you do this.

Note

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

The vgexport command makes an inactive volume group inaccessible to the system, which allows you to detach its physical volumes. The vgimport command makes a volume group accessible to a machine again after the vgexport command has made it inactive.

To move a volume group from one system to another, perform the following steps:

  1. Make sure that no users are accessing files on the active volumes in the volume group, then unmount the logical volumes.
  2. Use the -a n argument of the vgchange command to mark the volume group as inactive, which prevents any further activity on the volume group.
  3. Use the vgexport command to export the volume group. This prevents it from being accessed by the system from which you are removing it.

    After you export the volume group, the physical volume will show up as being in an exported volume group when you execute the pvscan command, as in the following example.

    # 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]
      ...

    When the system is next shut down, you can unplug the disks that constitute the volume group and connect them to the new system.

  4. When the disks are plugged into the new system, use the vgimport command to import the volume group, making it accessible to the new system.
  5. Activate the volume group with the -a y argument of the vgchange command.
  6. Mount the file system to make it available for use.

5.7. Removing physical volumes from a volume group

To remove unused physical volumes from a volume group, 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.

Before removing a physical volume from a volume group, you can make sure that the physical volume is not used by any logical volumes by using the pvdisplay command.

# pvdisplay /dev/hda1

-- Physical volume ---
PV Name               /dev/hda1
VG Name               myvg
PV Size               1.95 GB / NOT usable 4 MB [LVM: 122 KB]
PV#                   1
PV Status             available
Allocatable           yes (but full)
Cur LV                1
PE Size (KByte)       4096
Total PE              499
Free PE               0
Allocated PE          499
PV UUID               Sd44tK-9IRw-SrMC-MOkn-76iP-iftz-OVSen7

If the physical volume is still being used you will have to migrate the data to another physical volume using the pvmove command. Then use the vgreduce command to remove the physical volume.

The following command removes the physical volume /dev/hda1 from the volume group my_volume_group.

# vgreduce my_volume_group /dev/hda1

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.

If the physical volume that fails contains a mirror image of a logical volume of a mirror segment type, you can remove that image from the mirror with the vgreduce --removemissing --mirrorsonly --force command. This removes only the logical volumes that are mirror images from the physical volume.

5.8. Removing LVM volume groups

This procedure removes an existing volume group using the command-line LVM interface.

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 removing:

    [root@node-n]# vgchange --lockstop vg-name

    Wait for the lock to stop.

  2. To remove the volume group, use the vgremove utility:

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

Additional resources

  • The vgremove(8) man page

5.9. Additional resources

  • The vgchange(8) man page

Chapter 6. Managing LVM physical volumes

There are a variety of commands and procedures you can use to manage LVM physical volumes.

6.1. Scanning for block devices to use as physical volumes

You can scan for block devices that may be used as physical volumes with the lvmdiskscan command, as shown in the following example.

# lvmdiskscan
  /dev/ram0                    [       16.00 MB]
  /dev/sda                     [       17.15 GB]
  /dev/root                    [       13.69 GB]
  /dev/ram                     [       16.00 MB]
  /dev/sda1                    [       17.14 GB] LVM physical volume
  /dev/VolGroup00/LogVol01     [      512.00 MB]
  /dev/ram2                    [       16.00 MB]
  /dev/new_vg/lvol0            [       52.00 MB]
  /dev/ram3                    [       16.00 MB]
  /dev/pkl_new_vg/sparkie_lv   [        7.14 GB]
  /dev/ram4                    [       16.00 MB]
  /dev/ram5                    [       16.00 MB]
  /dev/ram6                    [       16.00 MB]
  /dev/ram7                    [       16.00 MB]
  /dev/ram8                    [       16.00 MB]
  /dev/ram9                    [       16.00 MB]
  /dev/ram10                   [       16.00 MB]
  /dev/ram11                   [       16.00 MB]
  /dev/ram12                   [       16.00 MB]
  /dev/ram13                   [       16.00 MB]
  /dev/ram14                   [       16.00 MB]
  /dev/ram15                   [       16.00 MB]
  /dev/sdb                     [       17.15 GB]
  /dev/sdb1                    [       17.14 GB] LVM physical volume
  /dev/sdc                     [       17.15 GB]
  /dev/sdc1                    [       17.14 GB] LVM physical volume
  /dev/sdd                     [       17.15 GB]
  /dev/sdd1                    [       17.14 GB] LVM physical volume
  7 disks
  17 partitions
  0 LVM physical volume whole disks
  4 LVM physical volumes

6.2. Setting the partition type for 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. For whole disk devices only the partition table must be erased, which will effectively destroy all data on that disk. You can remove an existing partition table by zeroing the first sector with the following command:

# dd if=/dev/zero of=PhysicalVolume bs=512 count=1

6.3. Resizing an LVM physical volume

If you need to change the size of an underlying block device for any reason, use the pvresize command to update LVM with the new size. You can execute this command while LVM is using the physical volume.

6.4. Removing physical volumes

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

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.

# pvremove /dev/ram15
  Labels on physical volume "/dev/ram15" successfully wiped

6.5. Adding physical volumes to a volume group

To add additional physical volumes to an existing volume group, use the vgextend command. The vgextend command increases a volume group’s capacity by adding one or more free physical volumes.

The following command adds the physical volume /dev/sdf1 to the volume group vg1.

# vgextend vg1 /dev/sdf1

6.6. Removing physical volumes from a volume group

To remove unused physical volumes from a volume group, 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.

Before removing a physical volume from a volume group, you can make sure that the physical volume is not used by any logical volumes by using the pvdisplay command.

# pvdisplay /dev/hda1

-- Physical volume ---
PV Name               /dev/hda1
VG Name               myvg
PV Size               1.95 GB / NOT usable 4 MB [LVM: 122 KB]
PV#                   1
PV Status             available
Allocatable           yes (but full)
Cur LV                1
PE Size (KByte)       4096
Total PE              499
Free PE               0
Allocated PE          499
PV UUID               Sd44tK-9IRw-SrMC-MOkn-76iP-iftz-OVSen7

If the physical volume is still being used you will have to migrate the data to another physical volume using the pvmove command. Then use the vgreduce command to remove the physical volume.

The following command removes the physical volume /dev/hda1 from the volume group my_volume_group.

# vgreduce my_volume_group /dev/hda1

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.

If the physical volume that fails contains a mirror image of a logical volume of a mirror segment type, you can remove that image from the mirror with the vgreduce --removemissing --mirrorsonly --force command. This removes only the logical volumes that are mirror images from the physical volume.

Chapter 7. Displaying LVM components

LVM provides a variety of ways to display the LVM components, as well as to customize the display. This sections summarizes the usage of the basic LVM display commands.

7.1. Displaying LVM Information with the lvm Command

The lvm command provides several built-in options that you can use to display information about LVM support and configuration.

  • lvm devtypes

    Displays the recognized built-in block device types

  • lvm formats

    Displays recognized metadata formats.

  • lvm help

    Displays LVM help text.

  • lvm segtypes

    Displays recognized logical volume segment types.

  • lvm tags

    Displays any tags defined on this host.

  • lvm version

    Displays the current version information.

7.2. Displaying physical volumes

There are three commands you can use to display properties of LVM physical volumes: pvs, pvdisplay, and pvscan.

The pvs command provides physical volume information in a configurable form, displaying one line per physical volume. The pvs command provides a great deal of format control, and is useful for scripting.

The pvdisplay command provides a verbose multi-line output for each physical volume. It displays physical properties (size, extents, volume group, and so on) in a fixed format.

The following example shows the output of the pvdisplay command for a single physical volume.

# pvdisplay
  --- Physical volume ---
  PV Name               /dev/sdc1
  VG Name               new_vg
  PV Size               17.14 GB / not usable 3.40 MB
  Allocatable           yes
  PE Size (KByte)       4096
  Total PE              4388
  Free PE               4375
  Allocated PE          13
  PV UUID               Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe

The pvscan command scans all supported LVM block devices in the system for physical volumes.

The following command shows all physical devices found:

# pvscan
 PV /dev/sdb2   VG vg0   lvm2 [964.00 MB / 0   free]
 PV /dev/sdc1   VG vg0   lvm2 [964.00 MB / 428.00 MB free]
 PV /dev/sdc2            lvm2 [964.84 MB]
 Total: 3 [2.83 GB] / in use: 2 [1.88 GB] / in no VG: 1 [964.84 MB]

You can define a filter in the lvm.conf file so that this command will avoid scanning specific physical volumes.

7.3. Displaying volume groups

There are two commands you can use to display properties of LVM volume groups: vgs and vgdisplay. The vgscan command, which scans all supported LVM block devices in the system for volume groups, can also be used to display the existing volume groups.

The vgs command provides volume group information in a configurable form, displaying one line per volume group. The vgs command provides a great deal of format control, and is useful for scripting.

The vgdisplay command displays volume group properties (such as size, extents, number of physical volumes, and so on) in a fixed form. The following example shows the output of the vgdisplay command for the volume group new_vg. If you do not specify a volume group, all existing volume groups are displayed.

# vgdisplay new_vg
  --- Volume group ---
  VG Name               new_vg
  System ID
  Format                lvm2
  Metadata Areas        3
  Metadata Sequence No  11
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               0
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size               51.42 GB
  PE Size               4.00 MB
  Total PE              13164
  Alloc PE / Size       13 / 52.00 MB
  Free  PE / Size       13151 / 51.37 GB
  VG UUID               jxQJ0a-ZKk0-OpMO-0118-nlwO-wwqd-fD5D32

The following example shows the output of the vgscan command.

# vgscan
Reading all physical volumes.  This may take a while...
Found volume group "new_vg" using metadata type lvm2
Found volume group "officevg" using metadata type lvm2

7.4. Displaying logical volumes

There are three commands you can use to display properties of LVM logical volumes: lvs, lvdisplay, and lvscan.

The lvs command provides logical volume information in a configurable form, displaying one line per logical volume. The lvs command provides a great deal of format control, and is useful for scripting.

The lvdisplay command displays logical volume properties (such as size, layout, and mapping) in a fixed format.

The following command shows the attributes of lvol2 in vg00. If snapshot logical volumes have been created for this original logical volume, this command shows a list of all snapshot logical volumes and their status (active or inactive) as well.

# lvdisplay -v /dev/vg00/lvol2

The lvscan command scans for all logical volumes in the system and lists them, as in the following example.

# lvscan
 ACTIVE                   '/dev/vg0/gfslv' [1.46 GB] inherit

Chapter 8. Customized reporting for LVM

LVM provides a wide range of configuration and command line options to produce customized reports and to filter the report’s output. For a full description of LVM reporting features and capabilities, see the lvmreport(7) man page.

You can produce concise and customizable reports of LVM objects with the pvs, lvs, and vgs commands. The reports that these commands generate include one line of output for each object. Each line contains an ordered list of fields of properties related to the object. There are five ways to select the objects to be reported: by physical volume, volume group, logical volume, physical volume segment, and logical volume segment.

You can report information about physical volumes, volume groups, logical volumes, physical volume segments, and logical volume segments all at once with the lvm fullreport command. For information on this command and its capabilities, see the lvm-fullreport(8) man page.

LVM supports log reports, which contain a log of operations, messages, and per-object status with complete object identification collected during LVM command execution. For further information about the LVM log report. see the lvmreport(7) man page.

8.1. Controlling the format of the LVM display

Whether you use the pvs, lvs, or vgs command determines the default set of fields displayed and the sort order. You can control the output of these commands with the following arguments:

  • You can change what fields are displayed to something other than the default by using the -o argument. For example, the following command displays only the physical volume name and size.

    # pvs -o pv_name,pv_size
    PV PSize
    /dev/sdb1 17.14G
    /dev/sdc1 17.14G
    /dev/sdd1 17.14G
  • You can append a field to the output with the plus sign (+), which is used in combination with the -o argument.

    The following example displays the UUID of the physical volume in addition to the default fields.

    # pvs -o +pv_uuid
    PV VG Fmt Attr PSize PFree PV UUID
    /dev/sdb1 new_vg lvm2 a- 17.14G 17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
    /dev/sdc1 new_vg lvm2 a- 17.14G 17.09G Joqlch-yWSj-kuEn-IdwM-01S9-X08M-mcpsVe
    /dev/sdd1 new_vg lvm2 a- 17.14G 17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-UqkCS
  • Adding the -v argument to a command includes some extra fields. For example, the pvs -v command will display the DevSize and PV UUID fields in addition to the default fields.

    # pvs -v
    Scanning for physical volume names
    PV VG Fmt Attr PSize PFree DevSize PV UUID
    /dev/sdb1 new_vg lvm2 a- 17.14G 17.14G 17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
    /dev/sdc1 new_vg lvm2 a- 17.14G 17.09G 17.14G Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe
    /dev/sdd1 new_vg lvm2 a- 17.14G 17.14G 17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-tUqkCS
  • The --noheadings argument suppresses the headings line. This can be useful for writing scripts.

    The following example uses the --noheadings argument in combination with the pv_name argument, which will generate a list of all physical volumes.

    # pvs --noheadings -o pv_name
    /dev/sdb1
    /dev/sdc1
    /dev/sdd1
  • The --separator separator argument uses separator to separate each field.

    The following example separates the default output fields of the pvs command with an equals sign (=).

    # pvs --separator =
    PV=VG=Fmt=Attr=PSize=PFree
    /dev/sdb1=new_vg=lvm2=a-=17.14G=17.14G
    /dev/sdc1=new_vg=lvm2=a-=17.14G=17.09G
    /dev/sdd1=new_vg=lvm2=a-=17.14G=17.14G

    To keep the fields aligned when using the separator argument, use the separator argument in conjunction with the --aligned argument.

    # pvs --separator = --aligned
    PV =VG =Fmt =Attr=PSize =PFree
    /dev/sdb1 =new_vg=lvm2=a- =17.14G=17.14G
    /dev/sdc1 =new_vg=lvm2=a- =17.14G=17.09G
    /dev/sdd1 =new_vg=lvm2=a- =17.14G=17.14G

You can use the -P argument of the lvs or vgs command to display information about a failed volume that would otherwise not appear in the output.

For a full listing of display arguments, see the pvs(8), vgs(8) and lvs(8) man pages.

Volume group fields can be mixed with either physical volume (and physical volume segment) fields or with logical volume (and logical volume segment) fields, but physical volume and logical volume fields cannot be mixed. For example, the following command will display one line of output for each physical volume.

# vgs -o +pv_name
  VG     #PV #LV #SN Attr   VSize  VFree  PV
  new_vg   3   1   0 wz--n- 51.42G 51.37G /dev/sdc1
  new_vg   3   1   0 wz--n- 51.42G 51.37G /dev/sdd1
  new_vg   3   1   0 wz--n- 51.42G 51.37G /dev/sdb1

8.2. LVM object display fields

This section provides a series of tables that list the information you can display about the LVM objects with the pvs, vgs, and lvs commands.

For convenience, a field name prefix can be dropped if it matches the default for the command. For example, with the pvs command, name means pv_name, but with the vgs command, name is interpreted as vg_name.

Executing the following command is the equivalent of executing pvs -o pv_free.

# pvs -o free
  PFree
  17.14G
  17.09G
  17.14G
Note

The number of characters in the attribute fields in pvs, vgs, and lvs output may increase in later releases. The existing character fields will not change position, but new fields may be added to the end. You should take this into account when writing scripts that search for particular attribute characters, searching for the character based on its relative position to the beginning of the field, but not for its relative position to the end of the field. For example, to search for the character p in the ninth bit of the lv_attr field, you could search for the string "^/…​…​..p/", but you should not search for the string "/*p$/".

Table 8.1, “The pvs Command Display Fields” lists the display arguments of the pvs command, along with the field name as it appears in the header display and a description of the field.

Table 8.1. The pvs Command Display Fields

ArgumentHeaderDescription

dev_size

DevSize

The size of the underlying device on which the physical volume was created

pe_start

1st PE

Offset to the start of the first physical extent in the underlying device

pv_attr

Attr

Status of the physical volume: (a)llocatable or e(x)ported.

pv_fmt

Fmt

The metadata format of the physical volume (lvm2 or lvm1)

pv_free

PFree

The free space remaining on the physical volume

pv_name

PV

The physical volume name

pv_pe_alloc_count

Alloc

Number of used physical extents

pv_pe_count

PE

Number of physical extents

pvseg_size

SSize

The segment size of the physical volume

pvseg_start

Start

The starting physical extent of the physical volume segment

pv_size

PSize

The size of the physical volume

pv_tags

PV Tags

LVM tags attached to the physical volume

pv_used

Used

The amount of space currently used on the physical volume

pv_uuid

PV UUID

The UUID of the physical volume

The pvs command displays the following fields by default: pv_name, vg_name, pv_fmt, pv_attr, pv_size, pv_free. The display is sorted by pv_name.

# pvs
  PV         VG     Fmt  Attr PSize  PFree
  /dev/sdb1  new_vg lvm2 a-   17.14G 17.14G
  /dev/sdc1  new_vg lvm2 a-   17.14G 17.09G
  /dev/sdd1  new_vg lvm2 a-   17.14G 17.13G

Using the -v argument with the pvs command adds the following fields to the default display: dev_size, pv_uuid.

# pvs -v
    Scanning for physical volume names
  PV         VG     Fmt  Attr PSize  PFree  DevSize PV UUID
  /dev/sdb1  new_vg lvm2 a-   17.14G 17.14G  17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
  /dev/sdc1  new_vg lvm2 a-   17.14G 17.09G  17.14G Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe
  /dev/sdd1  new_vg lvm2 a-   17.14G 17.13G  17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-tUqkCS

You can use the --segments argument of the pvs command to display information about each physical volume segment. A segment is a group of extents. A segment view can be useful if you want to see whether your logical volume is fragmented.

The pvs --segments command displays the following fields by default: pv_name, vg_name, pv_fmt, pv_attr, pv_size, pv_free, pvseg_start, pvseg_size. The display is sorted by pv_name and pvseg_size within the physical volume.

# pvs --segments
  PV         VG         Fmt  Attr PSize  PFree  Start SSize
  /dev/hda2  VolGroup00 lvm2 a-   37.16G 32.00M     0  1172
  /dev/hda2  VolGroup00 lvm2 a-   37.16G 32.00M  1172    16
  /dev/hda2  VolGroup00 lvm2 a-   37.16G 32.00M  1188     1
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G     0    26
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G    26    24
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G    50    26
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G    76    24
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G   100    26
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G   126    24
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G   150    22
  /dev/sda1  vg         lvm2 a-   17.14G 16.75G   172  4217
  /dev/sdb1  vg         lvm2 a-   17.14G 17.14G     0  4389
  /dev/sdc1  vg         lvm2 a-   17.14G 17.14G     0  4389
  /dev/sdd1  vg         lvm2 a-   17.14G 17.14G     0  4389
  /dev/sde1  vg         lvm2 a-   17.14G 17.14G     0  4389
  /dev/sdf1  vg         lvm2 a-   17.14G 17.14G     0  4389
  /dev/sdg1  vg         lvm2 a-   17.14G 17.14G     0  4389

You can use the pvs -a command to see devices detected by LVM that have not been initialized as LVM physical volumes.

# pvs -a
  PV                             VG     Fmt  Attr PSize  PFree
  /dev/VolGroup00/LogVol01                   --       0      0
  /dev/new_vg/lvol0                          --       0      0
  /dev/ram                                   --       0      0
  /dev/ram0                                  --       0      0
  /dev/ram2                                  --       0      0
  /dev/ram3                                  --       0      0
  /dev/ram4                                  --       0      0
  /dev/ram5                                  --       0      0
  /dev/ram6                                  --       0      0
  /dev/root                                  --       0      0
  /dev/sda                                   --       0      0
  /dev/sdb                                   --       0      0
  /dev/sdb1                      new_vg lvm2 a-   17.14G 17.14G
  /dev/sdc                                   --       0      0
  /dev/sdc1                      new_vg lvm2 a-   17.14G 17.09G
  /dev/sdd                                   --       0      0
  /dev/sdd1                      new_vg lvm2 a-   17.14G 17.14G

Table 8.2, “vgs Display Fields” lists the display arguments of the vgs command, along with the field name as it appears in the header display and a description of the field.

Table 8.2. vgs Display Fields

ArgumentHeaderDescription

lv_count

#LV

The number of logical volumes the volume group contains

max_lv

MaxLV

The maximum number of logical volumes allowed in the volume group (0 if unlimited)

max_pv

MaxPV

The maximum number of physical volumes allowed in the volume group (0 if unlimited)

pv_count

#PV

The number of physical volumes that define the volume group

snap_count

#SN

The number of snapshots the volume group contains

vg_attr

Attr

Status of the volume group: (w)riteable, (r)eadonly, resi(z)eable, e(x)ported, (p)artial and (c)lustered.

vg_extent_count

#Ext

The number of physical extents in the volume group

vg_extent_size

Ext

The size of the physical extents in the volume group

vg_fmt

Fmt

The metadata format of the volume group (lvm2 or lvm1)

vg_free

VFree

Size of the free space remaining in the volume group

vg_free_count

Free

Number of free physical extents in the volume group

vg_name

VG

The volume group name

vg_seqno

Seq

Number representing the revision of the volume group

vg_size

VSize

The size of the volume group

vg_sysid

SYS ID

LVM1 System ID

vg_tags

VG Tags

LVM tags attached to the volume group

vg_uuid

VG UUID

The UUID of the volume group

The vgs command displays the following fields by default: vg_name, pv_count, lv_count, snap_count, vg_attr, vg_size, vg_free. The display is sorted by vg_name.

# vgs
  VG     #PV #LV #SN Attr   VSize  VFree
  new_vg   3   1   1 wz--n- 51.42G 51.36G

Using the -v argument with the vgs command adds the following fields to the default display: vg_extent_size, vg_uuid.

# vgs -v
    Finding all volume groups
    Finding volume group "new_vg"
  VG     Attr   Ext   #PV #LV #SN VSize  VFree  VG UUID
  new_vg wz--n- 4.00M   3   1   1 51.42G 51.36G jxQJ0a-ZKk0-OpMO-0118-nlwO-wwqd-fD5D32

Table 8.3, “lvs Display Fields” lists the display arguments of the lvs command, along with the field name as it appears in the header display and a description of the field.

Note

In later releases of Red Hat Enterprise Linux, the output of the lvs command may differ, with additional fields in the output. The order of the fields, however, will remain the same and any additional fields will appear at the end of the display.

Table 8.3. lvs Display Fields

ArgumentHeaderDescription

* chunksize

* chunk_size

Chunk

Unit size in a snapshot volume

copy_percent

Copy%

The synchronization percentage of a mirrored logical volume; also used when physical extents are being moved with the pv_move command

devices

Devices

The underlying devices that make up the logical volume: the physical volumes, logical volumes, and start physical extents and logical extents

lv_ancestors

Ancestors

For thin pool snapshots, the ancestors of the logical volume

lv_descendants

Descendants

For thin pool snapshots, the descendants of the logical volume

lv_attr

Attr

The status of the logical volume. The logical volume attribute bits are as follows:

* Bit 1: Volume type: (m)irrored, (M)irrored without initial sync, (o)rigin, (O)rigin with merging snapshot, (r)aid, ®aid without initial sync, (s)napshot, merging (S)napshot, (p)vmove, (v)irtual, mirror or raid (i)mage, mirror or raid (I)mage out-of-sync, mirror (l)og device, under (c)onversion, thin (V)olume, (t)hin pool, (T)hin pool data, raid or thin pool m(e)tadata or pool metadata spare,

* Bit 2: Permissions: (w)riteable, (r)ead-only, ®ead-only activation of non-read-only volume

* Bit 3: Allocation policy: (a)nywhere, (c)ontiguous, (i)nherited, c(l)ing, (n)ormal. This is capitalized if the volume is currently locked against allocation changes, for example while executing the pvmove command.

* Bit 4: fixed (m)inor

* Bit 5: State: (a)ctive, (s)uspended, (I)nvalid snapshot, invalid (S)uspended snapshot, snapshot (m)erge failed, suspended snapshot (M)erge failed, mapped (d)evice present without tables, mapped device present with (i)nactive table

* Bit 6: device (o)pen

* Bit 7: Target type: (m)irror, (r)aid, (s)napshot, (t)hin, (u)nknown, (v)irtual. This groups logical volumes related to the same kernel target together. So, for example, mirror images, mirror logs as well as mirrors themselves appear as (m) if they use the original device-mapper mirror kernel driver, whereas the raid equivalents using the md raid kernel driver all appear as (r). Snapshots using the original device-mapper driver appear as (s), whereas snapshots of thin volumes using the thin provisioning driver appear as (t).

* Bit 8: Newly-allocated data blocks are overwritten with blocks of (z)eroes before use.

* Bit 9: Volume Health: (p)artial, (r)efresh needed, (m)ismatches exist, (w)ritemostly. (p)artial signifies that one or more of the Physical Volumes this Logical Volume uses is missing from the system. (r)efresh signifies that one or more of the Physical Volumes this RAID Logical Volume uses had suffered a write error. The write error could be due to a temporary failure of that Physical Volume or an indication that it is failing. The device should be refreshed or replaced. (m)ismatches signifies that the RAID logical volume has portions of the array that are not coherent. Inconsistencies are discovered by initiating a check operation on a RAID logical volume. (The scrubbing operations, check and repair, can be performed on a RAID Logical Volume by means of the lvchange command.) (w)ritemostly signifies the devices in a RAID 1 logical volume that have been marked write-mostly.

* Bit 10: s(k)ip activation: this volume is flagged to be skipped during activation.

lv_kernel_major

KMaj

Actual major device number of the logical volume (-1 if inactive)

lv_kernel_minor

KMIN

Actual minor device number of the logical volume (-1 if inactive)

lv_major

Maj

The persistent major device number of the logical volume (-1 if not specified)

lv_minor

Min

The persistent minor device number of the logical volume (-1 if not specified)

lv_name

LV

The name of the logical volume

lv_size

LSize

The size of the logical volume

lv_tags

LV Tags

LVM tags attached to the logical volume

lv_uuid

LV UUID

The UUID of the logical volume.

mirror_log

Log

Device on which the mirror log resides

modules

Modules

Corresponding kernel device-mapper target necessary to use this logical volume

move_pv

Move

Source physical volume of a temporary logical volume created with the pvmove command

origin

Origin

The origin device of a snapshot volume

* regionsize

* region_size

Region

The unit size of a mirrored logical volume

seg_count

#Seg

The number of segments in the logical volume

seg_size

SSize

The size of the segments in the logical volume

seg_start

Start

Offset of the segment in the logical volume

seg_tags

Seg Tags

LVM tags attached to the segments of the logical volume

segtype

Type

The segment type of a logical volume (for example: mirror, striped, linear)

snap_percent

Snap%

Current percentage of a snapshot volume that is in use

stripes

#Str

Number of stripes or mirrors in a logical volume

* stripesize

* stripe_size

Stripe

Unit size of the stripe in a striped logical volume

The lvs command provides the following display by default. The default display is sorted by vg_name and lv_name within the volume group.

# lvs
  LV     VG              Attr       LSize    Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  origin VG              owi-a-s---    1.00g
  snap   VG              swi-a-s---  100.00m      origin 0.00

A common use of the lvs command is to append devices to the command to display the underlying devices that make up the logical volume. This example also specifies the -a option to display the internal volumes that are components of the logical volumes, such as RAID mirrors, enclosed in brackets. This example includes a RAID volume, a striped volume, and a thinly-pooled volume.

# lvs -a -o +devices
  LV               VG            Attr       LSize   Pool   Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
  raid1            VG            rwi-a-r---   1.00g                                      100.00           raid1_rimage_0(0),raid1_rimage_1(0)
  [raid1_rimage_0] VG            iwi-aor---   1.00g                                                       /dev/sde1(7041)
  [raid1_rimage_1] VG            iwi-aor---   1.00g                                                       /dev/sdf1(7041)
  [raid1_rmeta_0]  VG            ewi-aor---   4.00m                                                       /dev/sde1(7040)
  [raid1_rmeta_1]  VG            ewi-aor---   4.00m                                                       /dev/sdf1(7040)
  stripe1          VG            -wi-a-----  99.95g                                                       /dev/sde1(0),/dev/sdf1(0)
  stripe1          VG            -wi-a-----  99.95g                                                       /dev/sdd1(0)
  stripe1          VG            -wi-a-----  99.95g                                                       /dev/sdc1(0)
  [lvol0_pmspare]  rhel_host-083 ewi-------   4.00m                                                       /dev/vda2(0)
  pool00           rhel_host-083 twi-aotz--  <4.79g               72.90  54.69                            pool00_tdata(0)
  [pool00_tdata]   rhel_host-083 Twi-ao----  <4.79g                                                       /dev/vda2(1)
  [pool00_tmeta]   rhel_host-083 ewi-ao----   4.00m                                                       /dev/vda2(1226)
  root             rhel_host-083 Vwi-aotz--  <4.79g pool00        72.90
  swap             rhel_host-083 -wi-ao---- 820.00m                                                       /dev/vda2(1227)

Using the -v argument with the lvs command adds the following fields to the default display: seg_count, lv_major, lv_minor, lv_kernel_major, lv_kernel_minor, lv_uuid.

# lvs -v
    Finding all logical volumes
  LV         VG     #Seg Attr   LSize  Maj Min KMaj KMin Origin Snap%  Move Copy%  Log Convert LV UUID
  lvol0      new_vg    1 owi-a- 52.00M  -1  -1 253  3                                          LBy1Tz-sr23-OjsI-LT03-nHLC-y8XW-EhCl78
  newvgsnap1 new_vg    1 swi-a-  8.00M  -1  -1 253  5    lvol0    0.20                         1ye1OU-1cIu-o79k-20h2-ZGF0-qCJm-CfbsIx

You can use the --segments argument of the lvs command to display information with default columns that emphasize the segment information. When you use the segments argument, the seg prefix is optional. The lvs --segments command displays the following fields by default: lv_name, vg_name, lv_attr, stripes, segtype, seg_size. The default display is sorted by vg_name, lv_name within the volume group, and seg_start within the logical volume. If the logical volumes were fragmented, the output from this command would show that.

# lvs --segments
  LV       VG         Attr   #Str Type   SSize
  LogVol00 VolGroup00 -wi-ao    1 linear  36.62G
  LogVol01 VolGroup00 -wi-ao    1 linear 512.00M
  lv       vg         -wi-a-    1 linear 104.00M
  lv       vg         -wi-a-    1 linear 104.00M
  lv       vg         -wi-a-    1 linear 104.00M
  lv       vg         -wi-a-    1 linear  88.00M

Using the -v argument with the lvs --segments command adds the following fields to the default display: seg_start, stripesize, chunksize.

# lvs -v --segments
    Finding all logical volumes
  LV         VG     Attr   Start SSize  #Str Type   Stripe Chunk
  lvol0      new_vg owi-a-    0  52.00M    1 linear     0     0
  newvgsnap1 new_vg swi-a-    0   8.00M    1 linear     0  8.00K

The following example shows the default output of the lvs command on a system with one logical volume configured, followed by the default output of the lvs command with the segments argument specified.

# lvs
  LV    VG     Attr   LSize  Origin Snap%  Move Log Copy%
  lvol0 new_vg -wi-a- 52.00M
# lvs --segments
  LV    VG     Attr   #Str Type   SSize
  lvol0 new_vg -wi-a-    1 linear 52.00M

8.3. Sorting LVM reports

Normally the entire output of the lvs, vgs, or pvs command has to be generated and stored internally before it can be sorted and columns aligned correctly. You can specify the --unbuffered argument to display unsorted output as soon as it is generated.

To specify an alternative ordered list of columns to sort on, use the -O argument of any of the reporting commands. It is not necessary to include these fields within the output itself.

The following example shows the output of the pvs command that displays the physical volume name, size, and free space.

# pvs -o pv_name,pv_size,pv_free
  PV         PSize  PFree
  /dev/sdb1  17.14G 17.14G
  /dev/sdc1  17.14G 17.09G
  /dev/sdd1  17.14G 17.14G

The following example shows the same output, sorted by the free space field.

# pvs -o pv_name,pv_size,pv_free -O pv_free
  PV         PSize  PFree
  /dev/sdc1  17.14G 17.09G
  /dev/sdd1  17.14G 17.14G
  /dev/sdb1  17.14G 17.14G

The following example shows that you do not need to display the field on which you are sorting.

# pvs -o pv_name,pv_size -O pv_free
  PV         PSize
  /dev/sdc1  17.14G
  /dev/sdd1  17.14G
  /dev/sdb1  17.14G

To display a reverse sort, precede a field you specify after the -O argument with the - character.

# pvs -o pv_name,pv_size,pv_free -O -pv_free
  PV         PSize  PFree
  /dev/sdd1  17.14G 17.14G
  /dev/sdb1  17.14G 17.14G
  /dev/sdc1  17.14G 17.09G

8.4. Specifying the units for an LVM report display

To specify the units for the LVM report display, use the --units argument of the report command. You can specify (b)ytes, (k)ilobytes, (m)egabytes, (g)igabytes, (t)erabytes, (e)xabytes, (p)etabytes, and (h)uman-readable. The default display is human-readable. You can override the default by setting the units parameter in the global section of the /etc/lvm/lvm.conf file.

The following example specifies the output of the pvs command in megabytes rather than the default gigabytes.

# pvs --units m
  PV         VG     Fmt  Attr PSize     PFree
  /dev/sda1         lvm2 --   17555.40M 17555.40M
  /dev/sdb1  new_vg lvm2 a-   17552.00M 17552.00M
  /dev/sdc1  new_vg lvm2 a-   17552.00M 17500.00M
  /dev/sdd1  new_vg lvm2 a-   17552.00M 17552.00M

By default, units are displayed in powers of 2 (multiples of 1024). You can specify that units be displayed in multiples of 1000 by capitalizing the unit specification (B, K, M, G, T, H).

The following command displays the output as a multiple of 1024, the default behavior.

# pvs
  PV         VG     Fmt  Attr PSize  PFree
  /dev/sdb1  new_vg lvm2 a-   17.14G 17.14G
  /dev/sdc1  new_vg lvm2 a-   17.14G 17.09G
  /dev/sdd1  new_vg lvm2 a-   17.14G 17.14G

The following command displays the output as a multiple of 1000.

#  pvs --units G
  PV         VG     Fmt  Attr PSize  PFree
  /dev/sdb1  new_vg lvm2 a-   18.40G 18.40G
  /dev/sdc1  new_vg lvm2 a-   18.40G 18.35G
  /dev/sdd1  new_vg lvm2 a-   18.40G 18.40G

You can also specify (s)ectors (defined as 512 bytes) or custom units.

The following example displays the output of the pvs command as a number of sectors.

# pvs --units s
  PV         VG     Fmt  Attr PSize     PFree
  /dev/sdb1  new_vg lvm2 a-   35946496S 35946496S
  /dev/sdc1  new_vg lvm2 a-   35946496S 35840000S
  /dev/sdd1  new_vg lvm2 a-   35946496S 35946496S

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/sdb1  new_vg lvm2 a-   4388.00U 4388.00U
  /dev/sdc1  new_vg lvm2 a-   4388.00U 4375.00U
  /dev/sdd1  new_vg lvm2 a-   4388.00U 4388.00U

8.5. Displaying LVM command output in JSON format

You can use the --reportformat option of the LVM display commands to display the output in JSON format.

The following example shows the output of the lvs in standard default format.

# lvs
  LV      VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  my_raid my_vg         Rwi-a-r---  12.00m                                    100.00
  root    rhel_host-075 -wi-ao----   6.67g
  swap    rhel_host-075 -wi-ao---- 820.00m

The following command shows the output of the same LVM configuration when you specify JSON format.

# lvs --reportformat json
  {
      "report": [
          {
              "lv": [
                  {"lv_name":"my_raid", "vg_name":"my_vg", "lv_attr":"Rwi-a-r---", "lv_size":"12.00m", "pool_lv":"", "origin":"", "data_percent":"", "metadata_percent":"", "move_pv":"", "mirror_log":"", "copy_percent":"100.00", "convert_lv":""},
                  {"lv_name":"root", "vg_name":"rhel_host-075", "lv_attr":"-wi-ao----", "lv_size":"6.67g", "pool_lv":"", "origin":"", "data_percent":"", "metadata_percent":"", "move_pv":"", "mirror_log":"", "copy_percent":"", "convert_lv":""},
                  {"lv_name":"swap", "vg_name":"rhel_host-075", "lv_attr":"-wi-ao----", "lv_size":"820.00m", "pool_lv":"", "origin":"", "data_percent":"", "metadata_percent":"", "move_pv":"", "mirror_log":"", "copy_percent":"", "convert_lv":""}
              ]
          }
      ]
  }

You can also set the report format as a configuration option in the /etc/lvm/lvm.conf file, using the output_format setting. The --reportformat setting of the command line, however, takes precedence over this setting.

8.6. Displaying the LVM command log

Both report-oriented and processing-oriented LVM commands can report the command log if this is enabled with the log/report_command_log configuration setting. You can determine the set of fields to display and to sort by for this report.

The following examples configures LVM to generate a complete log report for LVM commands. In this example, you can see that both logical volumes lvol0 and lvol1 were successfully processed, as was the volume group VG that contains the volumes.

# lvmconfig --type full log/command_log_selection
command_log_selection="all"

# lvs
  Logical Volume
  ==============
  LV    LSize Cpy%Sync
  lvol1 4.00m 100.00
  lvol0 4.00m

  Command Log
  ===========
  Seq LogType Context    ObjType ObjName ObjGrp  Msg     Errno RetCode
    1 status  processing lv      lvol0   vg      success     0       1
    2 status  processing lv      lvol1   vg      success     0       1
    3 status  processing vg      vg              success     0       1

# lvchange -an vg/lvol1
  Command Log
  ===========
  Seq LogType Context    ObjType ObjName ObjGrp  Msg     Errno RetCode
    1 status  processing lv      lvol1   vg      success     0       1
    2 status  processing vg      vg              success     0       1

For further information on configuring LVM reports and command logs, see the lvmreport man page.

Chapter 9. Configuring RAID logical volumes

LVM supports RAID0/1/4/5/6/10.

Note

RAID logical volumes are not cluster-aware. While RAID logical volumes can be created and activated exclusively on one machine, they cannot be activated simultaneously on more than one machine.

To create a RAID logical volume, you specify a raid type as the --type argument of the lvcreate command. Table 9.1, “RAID Segment Types” describes the possible RAID segment types. After you have created a RAID logical volume with LVM, you can activate, change, remove, display, and use the volume just as you would any other LVM logical volume.

Table 9.1. 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 but you do not specify striping.

raid4

RAID4 dedicated parity disk

raid5

Same as raid5_ls

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.

* Rotating parity 0 with data restart

raid5_rs

* RAID5 right symmetric.

* Rotating parity N with data restart

raid6

Same as raid6_zr

raid6_zr

* RAID6 zero restart

* 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 and you specify a 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 will be lost if any of the data subvolumes fail.

For most users, specifying one of the five available primary types (raid1, raid4, raid5, raid6, raid10) should be sufficient. For more information on the different algorithms used by RAID 5/6, see chapter four of the Common RAID Disk Data Format Specification at http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf.

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. 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 (plus 1 implicit parity device) RAID4 results in 4 metadata subvolumes (lv_rmeta_0, lv_rmeta_1, lv_rmeta_2, and lv_rmeta_3) and 4 data subvolumes (lv_rimage_0, lv_rimage_1, lv_rimage_2, and lv_rimage_3).

Note

You can generate commands to create logical volumes on RAID storage with the LVM RAID Calculator application. This application uses the information you input about your current or planned storage to generate these commands. The LVM RAID Calculator application can be found at https://access.redhat.com/labs/lvmraidcalculator/.

9.1. Creating RAID logical volumes

This section provides example commands that create different types of RAID logical volume.

You can create RAID1 arrays with different numbers of copies according to the value you specify for the -m argument. Similarly, you specify the number of stripes for a RAID 4/5/6 logical volume with the -i argument. You can also specify the stripe size with the -I argument.

The following command creates a 2-way RAID1 array named my_lv in the volume group my_vg that is one gigabyte in size.

# lvcreate --type raid1 -m 1 -L 1G -n my_lv my_vg

The following command creates a RAID5 array (3 stripes + 1 implicit parity drive) named my_lv in the volume group my_vg that is one gigabyte in size. Note that you specify the number of stripes just as you do for 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

The following command creates a RAID6 array (3 stripes + 2 implicit parity drives) named my_lv in the volume group my_vg that is one gigabyte in size.

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

9.2. Creating a RAID0 (striped) logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size.

The format for the command to create a RAID0 volume is as follows.

lvcreate --type raid0[_meta] --stripes Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath ...]

Table 9.2. RAID0 Command Creation parameters

ParameterDescription

--type raid0[_meta]

Specifying raid0 creates a RAID0 volume without metadata volumes. Specifying raid0_meta creates a RAID0 volume with metadata volumes. Because RAID0 is non-resilient, it does not have to 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, however, and 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.

This example procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks at /dev/sda1, /dev/sdb1, and /dev/sdc1.

  1. Label the disks you will use in the volume group as LVM physical volumes with the pvcreate command.

    Warning

    This command destroys any data on /dev/sda1, /dev/sdb1, and /dev/sdc1.

    # pvcreate /dev/sda1 /dev/sdb1 /dev/sdc1
      Physical volume "/dev/sda1" successfully created
      Physical volume "/dev/sdb1" successfully created
      Physical volume "/dev/sdc1" successfully created
  2. Create the volume group myvg. The following command creates the volume group myvg.

    # vgcreate myvg /dev/sda1 /dev/sdb1 /dev/sdc1
      Volume group "myvg" successfully created

    You can use the vgs command to display the attributes of the new volume group.

    # vgs
      VG   #PV #LV #SN Attr   VSize  VFree
      myvg   3   0   0 wz--n- 51.45G 51.45G
  3. Create a RAID0 logical volume from the volume group you have created. The following command creates the RAID0 volume mylv from the volume group myvg. This example creates a logical volume that is 2 gigabytes in size, with three stripes and a stripe size of 4 kilobytes.

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

    # mkfs.ext4 /dev/myvg/mylv
    mke2fs 1.44.3 (10-July-2018)
    Creating filesystem with 525312 4k blocks and 131376 inodes
    Filesystem UUID: 9d4c0704-6028-450a-8b0a-8875358c0511
    Superblock backups stored on blocks:
            32768, 98304, 163840, 229376, 294912
    
    Allocating group tables: done
    Writing inode tables: done
    Creating journal (16384 blocks): done
    Writing superblocks and filesystem accounting information: done

    The following commands mount the logical volume and report the file system disk space usage.

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

9.3. Controlling the rate at which RAID volumes are initialized

When you create RAID10 logical volumes, the background I/O required to initialize the logical volumes with a sync operation can crowd out 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. You control the rate at which sync operations are performed by setting the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvcreate command. You specify these options as follows.

  • --maxrecoveryrate Rate[bBsSkKmMgG]

    Sets the maximum recovery rate for a RAID logical volume so that it will not crowd out nominal I/O operations. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed. 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. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed.

The following command creates a 2-way RAID10 array with 3 stripes that is 10 gigabytes in size with a maximum recovery rate of 128 kiB/sec/device. The array is named my_lv and is in the volume group my_vg.

# lvcreate --type raid10 -i 2 -m 1 -L 10G --maxrecoveryrate 128 -n my_lv my_vg

You can also specify minimum and maximum recovery rates for a RAID scrubbing operation.

9.4. Converting a Linear device to a RAID device

You can convert an existing linear logical volume to a RAID device by using the --type argument of the lvconvert command.

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

Since RAID logical volumes are composed of metadata and data subvolume pairs, when you convert a linear device to a RAID1 array, a new metadata subvolume is created and associated with the original logical volume on (one of) the same physical volumes that the linear volume is on. The additional images are added in metadata/data subvolume pairs. For example, if the original device is as follows:

# lvs -a -o name,copy_percent,devices my_vg
  LV     Copy%  Devices
  my_lv         /dev/sde1(0)

After conversion to a 2-way RAID1 array the device contains the following data and metadata subvolume pairs:

# lvconvert --type raid1 -m 1 my_vg/my_lv
# 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)

If the metadata image that pairs with the original logical volume cannot be placed on the same physical volume, the lvconvert will fail.

9.5. 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 with the lvconvert command by specifying 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.

The following example displays 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)

The following command converts the LVM RAID1 logical volume my_vg/my_lv to an LVM linear device.

# lvconvert -m0 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV      Copy%  Devices
  my_lv          /dev/sde1(1)

When you convert an LVM RAID1 logical volume to an LVM linear volume, you can specify which physical volumes to remove. The following example shows the layout of an LVM RAID1 logical volume made up of two images: /dev/sda1 and /dev/sdb1. In this example, the lvconvert command specifies that you want to remove /dev/sda1, leaving /dev/sdb1 as the physical volume that makes up the linear device.

# 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/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# lvconvert -m0 my_vg/my_lv /dev/sda1
# lvs -a -o name,copy_percent,devices my_vg
  LV    Copy%  Devices
  my_lv        /dev/sdb1(1)

9.6. Converting a mirrored LVM device to a RAID1 device

You can convert an existing mirrored LVM device with a segment type of mirror to a RAID1 LVM device with the lvconvert command by specifying the --type raid1 argument. This renames the mirror subvolumes (mimage) to RAID subvolumes (rimage). In addition, the mirror log is removed and metadata subvolumes (rmeta) are created for the data subvolumes on the same physical volumes as the corresponding data subvolumes.

The following example shows 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)

The following command converts the mirrored logical volume my_vg/my_lv to a RAID1 logical volume.

# lvconvert --type raid1 my_vg/my_lv
# 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)

9.7. Resizing 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 as 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 as when you create a striped RAID logical volume.
  • You can change the number of stripes on a striped RAID logical volume (raid4/5/6/10) 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 that allows you to change attributes of a RAID logical volume while keeping the same RAID level. For information on RAID reshaping and examples of using the lvconvert command to reshape a RAID logical volume, see the lvmraid(7) man page.

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

You can change the number of images in an existing RAID1 array just as you can change the number of images in the earlier implementation of LVM mirroring. Use the lvconvert command to specify the number of additional metadata/data subvolume pairs to add or remove.

When you add images to a RAID1 device with the lvconvert command, you can specify the total number of images for the resulting device, or you can specify how many images to add to the device. You can also optionally specify on which physical volumes the new metadata/data image pairs will reside.

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).

The format for the command to add images to a RAID1 volume is as follows:

lvconvert -m new_absolute_count vg/lv [removable_PVs]
lvconvert -m +num_additional_images vg/lv [removable_PVs]

For example, the following command displays 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)

The following command converts the 2-way RAID1 device my_vg/my_lv to a 3-way RAID1 device:

# lvconvert -m 2 my_vg/my_lv
# 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_2(0)
  [my_lv_rimage_0]        /dev/sde1(0)
  [my_lv_rimage_1]        /dev/sdf1(1)
  [my_lv_rimage_2]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sde1(256)
  [my_lv_rmeta_1]         /dev/sdf1(0)
  [my_lv_rmeta_2]         /dev/sdg1(0)

When you add an image to a RAID1 array, you can specify which physical volumes to use for the image. The following command converts the 2-way RAID1 device my_vg/my_lv to a 3-way RAID1 device, specifying that the physical volume /dev/sdd1 be used for the array:

# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv             56.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# lvconvert -m 2 my_vg/my_lv /dev/sdd1
# 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_2(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rimage_2]        /dev/sdd1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
  [my_lv_rmeta_2]         /dev/sdd1(0)

To remove images from a RAID1 array, use the following command. When you remove images from a RAID1 device with the lvconvert command, you can specify the total number of images for the resulting device, or you can specify how many images to remove from the device. You can also optionally specify the physical volumes from which to remove the device.

lvconvert -m new_absolute_count vg/lv [removable_PVs]
lvconvert -m -num_fewer_images vg/lv [removable_PVs]

Additionally, when an image and its associated metadata subvolume volume are removed, any higher-numbered images will be shifted down to fill the slot. If you remove lv_rimage_1 from a 3-way RAID1 array that consists of lv_rimage_0, lv_rimage_1, and lv_rimage_2, this 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.

The following example shows the layout of a 3-way RAID1 logical volume my_vg/my_lv.

# 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/sde1(1)
  [my_lv_rimage_1]        /dev/sdf1(1)
  [my_lv_rimage_2]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sde1(0)
  [my_lv_rmeta_1]         /dev/sdf1(0)
  [my_lv_rmeta_2]         /dev/sdg1(0)

The following command converts the 3-way RAID1 logical volume into a 2-way RAID1 logical volume.

# lvconvert -m1 my_vg/my_lv
# 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)

The following command converts the 3-way RAID1 logical volume into a 2-way RAID1 logical volume, specifying the physical volume that contains the image to remove as /dev/sde1.

# lvconvert -m1 my_vg/my_lv /dev/sde1
# 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/sdf1(1)
  [my_lv_rimage_1]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sdf1(0)
  [my_lv_rmeta_1]         /dev/sdg1(0)

9.9. 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.

The format of the command to split off a RAID image is as follows:

lvconvert --splitmirrors count -n splitname vg/lv [removable_PVs]

Just as when you are removing a RAID image from an existing RAID1 logical volume, when you remove 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.

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

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

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

# 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/sde1(1)
  [my_lv_rimage_1]        /dev/sdf1(1)
  [my_lv_rimage_2]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sde1(0)
  [my_lv_rmeta_1]         /dev/sdf1(0)
  [my_lv_rmeta_2]         /dev/sdg1(0)
# lvconvert --splitmirror 1 -n new my_vg/my_lv
# 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)
  new                     /dev/sdg1(1)

9.10. Splitting and Merging a RAID Image

You can temporarily split off an image of a RAID1 array for read-only use while keeping track of any changes by using the --trackchanges argument in conjunction with the --splitmirrors argument of the lvconvert command. This allows you to merge the image back into the array at a later time while resyncing only those portions of the array that have changed since the image was split.

The format for the lvconvert command to split off a RAID image is as follows.

lvconvert --splitmirrors count --trackchanges vg/lv [removable_PVs]

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 with the --trackchanges argument specified by executing a subsequent lvconvert command with the --merge argument. When you merge the image, only the portions of the array that have changed since the image was split are resynced.

The format for the lvconvert command to merge a RAID image is as follows.

lvconvert --merge raid_image

The following example creates a RAID1 logical volume and then splits off an image from that volume while tracking changes to the remaining array.

# lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
  Logical volume "my_lv" created
# 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)
# 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
# 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)

The following example splits off an image from a RAID1 volume while tracking changes to the remaining array, then merges the volume back into the array.

# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  lv_rimage_1 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_1' to merge back into my_lv
# 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)
# lvconvert --merge my_vg/my_lv_rimage_1
  my_vg/my_lv_rimage_1 successfully merged back into my_vg/my_lv
# 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)

9.11. Setting a RAID fault policy

LVM RAID handles device failures in an automatic fashion based on the preferences defined by the raid_fault_policy field in the lvm.conf file.

  • If the raid_fault_policy field is set to allocate, the system will attempt to replace the failed device with a spare device from the volume group. If there is no available spare device, this will be reported to the system log.
  • If the raid_fault_policy field is set to warn, the system will produce a warning and the log will indicate that a device has failed. This allows the user to determine the course of action to take.

As long as there are enough devices remaining to support usability, the RAID logical volume will continue to operate.

9.11.1. The allocate RAID Fault Policy

In the following example, the raid_fault_policy field has been set to allocate in the lvm.conf file. The RAID logical volume is laid out as follows.

# 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/sde1(1)
  [my_lv_rimage_1]        /dev/sdf1(1)
  [my_lv_rimage_2]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sde1(0)
  [my_lv_rmeta_1]         /dev/sdf1(0)
  [my_lv_rmeta_2]         /dev/sdg1(0)

If the /dev/sde device fails, the system log will display error messages.

# grep lvm /var/log/messages
Jan 17 15:57:18 bp-01 lvm[8599]: Device #0 of raid1 array, my_vg-my_lv, has failed.
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
250994294784: Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
250994376704: Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at 0:
Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
4096: Input/output error
Jan 17 15:57:19 bp-01 lvm[8599]: Couldn't find device with uuid
3lugiV-3eSP-AFAR-sdrP-H20O-wM2M-qdMANy.
Jan 17 15:57:27 bp-01 lvm[8599]: raid1 array, my_vg-my_lv, is not in-sync.
Jan 17 15:57:36 bp-01 lvm[8599]: raid1 array, my_vg-my_lv, is now in-sync.

Since the raid_fault_policy field has been set to allocate, the failed device is replaced with a new device from the volume group.

# lvs -a -o name,copy_percent,devices 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/sdf1(1)
  [lv_rimage_2]        /dev/sdg1(1)
  [lv_rmeta_0]         /dev/sdh1(0)
  [lv_rmeta_1]         /dev/sdf1(0)
  [lv_rmeta_2]         /dev/sdg1(0)

Note that even though the failed device has been replaced, the display still indicates that LVM could not find the failed device. This is because, although the failed device has been removed from the RAID logical volume, the failed device has not yet been removed from the volume group. To remove the failed device from the volume group, you can execute vgreduce --removemissing VG.

If the raid_fault_policy has been set to allocate but there are no spare devices, the allocation will fail, leaving the logical volume as it is. If the allocation fails, you have the option of fixing the drive, then initiating recovery of the failed device with the --refresh option of the lvchange command. Alternately, you can replace the failed device.

9.11.2. The warn RAID Fault Policy

In the following example, the raid_fault_policy field has been set to warn in the lvm.conf file. The RAID logical volume is laid out as follows.

# 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/sdh1(1)
  [my_lv_rimage_1]        /dev/sdf1(1)
  [my_lv_rimage_2]        /dev/sdg1(1)
  [my_lv_rmeta_0]         /dev/sdh1(0)
  [my_lv_rmeta_1]         /dev/sdf1(0)
  [my_lv_rmeta_2]         /dev/sdg1(0)

If the /dev/sdh device fails, the system log will display 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.

9.12. Replacing a RAID device in a logical volume

You can replace a RAID device in a logical volume with the lvconvert command.

  • If there has been no failure on a RAID device, use the --replace argument of the lvconvert command to replace the device.
  • If the RAID device has failed, use the --repair argument of the lvconvert command to replace the failed device.

9.12.1. Replacing a RAID device that has not failed

To replace a RAID device in a logical volume, use the --replace argument of the lvconvert command. Note that this command will not work if the RAID device has failed.

The format for the lvconvert --replace command is as follows.

lvconvert --replace dev_to_remove vg/lv [possible_replacements]

The following example creates a RAID1 logical volume and then replaces a device in that volume.

# lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
  Logical volume "my_lv" created
# 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)
# lvconvert --replace /dev/sdb2 my_vg/my_lv
# 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)

The following example creates a RAID1 logical volume and then replaces a device in that volume, specifying which physical volume to use for the replacement.

# lvcreate --type raid1 -m 1 -L 100 -n my_lv my_vg
  Logical volume "my_lv" created
# 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/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# pvs
  PV          VG       Fmt  Attr PSize    PFree
  /dev/sda1   my_vg    lvm2 a--  1020.00m  916.00m
  /dev/sdb1   my_vg    lvm2 a--  1020.00m  916.00m
  /dev/sdc1   my_vg    lvm2 a--  1020.00m 1020.00m
  /dev/sdd1   my_vg    lvm2 a--  1020.00m 1020.00m
# lvconvert --replace /dev/sdb1 my_vg/my_lv /dev/sdd1
# 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)

You can replace more than one RAID device at a time by specifying multiple replace arguments, as in the following example.

# lvcreate --type raid1 -m 2 -L 100 -n my_lv my_vg
  Logical volume "my_lv" created
# 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/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rimage_2]        /dev/sdc1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
  [my_lv_rmeta_2]         /dev/sdc1(0)
# lvconvert --replace /dev/sdb1 --replace /dev/sdc1 my_vg/my_lv
# 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)

9.12.2. Replacing a failed RAID device in a logical volume

RAID is not like traditional LVM mirroring. LVM mirroring required failed devices to be removed or the mirrored logical volume would hang. RAID arrays can keep on running with failed devices. In fact, for RAID types other than RAID1, removing a device would mean converting to a lower level RAID (for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0). Therefore, rather than removing a failed device unconditionally and potentially allocating a replacement, LVM allows you to replace a failed device in a RAID volume in a one-step solution by using the --repair argument of the lvconvert command.

In the following example, a RAID logical volume is laid out as follows.

# 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/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)

If the /dev/sdc device fails, the output of the lvs command is as follows.

# lvs -a -o 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)

Use the following commands to replace the failed device and display the logical volume.

# 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.
# lvs -a -o 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)

Note that even though the failed device has been replaced, the display still indicates that LVM could not find the failed device. This is because, although the failed device has been removed from the RAID logical volume, the failed device has not yet been removed from the volume group. To remove the failed device from the volume group, you can execute vgreduce --removemissing VG.

If the device failure is a transient failure or you are able to repair the device that failed, you can initiate recovery of the failed device with the --refresh option of the lvchange command.

The following command refreshes a logical volume.

# lvchange --refresh my_vg/my_lv

9.13. Checking data coherency in a RAID logical volume (RAID scrubbing)

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.

You initiate a RAID scrubbing operation with the --syncaction option of the lvchange command. You specify either a check or repair operation. A check operation goes over the array and records the number of discrepancies in the array but does not repair them. A repair operation corrects the discrepancies as it finds them.

The format of the command to scrub a RAID logical volume is as follows:

lvchange --syncaction {check|repair} vg/raid_lv
Note

The lvchange --syncaction repair vg/raid_lv operation does not perform the same function as the lvconvert --repair vg/raid_lv operation. The lvchange --syncaction repair operation initiates a background synchronization operation on the array, while the lvconvert --repair operation is designed to repair/replace failed devices in a mirror or RAID logical volume.

In support of the RAID scrubbing operation, the lvs command supports two new printable fields: raid_sync_action and raid_mismatch_count. These fields are not printed by default. To display these fields you specify them with the -o parameter of the lvs, as follows.

lvs -o +raid_sync_action,raid_mismatch_count vg/lv

The raid_sync_action field displays the current synchronization operation that the raid volume is performing. It can be one of the following values:

  • idle: All sync operations complete (doing nothing)
  • resync: Initializing an array or recovering after a machine failure
  • 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 operation.

The Cpy%Sync field of the lvs command now prints the progress of any of the raid_sync_action operations, including check and repair.

The lv_attr field of the lvs command output now provides additional indicators in support of the RAID scrubbing operation. Bit 9 of this field displays the health of the logical volume, and it now supports the following indicators.

  • (m)ismatches indicates that there are discrepancies in a RAID logical volume. This character is shown after a scrubbing operation has detected that portions of the RAID are not coherent.
  • (r)efresh indicates that a device in a RAID array has suffered a failure and the kernel regards it as failed, even though LVM can read the device label and considers the device to be operational. The logical volume should be (r)efreshed to notify the kernel that the device is now available, or the device should be (r)eplaced if it is suspected of having failed.

When you perform a RAID scrubbing operation, the background I/O required by the sync operations can crowd out other I/O operations to LVM devices, such as updates to volume group metadata. This can cause the other LVM operations to slow down. You can control the rate at which the RAID logical volume is scrubbed by implementing recovery throttling.

You control the rate at which sync operations are performed by setting the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvchange command. You specify these options as follows.

  • --maxrecoveryrate Rate[bBsSkKmMgG]

    Sets the maximum recovery rate for a RAID logical volume so that it will not crowd out nominal I/O operations. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed. 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. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed.

9.14. Converting a RAID level (RAID takeover)

LVM supports Raid takeover, which means converting a RAID logical volume from one RAID level to another (such as from RAID 5 to RAID 6). Changing the RAID level is usually done to increase or decrease resilience to device failures or to restripe logical volumes. You use the lvconvert for RAID takeover. For information on RAID takeover and for examples of using the lvconvert to convert a RAID logical volume, see the lvmraid(7) man page.

9.15. Changing attributes of a RAID volume (RAID reshape)

RAID reshaping means changing attributes of a RAID logical volume while keeping the same RAID level. Some attributes you can change include RAID layout, stripe size, and number of stripes. For information on RAID reshaping and examples of using the lvconvert command to reshape a RAID logical volume, see the lvmraid(7) man page.

9.16. Controlling 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 format for using these parameters is as follows.

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

    Marks a device in a RAID1 logical volume as write-mostly. All reads to these drives will be avoided unless necessary. Setting this parameter keeps the number of I/O operations to the drive to a minimum. By default, the write-mostly attribute is set to yes for the specified physical volume in the logical volume. It is possible to remove the write-mostly flag by appending :n to the physical volume or to toggle the value by specifying :t. The --writemostly argument can be specified more than one time in a single command, making it possible to toggle the write-mostly attributes for all the physical volumes in a logical volume at once.

  • --[raid]writebehind IOCount

    Specifies the maximum number of outstanding writes that are allowed to devices in a RAID1 logical volume that are marked as write-mostly. Once this value is exceeded, writes become synchronous, causing all writes to the constituent devices to complete before the array signals the write has completed. Setting the value to zero clears the preference and allows the system to choose the value arbitrarily.

9.17. Changing the region size on a RAID logical volume

When you create a RAID logical volume, the region size for the logical volume will be the value of the raid_region_size parameter in the /etc/lvm/lvm.conf file. You can override this default value with the -R option of the lvcreate command.

After you have created a RAID logical volume, you can change the region size of the volume with the -R option of the lvconvert command. The following example changes the region size of logical volume vg/raidlv to 4096K. The RAID volume must be synced in order to change the region size.

# lvconvert -R 4096K vg/raid1
Do you really want to change the region_size 512.00 KiB of LV vg/raid1 to 4.00 MiB? [y/n]: y
  Changed region size on RAID LV vg/raid1 to 4.00 MiB.

Chapter 10. Snapshot logical volumes

The LVM snapshot feature provides the ability to create virtual images of a device at a particular instant without causing a service interruption.

10.1. 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.

Note

LVM supports thinly-provisioned snapshots.

Because 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.

Note

Snapshot copies of a file system are virtual copies, not an actual media backup for a file system. Snapshots do not provide a substitute for a backup procedure.

The size of the snapshot governs the amount of space set aside for storing the changes to the origin volume. For example, if you made a snapshot and then completely overwrote the origin the snapshot would have to be at least as big as the origin volume to hold the changes. You need to dimension a snapshot according to the expected level of change. So 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 that sees a greater number of writes, such as /home.

If a snapshot runs full, the snapshot becomes invalid, since it can no longer track changes on the origin volume. You should regularly monitor the size of the snapshot. Snapshots are fully resizable, however, so if you have the storage capacity you can increase the size of the snapshot volume to prevent it from getting dropped. Conversely, if you find that 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.

When you create a snapshot file system, full read and write access to the origin stays possible. If a chunk on a snapshot is changed, that chunk is marked and never gets copied from the original volume.

There are several uses for the snapshot feature:

  • Most typically, a snapshot is taken 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 whether the original file system requires file system repair.
  • Because the snapshot is read/write, you can test applications against production data by taking a snapshot and running tests against the snapshot, leaving the real data untouched.
  • You can create LVM volumes for use with Red Hat Virtualization. LVM snapshots can be used 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.

You can use the --merge option of the lvconvert command to merge a snapshot into its origin volume. One use for this feature is to perform system rollback if you have lost data or files or otherwise need to restore your system to a previous state. After you merge the snapshot volume, the resulting logical volume will have the origin volume’s name, minor number, and UUID and the merged snapshot is removed.

10.2. Creating snapshot volumes

Use the -s argument of the lvcreate command to create a snapshot volume. A snapshot volume is writable.

Note

LVM snapshots are not supported across the nodes in a cluster. 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.

Note

Snapshots are supported for RAID logical volumes.

LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. If you specify a snapshot volume that is larger than this, the system will create a snapshot volume that is only as large as will be needed for the size of the origin.

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

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

  1. Create a logical volume named origin from the volume group VG.

    # lvcreate -L 1G -n origin VG
      Logical volume "origin" created.
  2. Create a snapshot logical volume of /dev/VG/origin that is 100 MB in size named snap. If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in order to access the contents of the file system to run a backup while the original file system continues to get updated.

    # lvcreate --size 100M --snapshot --name snap /dev/VG/origin
      Logical volume "snap" created.
  3. Display the status of logical volume /dev/VG/origin, showing all snapshot logical volumes and their status (active or inactive).

    # lvdisplay /dev/VG/origin
      --- Logical volume ---
      LV Path                /dev/VG/origin
      LV Name                origin
      VG Name                VG
      LV UUID                EsFoBp-CB9H-Epl5-pUO4-Yevi-EdFS-xtFnaF
      LV Write Access        read/write
      LV Creation host, time host-083.virt.lab.msp.redhat.com, 2019-04-11 14:45:06 -0500
      LV snapshot status     source of
                             snap [active]
      LV Status              available
      # open                 0
      LV Size                1.00 GiB
      Current LE             256
      Segments               1
      Allocation             inherit
      Read ahead sectors     auto
      - currently set to     8192
      Block device           253:6
  4. The lvs command, by default, displays the origin volume and the current percentage of the snapshot volume being used. The following example shows the default output for the lvs command after you have created the snapshot volume, with a display that includes the devices that constitute the logical volumes.
# lvs -a -o +devices
  LV              VG            Attr       LSize   Pool   Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
  origin          VG            owi-a-s---   1.00g                                                       /dev/sde1(0)
  snap            VG            swi-a-s--- 100.00m        origin 0.00                                    /dev/sde1(256)
Warning

Because the snapshot increases in size as the origin volume changes, it is important to monitor the percentage of the snapshot volume regularly with the lvs command to be sure it does not fill. A snapshot that is 100% full is lost completely, as a write to unchanged parts of the origin would be unable to succeed without corrupting the snapshot.

In addition to the snapshot itself being invalidated when full, any mounted file systems on that snapshot device are forcibly unmounted, avoiding the inevitable file system errors upon access to the mount point. In addition, you can specify the snapshot_autoextend_threshold option in the lvm.conf file. This option allows automatic extension of a snapshot whenever the remaining snapshot space drops below the threshold you set. This feature requires that there be unallocated space in the volume group.

LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. Similarly, automatic extension of a snapshot will 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.

Information on setting snapshot_autoextend_threshold and snapshot_autoextend_percent is provided in the /etc/lvm/lvm.conf file itself.

10.3. Merging snapshot volumes

You can use the --merge option of the lvconvert command to merge a snapshot into its origin volume. If both the origin and snapshot volume are not open, the merge will start immediately. Otherwise, the merge will start the first time either the origin or snapshot are activated and both are closed. Merging a snapshot into an origin that cannot be closed, for example a root file system, is deferred until the next time the origin volume is activated. When merging starts, the resulting logical volume will have the origin’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.

The following command merges snapshot volume vg00/lvol1_snap into its origin.

# lvconvert --merge vg00/lvol1_snap

You can specify multiple snapshots on the command line, or you can use LVM object tags to specify that multiple snapshots be merged to their respective origins. In the following example, logical volumes vg00/lvol1, vg00/lvol2, and vg00/lvol3 are all tagged with the tag @some_tag. The following command merges the snapshot logical volumes for all three volumes serially: vg00/lvol1, then vg00/lvol2, then vg00/lvol3. If the --background option were used, all snapshot logical volume merges would start in parallel.

# lvconvert --merge @some_tag

For further information on the lvconvert --merge command, see the lvconvert(8) man page.

Chapter 11. Creating and managing thinly-provisioned logical volumes (thin volumes)

Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available extents.

11.1. Thinly-provisioned logical volumes (thin volumes)

Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available extents. Using thin provisioning, you can manage a storage pool of free space, known as a thin pool, which can be allocated to an arbitrary number of devices when needed by applications. You can then create devices that can be bound to the thin pool for later allocation when an application actually writes to the logical volume. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space.

Note

Thin volumes are not supported across the nodes in a cluster. The thin pool and all its thin volumes must be exclusively activated on only one cluster node.

By using thin provisioning, a storage administrator can overcommit the physical storage, often avoiding the need to purchase additional storage. For example, if ten users each request a 100GB file system for their application, the storage administrator 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. 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.

To make sure that all available space can be used, LVM supports data discard. This allows for re-use of the space that was formerly used by a discarded file or other block range.

Thin volumes provide support for a new implementation of copy-on-write (COW) snapshot logical volumes, which allow many virtual devices to share the same data in the thin pool.

11.2. Creating thinly-provisioned logical volumes

This procedure provides an overview of the basic commands you use to create and grow thinly-provisioned logical volumes. For detailed information on LVM thin provisioning as well as information on using the LVM commands and utilities with thinly-provisioned logical volumes, see the lvmthin(7) man page.

To create a thin volume, perform the following tasks:

  1. Create a volume group with the vgcreate command.
  2. Create a thin pool with the lvcreate command.
  3. Create a thin volume in the thin pool with the lvcreate command.

You can use the -T (or --thin) option of the lvcreate command to create either a thin pool or a thin volume. You can also use -T option of the lvcreate command to create both a thin pool and a thin volume in that pool at the same time with a single command.

The following command uses the -T option of the lvcreate command to create a thin pool named mythinpool in the volume group vg001 and that is 100M in size. 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 deduces what type of device is to be created from the other options the command specifies.

# 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.
# lvs
  LV         VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  mythinpool vg001 twi-a-tz-- 100.00m             0.00   10.84

The following command uses the -T option of the lvcreate command to create a thin volume named thinvolume in the thin pool vg001/mythinpool. Note that in this case you are specifying virtual size, and that you are specifying a virtual size for the volume that is greater than the pool that contains it.

# 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.
# lvs
  LV          VG       Attr     LSize   Pool       Origin Data%  Move Log Copy%  Convert
  mythinpool  vg001    twi-a-tz 100.00m                     0.00
  thinvolume  vg001    Vwi-a-tz   1.00g mythinpool          0.00

The following command uses the -T option of the lvcreate command to create a thin pool and a thin volume in that pool by specifying both a size and a virtual size argument for the lvcreate command. This command creates a thin pool named mythinpool in the volume group vg001 and it also creates a thin volume named thinvolume in that pool.

# 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.
# lvs
  LV         VG    Attr       LSize   Pool       Origin Data%  Meta%  Move Log Cpy%Sync Convert
  mythinpool vg001 twi-aotz-- 100.00m                   0.00   10.94
  thinvolume vg001 Vwi-a-tz--   1.00g mythinpool        0.00

You can also create a thin pool by specifying the --thinpool parameter of the lvcreate command. Unlike the -T option, the --thinpool parameter requires an argument, which is the name of the thin pool logical volume that you are creating. The following example specifies the --thinpool parameter of the lvcreate command to create a thin pool named mythinpool in the volume group vg001 and 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.
# lvs
  LV         VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  mythinpool vg001 twi-a-tz-- 100.00m             0.00   10.84

Striping is supported for pool creation. The following command creates a 100M thin pool named pool in volume group vg001 with two 64 kB stripes and a chunk size of 256 kB. It also creates a 1T thin volume, vg00/thin_lv.

# lvcreate -i 2 -I 64 -c 256 -L 100M -T vg00/pool -V 1T --name thin_lv

You can extend the size of a thin volume with the lvextend command. You cannot, however, reduce the size of a thin pool.

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

# lvextend -L+100M vg001/mythinpool
  Extending logical volume mythinpool to 200.00 MiB
  Logical volume mythinpool successfully resized
# lvs
  LV           VG       Attr     LSize   Pool     Origin Data%  Move Log Copy%  Convert
  mythinpool   vg001    twi-a-tz 200.00m                   0.00
  thinvolume   vg001    Vwi-a-tz   1.00g mythinpool          0.00

As with other types of logical volumes, you can rename the volume with the lvrename, you can remove the volume with the lvremove, and you can display information about the volume with the lvs and lvdisplay commands.

By default, the lvcreate command sets the size of the thin pool’s metadata logical volume according to the formula (Pool_LV_size / Pool_LV_chunk_size * 64). If you will have large numbers of snapshots or if you have have small chunk sizes for your thin pool and thus 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 with 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.

You can use the --thinpool parameter of the lvconvert command to convert an existing logical volume to a thin pool volume. When you convert an existing logical volume to a thin pool volume, you must use the --poolmetadata parameter in conjunction with the --thinpool parameter of the lvconvert to convert an existing logical volume to the thin pool volume’s metadata volume.

Note

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

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.

11.3. Thinly-provisioned snapshot volumes

Red Hat Enterprise Linux provides support for thinly-provisioned snapshot volumes. Thin snapshot volumes allow many virtual devices to be stored on the same data volume. This simplifies administration and allows for the sharing of data between snapshot volumes.

As for all LVM snapshot volumes, as well as 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.

Thin snapshot volumes provide the following benefits:

  • A thin snapshot volume can reduce disk usage when there are multiple snapshots of the same origin volume.
  • If there are multiple snapshots of the same origin, then a write to the origin will cause one COW operation to preserve the data. Increasing the number of snapshots of the origin should yield no major slowdown.
  • Thin snapshot volumes can be used as a logical volume origin for another snapshot. This allows for an arbitrary depth of recursive snapshots (snapshots of snapshots of snapshots…​).
  • A snapshot of a thin logical volume also creates a thin logical volume. This consumes no data space until a COW operation is required, or until the snapshot itself is written.
  • A thin snapshot volume does not need to be activated with its origin, so a user may have only the origin active while there are many inactive snapshot volumes of the origin.
  • When you delete the origin of a thinly-provisioned snapshot volume, each snapshot of that origin volume becomes an independent thinly-provisioned volume. This means that instead of merging a snapshot with its origin volume, you may choose to delete the origin volume and then create a new thinly-provisioned snapshot using that independent volume as the origin volume for the new snapshot.

Although there are many advantages to using thin snapshot volumes, there are some use cases for which the older LVM snapshot volume feature may be more appropriate to your needs:

  • You cannot change the chunk size of a thin pool. If the thin pool has a large chunk size (for example, 1MB) and you require a short-living snapshot for which a chunk size that large is not efficient, you may elect to use the older snapshot feature.
  • You cannot limit the size of a thin snapshot volume; the snapshot will use all of the space in the thin pool, if necessary. This may not be appropriate for your needs.

In general, you should consider the specific requirements of your site when deciding which snapshot format to use.

11.4. Creating thinly-provisioned snapshot volumes

Red Hat Enterprise Linux provides support for thinly-provisioned snapshot volumes.

Note

This section provides an overview of the basic commands you use to create and grow thinly-provisioned snapshot volumes. For detailed information on LVM thin provisioning as well as information on using the LVM commands and utilities with thinly-provisioned logical volumes, see the lvmthin(7) man page.

Important

When creating a thin snapshot volume, you 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.

You can specify a name for the snapshot volume with the --name option of the lvcreate command. The following command creates a thinly-provisioned snapshot volume of the thinly-provisioned logical volume vg001/thinvolume that is named mysnapshot1.

# 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

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.

By default, a snapshot volume is skipped during normal activation commands. For information on controlling the activation of a logical volume, see Logical volume activation.

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.

To create a thinly-provisioned snapshot of an external origin, you must specify the --thinpool option. The following command creates a thin snapshot volume of the read-only inactive volume 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 will use the existing thin pool vg001/pool. Because the origin volume must be in the same volume group as the snapshot volume, you do not need to 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, as in the following command.

# lvcreate -s vg001/mythinsnap --name my2ndthinsnap

You can display a list of all ancestors and descendants of a thin snapshot logical volume by specifying the lv_ancestors and lv_descendants reporting fields of the lvs command.

In the following example:

  • stack1 is an origin volume in volume group vg001.
  • stack2 is a snapshot of stack1
  • stack3 is a snapshot of stack2
  • stack4 is a snapshot of stack3

Additionally:

  • stack5 is also a snapshot of stack2
  • stack6 is a snapshot of stack5
$ lvs -o name,lv_ancestors,lv_descendants vg001
  LV      Ancestors              Descendants
  stack1                         stack2,stack3,stack4,stack5,stack6
  stack2  stack1                 stack3,stack4,stack5,stack6
  stack3  stack2,stack1          stack4
  stack4  stack3,stack2,stack1
  stack5  stack2,stack1          stack6
  stack6  stack5,stack2,stack1
  pool
Note

The lv_ancestors and lv_descendants fields display existing dependencies but do not track removed entries which can break a dependency chain if the entry was removed from the middle of the chain. For example, if you remove the logical volume stack3 from this sample configuration, the display is as follows.

$ lvs -o name,lv_ancestors,lv_descendants vg001
  LV      Ancestors              Descendants
  stack1                         stack2,stack5,stack6
  stack2  stack1                 stack5,stack6
  stack4
  stack5  stack2,stack1          stack6
  stack6  stack5,stack2,stack1
  pool

You can configure your system to track and display logical volumes that have been removed, and you can display the full dependency chain that includes those volumes by specifying the lv_ancestors_full and lv_descendants_full fields.

11.5. Tracking and displaying thin snapshot volumes that have been removed

You can configure your 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 your 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 logical volume that has been removed, 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.

When a volume is removed, the historical logical volume name acquires a hypen as a prefix. For example, when you remove the logical volume lvol1, the name of the historical volume is -lvol1. A historical logical volume cannot be reactivated.

Even when the record_lvs_history metadata option enabled, you can prevent the retention of historical logical volumes on an individual basis when you remove a logical volume by specifying the --nohistory option of the lvremove command.

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.

The following series of commands provides examples of how you can display and manage historical logical volumes.

  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. Enter the following command 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.

      Note that even though the example lvs display command includes the -H option, no thin snapshot volume has yet been removed and there are no historical logical volumes to display.

      # 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
  3. Remove logical volume lvol3 from the snapshot chain, then run the following lvs command again to see how historical logical volumes are displayed, along with their ancestors and descendants.

    # lvremove -f vg/lvol3
      Logical volume "lvol3" successfully removed
    # 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
  4. You can use the lv_time_removed reporting field to display the time a historical volume was removed.

    # 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
  5. You can reference historical logical volumes individually in a display command by specifying the vgname/lvname format, as in the following example. Note that the fifth bit in the lv_attr field is set to h to indicate the volume is a historical volume.

    # lvs -H vg/-lvol3
      LV     VG   Attr       LSize
      -lvol3 vg   ----h-----    0
  6. 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
  7. Run the following commands to 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
  8. To remove a historical logical volume completely, you can run the lvremove command again, 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
  9. A historical logical volumes is retained as long as there is a chain that includes live volumes in its descendants. This means that removing a historical logical volume also removes all of the logical volumes in the chain if no existing descendant is linked to them, as shown in the following example.

    # lvremove -f vg/lvol4
      Automatically removing historical logical volume vg/-lvol1.
      Automatically removing historical logical volume vg/-lvol2.
      Automatically removing historical logical volume vg/-lvol4.
      Logical volume "lvol4" successfully removed

Chapter 12. LVM cache logical volumes

LVM provides full support for LVM cache logical volumes. A cache logical volume uses a small logical volume consisting of fast block devices (such as SSD drives) to improve the performance of a larger and slower logical volume by storing the frequently used blocks on the smaller, faster logical volume.

12.1. Cache volume types

LVM caching uses the following LVM logical volume types. All of these associated logical volumes must be in the same volume group.

  • Origin logical volume — the large, slow logical volume
  • Cache pool logical volume — the small, fast logical volume, which is composed of two devices: the cache data logical volume, and the cache metadata logical volume
  • Cache data logical volume — the logical volume containing the data blocks for the cache pool logical volume
  • Cache metadata logical volume — the logical volume containing the metadata for the cache pool logical volume, which holds the accounting information that specifies where data blocks are stored (for example, on the origin logical volume or the cache data logical volume).
  • Cache logical volume — the logical volume containing the origin logical volume and the cache pool logical volume. This is the resultant usable device which encapsulates the various cache volume components.

12.2. Creating an LVM cache logical volume

The following procedure creates an LVM cache logical volume.

  1. Create a volume group that contains a slow physical volume and a fast physical volume. In this example. /dev/sde1 is a slow device and /dev/sdf1 is a fast device and both devices are contained in volume group VG.

    # pvcreate /dev/sde1
    # pvcreate /dev/sdf1
    # vgcreate VG /dev/sde1 /dev/sdf1
  2. Create the origin volume. This example creates an origin volume named lv that is ten gigabytes in size and that consists of /dev/sde1, the slow physical volume.

    # lvcreate -L 10G -n lv VG /dev/sde1
  3. Create the cache pool logical volume. This example creates the cache pool logical volume named cpool on the fast device /dev/sdf1, which is part of the volume group VG. The cache pool logical volume this command creates consists of the hidden cache data logical volume cpool_cdata and the hidden cache metadata logical volume cpool_cmeta.

    # lvcreate --type cache-pool -L 5G -n cpool VG /dev/sdf1
      Using default stripesize 64.00 KiB.
      Logical volume "cpool" created.
    # lvs -a -o name,size,attr,devices VG
      LV              LSize  Attr       Devices
      cpool            5.00g Cwi---C--- cpool_cdata(0)
      [cpool_cdata]    5.00g Cwi------- /dev/sdf1(4)
      [cpool_cmeta]    8.00m ewi------- /dev/sdf1(2)
      lv              10.00g -wi-a----- /dev/sde1(0)
      [lvol0_pmspare]  8.00m ewi------- /dev/sdf1(0)

    For more complicated configurations you may need to create the cache data and the cache metadata logical volumes individually and then combine the volumes into a cache pool logical volume. For information on this procedure, see the lvmcache(7) man page.

  4. Create the cache logical volume by linking the cache pool logical volume to the origin logical volume. The resulting user-accessible cache logical volume takes the name of the origin logical volume. The origin logical volume becomes a hidden logical volume with _corig appended to the original name. Note that this conversion can be done live, although you must ensure you have performed a backup first.

    # lvconvert --type cache --cachepool cpool VG/lv
      Logical volume cpool is now cached.
    # lvs -a -o name,size,attr,devices VG
        LV              LSize  Attr       Devices
        [cpool]          5.00g Cwi---C--- cpool_cdata(0)
        [cpool_cdata]    5.00g Cwi-ao---- /dev/sdf1(4)
        [cpool_cmeta]    8.00m ewi-ao---- /dev/sdf1(2)
        lv              10.00g Cwi-a-C--- lv_corig(0)
        [lv_corig]      10.00g owi-aoC--- /dev/sde1(0)
        [lvol0_pmspare]  8.00m ewi------- /dev/sdf1(0)
  5. Optionally, you can convert the cached logical volume to a thin pool logical volume. Note that any thin logical volumes created from the pool will share the cache.

    The following command uses the fast device, /dev/sdf1, for allocating the thin pool metadata (lv_tmeta). This is the same device that is used by the cache pool volume, which means that the thin pool metadata volume shares that device with both the cache data logical volume cpool_cdata and the cache metadata logical volume cpool_cmeta.

    # lvconvert --type thin-pool VG/lv /dev/sdf1
       WARNING: Converting logical volume VG/lv to thin pool's data volume with metadata wiping.
       THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
     Do you really want to convert VG/lv? [y/n]: y
       Converted VG/lv to thin pool.
    # lvs -a -o name,size,attr,devices vg
       LV               LSize  Attr       Devices
       [cpool]           5.00g Cwi---C--- cpool_cdata(0)
       [cpool_cdata]     5.00g Cwi-ao---- /dev/sdf1(4)
       [cpool_cmeta]     8.00m ewi-ao---- /dev/sdf1(2)
       lv               10.00g twi-a-tz-- lv_tdata(0)
       [lv_tdata]       10.00g Cwi-aoC--- lv_tdata_corig(0)
       [lv_tdata_corig] 10.00g owi-aoC--- /dev/sde1(0)
       [lv_tmeta]       12.00m ewi-ao---- /dev/sdf1(1284)
       [lvol0_pmspare]  12.00m ewi------- /dev/sdf1(0)
       [lvol0_pmspare]  12.00m ewi------- /dev/sdf1(1287)

For further information on LVM cache volumes, including additional administrative examples, see the lvmcache(7) man page.

Chapter 13. Logical volume activation

A logical volume that is an active state can be used through a block device. A logical volume that is activated is accessible and is subject to change. When you create a logical volume it is activated by default.

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

The format for the command to deactivate an individual logical volume is as follows.

lvchange -an vg/lv

The format for the command to activate an individual logical volume is as follows.

lvchange -ay vg/lv

You can and 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 format for the command to deactivate all of the logical volumes in a volume group is as follows.

vgchange -an vg

The format for the command to activate all of the logical volumes in a volume group is as follows.

vgchange -ay vg

13.1. Controlling autoactivation of logical volumes

Autoactivation of a logical volume refers to the event-based automatic activation of a logical volume during system startup. As devices become available on the system (device online events), systemd/udev runs the lvm2-pvscan service for each device. This service runs the pvscan --cache -aay device command, which reads the named device. If the device belongs to a volume group, the pvscan command will check if all of the physical volumes for that volume group are present on the system. If so, the command will activate logical volumes in that volume group.

You can use the following configuration options in the /etc/lvm/lvm.conf configuration file to control autoactivation of logical volumes.

  • global/event_activation

    When event_activation is disabled, systemd/udev will autoactivate logical volume only on whichever physical volumes are present during system startup. If all physical volumes have not appeared yet, then some logical volumes may not be autoactivated.

  • activation/auto_activation_volume_list

    Setting auto_activation_volume_list to an empty list disables autoactivation entirely. Setting auto_activation_volume_list to specific logical volumes and volume groups limits autoactivation to those logical volumes.

For information on setting these options, see the /etc/lvm/lvm.conf configuration file.

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 on 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.

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

  • You can turn off the activation skip flag when creating a logical volume by specifying the -kn or --setactivationskip n option of the lvcreate command.
  • You can turn off the activation skip flag for an existing logical volume by specifying the -kn or --setactivationskip n option of the lvchange command.
  • You can turn on the activation skip flag on again for a volume where it has been turned off with the -ky or --setactivationskip y option of the lvchange command.

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 --type thin -n SnapLV -kn -s ThinLV --thinpool VG/ThinPoolLV

The following command removes the activation skip flag from a snapshot logical volume.

# lvchange -kn VG/SnapLV

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|e

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 configure which logical volumes with missing devices are activated by setting the activation_mode parameter with the lvchange command to one of the following values.

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 activation_mode is determined by the activation_mode setting in the /etc/lvm/lvm.conf file. For further information, see the lvmraid(7) man page.

Chapter 14. Controlling LVM device scanning

You can control LVM device scanning by configuring filters in the /etc/lvm/lvm.conf file. The filters in the lvm.conf file consist of a series of simple regular expressions that get applied to the device names in the /dev directory to decide whether to accept or reject each block device found.

14.1. Configuring filters to control device scanning

The following examples show the use of filters to control which devices LVM scans. Note that some of these examples do not necessarily represent recommended practice, as the regular expressions are matched freely against the complete pathname. For example, a/loop/ is equivalent to a/.*loop.*/ and would match /dev/solooperation/lvol1.

The following filter adds all discovered devices, which is the default behavior as there is no filter configured in the configuration file:

filter = [ "a/.*/" ]

The following filter removes the cdrom device in order to avoid delays if the drive contains no media:

filter = [ "r|/dev/cdrom|" ]

The following filter adds all loop and removes all other block devices:

filter = [ "a/loop.*/", "r/.*/" ]

The following filter adds all loop and IDE and removes all other block devices:

filter =[ "a|loop.*|", "a|/dev/hd.*|", "r|.*|" ]

The following filter adds just partition 8 on the first IDE drive and removes all other block devices:

filter = [ "a|^/dev/hda8$|", "r/.*/" ]

14.2. Controlling whether LVM commands scan logical volumes

By default, LVM commands do not scan the logical volumes on your system. This default behavior provides the following advantages:

  • If there are many active logical volumes on the system, every LVM command would require additional time, negatively impacting performance and causing unwanted delays or timeouts.
  • If logical volumes contain physical volumes from a guest VM image, the host usually does not want to scan or use those layered physical volumes which belong to the guest. Note, however, that in the cases where a guest VM’s physical volume exists directly on an SCSI device visible to the host, in order to prevent LVM on the host from accessing those physical volumes you will need to configure a filter, as described in Configuring filters to control device scanning.

Scanning logical volumes may be necessary when layering physical volumes on top of logical volumes is intentional. This will allow the pvcreate command to be run on a logical volume. To configure LVM to scan all logical volumes, set the scan_lvs configuration option in the /etc/lvm/lvm.conf file to scan_lvs=1. To restrict which logical volumes LVM commands scan, you can then set up device filters in the /etc/lvm/lvm.conf configuration file, as described in Configuring filters to control device scanning.

Chapter 15. Controlling LVM allocation

By default, a volume group allocates physical extents according to common-sense rules such as not placing parallel stripes on the same physical volume. This is the normal allocation policy. You can use the --alloc argument of the vgcreate command to specify an allocation policy of contiguous, anywhere, or cling. 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. LVM allocation policies

When an LVM operation needs to allocate physical extents for one or more logical volumes, 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 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 logical volume or volume group. For each policy, working from the lowest-numbered logical extent of the empty logical volume 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:

  • An allocation policy of contiguous requires that the physical location of any logical extent that is not the first logical extent of a logical volume is adjacent to the physical location of the logical extent immediately preceding it.

    When a logical volume is striped or mirrored, the contiguous allocation restriction is applied independently to each stripe or mirror image (leg) that needs space.

  • An allocation policy of cling requires that the physical volume used for any logical extent be added to an existing logical volume that is already in use by at least one logical extent earlier in that logical volume. If the configuration parameter allocation/cling_tag_list is defined, then two physical volumes are considered to match if any of the listed tags is present on both physical volumes. This allows groups of physical volumes with similar properties (such as their physical location) to be tagged and treated as equivalent for allocation purposes.

    When a Logical Volume is striped or mirrored, the cling allocation restriction is applied independently to each stripe or mirror image (leg) that needs space.

  • An allocation policy of normal will not choose a physical extent that shares the same physical volume as a logical extent already allocated to a parallel logical volume (that is, a different stripe or mirror image/leg) at the same offset within that parallel logical volume.

    When allocating a mirror log at the same time as logical volumes to hold the mirror data, an allocation policy of normal will first try to select different physical volumes for the log and the data. If that is not possible and the allocation/mirror_logs_require_separate_pvs configuration parameter is set to 0, it will then allow the log to share physical volume(s) with part of the data.

    Similarly, when allocating thin pool metadata, an allocation policy of normal will follow the same considerations as for allocation of a mirror log, based on the value of the allocation/thin_pool_metadata_require_separate_pvs configuration parameter.

  • 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 physical volume.

The allocation policies can be changed using the vgchange command.

Note

If you rely upon any layout behavior beyond that documented in this section according to the defined allocation policies, you should note that this might change in future versions of the code. 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 it is important to obtain a specific layout for a particular Logical Volume, then you should 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.

To view the way the allocation process currently works in any specific case, you can read the debug logging output, for example by adding the -vvvv option to a command.

15.2. 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 may be necessary if there are disk errors, or if you will be removing the physical volume.

The following command disallows the allocation of physical extents on /dev/sdk1.

# pvchange -x n /dev/sdk1

You can also use the -xy arguments of the pvchange command to allow allocation where it had previously been disallowed.

15.3. Extending a logical volume with the cling allocation policy

When extending an LVM volume, you can use the --alloc cling option of the lvextend command to specify the cling allocation policy. This policy will choose space on the same physical volumes as the last segment of the existing logical volume. If there is insufficient space on the physical volumes and a list of tags is defined in the /etc/lvm/lvm.conf file, LVM will check whether any of the tags are attached to the physical volumes and seek to match those physical volume tags between existing extents and new extents.

For example, if you have logical volumes that are mirrored between two sites within a single volume group, you can tag the physical volumes according to where they are situated by tagging the physical volumes with @site1 and @site2 tags. You can then specify the following line in the lvm.conf file:

cling_tag_list = [ "@site1", "@site2" ]

In the following example, the lvm.conf file has been modified to contain the following line:

cling_tag_list = [ "@A", "@B" ]

Also in this example, a volume group taft has been created that consists of the physical volumes /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1, /dev/sdf1, /dev/sdg1, and /dev/sdh1. These physical volumes have been tagged with tags A, B, and C. The example does not use the C tag, but this will show that LVM uses the tags to select which physical volumes to use for the mirror legs.

# pvs -a -o +pv_tags /dev/sd[bcdefgh]
  PV         VG   Fmt  Attr PSize  PFree  PV Tags
  /dev/sdb1  taft lvm2 a--  15.00g 15.00g A
  /dev/sdc1  taft lvm2 a--  15.00g 15.00g B
  /dev/sdd1  taft lvm2 a--  15.00g 15.00g B
  /dev/sde1  taft lvm2 a--  15.00g 15.00g C
  /dev/sdf1  taft lvm2 a--  15.00g 15.00g C
  /dev/sdg1  taft lvm2 a--  15.00g 15.00g A
  /dev/sdh1  taft lvm2 a--  15.00g 15.00g A

The following command creates a 10 gigabyte mirrored volume from the volume group taft.

# lvcreate --type raid1 -m 1 -n mirror --nosync -L 10G taft
  WARNING: New raid1 won't be synchronised. Don't read what you didn't write!
  Logical volume "mirror" created

The following command shows which devices are used for the mirror legs and RAID metadata subvolumes.

# lvs -a -o +devices
  LV                VG   Attr       LSize  Log Cpy%Sync Devices
  mirror            taft Rwi-a-r--- 10.00g       100.00 mirror_rimage_0(0),mirror_rimage_1(0)
  [mirror_rimage_0] taft iwi-aor--- 10.00g              /dev/sdb1(1)
  [mirror_rimage_1] taft iwi-aor--- 10.00g              /dev/sdc1(1)
  [mirror_rmeta_0]  taft ewi-aor---  4.00m              /dev/sdb1(0)
  [mirror_rmeta_1]  taft ewi-aor---  4.00m              /dev/sdc1(0)

The following command extends the size of the mirrored volume, using the cling allocation policy to indicate that the mirror legs should be extended using physical volumes with the same tag.

# lvextend --alloc cling -L +10G taft/mirror
  Extending 2 mirror images.
  Extending logical volume mirror to 20.00 GiB
  Logical volume mirror successfully resized

The following display command shows that the mirror legs have been extended using physical volumes with the same tag as the leg. Note that the physical volumes with a tag of C were ignored.

# lvs -a -o +devices
  LV                VG   Attr       LSize  Log Cpy%Sync Devices
  mirror            taft Rwi-a-r--- 20.00g       100.00 mirror_rimage_0(0),mirror_rimage_1(0)
  [mirror_rimage_0] taft iwi-aor--- 20.00g              /dev/sdb1(1)
  [mirror_rimage_0] taft iwi-aor--- 20.00g              /dev/sdg1(0)
  [mirror_rimage_1] taft iwi-aor--- 20.00g              /dev/sdc1(1)
  [mirror_rimage_1] taft iwi-aor--- 20.00g              /dev/sdd1(0)
  [mirror_rmeta_0]  taft ewi-aor---  4.00m              /dev/sdb1(0)
  [mirror_rmeta_1]  taft ewi-aor---  4.00m              /dev/sdc1(0)

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