Deduplicating and compressing logical volumes on RHEL

Red Hat Enterprise Linux 9.0 Beta

Using VDO to increase LVM storage capacity

Red Hat Customer Content Services

Abstract

This document explains how to use the Virtual Data Optimizer (VDO) feature in LVM to manage deduplicated and compressed logical volumes on RHEL.

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Chapter 1. Introduction to VDO on LVM

The Virtual Data Optimizer (VDO) feature provides inline block-level deduplication, compression, and thin provisioning for storage. You can manage VDO as a type of LVM logical volumes (LVs), similar to LVM thinly provisioned volumes.

VDO volumes on LVM (LVM-VDO) are composed of the following LVs:

VDO pool LV

This is the backing physical device that stores, deduplicates, and compresses data for the VDO LV. The VDO pool LV sets the physical size of the VDO volume, which is the amount of data that VDO can store on the disk.

Currently, each VDO pool LV can hold only one VDO LV. As a result, VDO deduplicated and compresses each VDO LV separately. In other words, VDO cannot deduplicate or compress a piece of data that is shared between several VDO LV.

VDO LV
This is the virtual, provisioned device on top of the VDO pool LV. The VDO LV sets the provisioned, logical size of the VDO volume, which is the amount of data that applications can write to the volume before deduplication and compression occurs.

Table 1.1. A comparison of components in VDO on LVM and LVM thin provisioning

 Physical deviceProvisioned device

VDO on LVM

VDO pool LV

VDO LV

LVM thin provisioning

Thin pool

Thin volume

Since VDO is thinly provisioned, the file system and applications only see the logical space in use and are not aware of the actual physical space available. Use scripting to monitor the actual available space and generate an alert if use exceeds a threshold: for example, when the VDO pool LV is 80% full.

Chapter 2. LVM-VDO requirements

VDO on LVM has certain requirements on its placement and your system resources.

2.1. VDO memory requirements

Each VDO volume has two distinct memory requirements:

The VDO module

VDO requires a fixed 38 MB of RAM and several variable amounts:

  • 1.15 MB of RAM for each 1 MB of configured block map cache size. The block map cache requires a minimum of 150MB RAM.
  • 1.6 MB of RAM for each 1 TB of logical space.
  • 268 MB of RAM for each 1 TB of physical storage managed by the volume.
The UDS index

The Universal Deduplication Service (UDS) requires a minimum of 250 MB of RAM, which is also the default amount that deduplication uses. You can configure the value when formatting a VDO volume, because the value also affects the amount of storage that the index needs.

The memory required for the UDS index is determined by the index type and the required size of the deduplication window:

Index typeDeduplication windowNote

Dense

1 TB per 1 GB of RAM

A 1 GB dense index is generally sufficient for up to 4 TB of physical storage.

Sparse

10 TB per 1 GB of RAM

A 1 GB sparse index is generally sufficient for up to 40 TB of physical storage.

The UDS Sparse Indexing feature is the recommended mode for VDO. It relies on the temporal locality of data and attempts to retain only the most relevant index entries in memory. With the sparse index, UDS can maintain a deduplication window that is ten times larger than with dense, while using the same amount of memory.

Although the sparse index provides the greatest coverage, the dense index provides more deduplication advice. For most workloads, given the same amount of memory, the difference in deduplication rates between dense and sparse indexes is negligible.

2.2. VDO storage space requirements

You can configure a VDO volume to use up to 256 TB of physical storage. Only a certain part of the physical storage is usable to store data. This section provides the calculations to determine the usable size of a VDO-managed volume.

VDO requires storage for two types of VDO metadata and for the UDS index:

  • The first type of VDO metadata uses approximately 1 MB for each 4 GB of physical storage plus an additional 1 MB per slab.
  • The second type of VDO metadata consumes approximately 1.25 MB for each 1 GB of logical storage, rounded up to the nearest slab.
  • The amount of storage required for the UDS index depends on the type of index and the amount of RAM allocated to the index. For each 1 GB of RAM, a dense UDS index uses 17 GB of storage, and a sparse UDS index will use 170 GB of storage.

2.3. Examples of VDO requirements by physical size

The following tables provide approximate system requirements of VDO based on the physical size of the underlying volume. Each table lists requirements appropriate to the intended deployment, such as primary storage or backup storage.

The exact numbers depend on your configuration of the VDO volume.

Primary storage deployment

In the primary storage case, the UDS index is between 0.01% to 25% the size of the physical size.

Table 2.1. Storage and memory requirements for primary storage

Physical sizeRAM usage: UDSRAM usage: VDODisk usageIndex type

10GB–1TB

250MB

472MB

2.5GB

Dense

2–10TB

1GB

3GB

10GB

Dense

250MB

22GB

Sparse

11–50TB

2GB

14GB

170GB

Sparse

51–100TB

3GB

27GB

255GB

Sparse

101–256TB

12GB

69GB

1020GB

Sparse

Backup storage deployment

In the backup storage case, the UDS index covers the size of the backup set but is not bigger than the physical size. If you expect the backup set or the physical size to grow in the future, factor this into the index size.

Table 2.2. Storage and memory requirements for backup storage

Physical sizeRAM usage: UDSRAM usage: VDODisk usageIndex type

10GB–1TB

250MB

472MB

2.5 GB

Dense

2–10TB

2GB

3GB

170GB

Sparse

11–50TB

10GB

14GB

850GB

Sparse

51–100TB

20GB

27GB

1700GB

Sparse

101–256TB

26GB

69GB

3400GB

Sparse

2.4. Placement of LVM-VDO in the storage stack

You must place certain storage layers under a VDO logical volume and others above it.

You can place thick-provisioned layers on top of VDO, but you cannot rely on the guarantees of thick provisioning in that case. Because the VDO layer is thin-provisioned, the effects of thin provisioning apply to all layers above it. If you do not monitor the VDO volume, you might run out of physical space on thick-provisioned volumes above VDO.

The supported placement of the following layers is under VDO. Do not place them above VDO:

  • DM Multipath
  • DM Crypt
  • Software RAID (LVM or MD RAID)

The following configurations are not supported:

  • VDO on top of a loopback device
  • Encrypted volumes on top of VDO
  • Partitions on a VDO volume
  • RAID, such as LVM RAID, MD RAID, or any other type, on top of a VDO volume
  • Deploying Ceph Storage on LVM-VDO

Chapter 3. Creating a deduplicated and compressed logical volume

You can create an LVM logical volume that uses the VDO feature to deduplicate and compress data.

3.1. LVM-VDO deployment scenarios

You can deploy VDO on LVM (LVM-VDO) in a variety of ways to provide deduplicated storage for:

  • block access
  • file access
  • local storage
  • remote storage

Because LVM-VDO exposes its deduplicated storage as a regular logical volume (LV), you can use it with standard file systems, iSCSI and FC target drivers, or as unified storage.

Note

Deploying Ceph Storage on LVM-VDO is currently not supported.

KVM

You can deploy LVM-VDO on a KVM server configured with Direct Attached Storage.

LVM-VDO deployment with KVM
File systems

You can create file systems on top of a VDO LV and expose them to NFS or CIFS users with the NFS server or Samba.

Deduplicated NAS
iSCSI target

You can export the entirety of the VDO LV as an iSCSI target to remote iSCSI initiators.

Deduplicated block storage target
Encryption

Device Mapper (DM) mechanisms such as DM Crypt are compatible with VDO. Encrypting a VDO LV volumes helps ensure data security, and any file systems above the VDO LV are still deduplicated.

LVM-VDO with encryption
Important

Applying the encryption layer above the VDO LV results in little if any data deduplication. Encryption makes duplicate blocks different before VDO can deduplicate them.

Always place the encryption layer below the VDO LV.

3.2. The physical and logical size of an LVM-VDO volume

This section describes the physical size, available physical size, and logical size that VDO can utilize.

Physical size

This is the same size as the physical extents allocated to the VDO pool LV. VDO uses this storage for:

  • User data, which might be deduplicated and compressed
  • VDO metadata, such as the UDS index
Available physical size

This is the portion of the physical size that VDO is able to use for user data.

It is equivalent to the physical size minus the size of the metadata, rounded down to a multiple of the slab size.

Logical Size

This is the provisioned size that the VDO LV presents to applications. It is usually larger than the available physical size. VDO currently supports any logical size up to 254 times the size of the physical volume with an absolute maximum logical size of 4 PB.

When you set up a VDO logical volume (LV), you specify the amount of logical storage that the VDO LV presents. When hosting active VMs or containers, Red Hat recommends provisioning storage at a 10:1 logical to physical ratio, that is, if you are utilizing 1 TB of physical storage, you would present it as 10 TB of logical storage.

If you do not specify the --virtualsize option, VDO provisions the volume to a 1:1 ratio. For example, if you put a VDO LV on top of a 20 GB VDO pool LV, VDO reserves 2.5 GB for the UDS index, if the default index size is used. The remaining 17.5 GB is provided for the VDO metadata and user data. As a result, the available storage to consume is not more than 17.5 GB, and can be less due to metadata that makes up the actual VDO volume.

3.3. Slab size in VDO

The physical storage of the VDO volume is divided into a number of slabs. Each slab is a contiguous region of the physical space. All of the slabs for a given volume have the same size, which can be any power of 2 multiple of 128 MB up to 32 GB.

The default slab size is 2 GB in order to facilitate evaluating VDO on smaller test systems. A single VDO volume can have up to 8192 slabs. Therefore, in the default configuration with 2 GB slabs, the maximum allowed physical storage is 16 TB. When using 32 GB slabs, the maximum allowed physical storage is 256 TB. VDO always reserves at least one entire slab for metadata, and therefore, the reserved slab cannot be used for storing user data.

Slab size has no effect on the performance of the VDO volume.

Table 3.1. Recommended VDO slab sizes by physical volume size

Physical volume sizeRecommended slab size

10–99 GB

1 GB

100 GB – 1 TB

2 GB

2–256 TB

32 GB

You can control the slab size by providing the --config 'allocation/vdo_slab_size_mb=size-in-megabytes' option to the lvcreate command.

3.4. Installing VDO

This procedure installs software necessary to create, mount, and manage VDO volumes.

Procedure

  • Install the vdo and kmod-kvdo packages:

    # yum install vdo kmod-kvdo

3.5. Creating an LVM-VDO volume

This procedure creates an VDO logical volume (LV) on a VDO pool LV.

Prerequisites

  • Install the VDO software. For more information, see Installing VDO.
  • An LVM volume group with free storage capacity exists on your system.

Procedure

  1. Pick a name for your VDO LV, such as vdo1. You must use a different name and device for each VDO LV on the system.

    In all the following steps, replace vdo-name with the name.

  2. Create the VDO LV:

    # lvcreate --type vdo \
               --name vdo-name
               --size physical-size
               --virtualsize logical-size \
               vg-name
    • Replace vg-name with the name of an existing LVM volume group where you want to place the VDO LV.
    • Replace logical-size with the amount of logical storage that the VDO LV will present.
    • If the physical size is larger than 16TiB, add the following option to increase the slab size on the volume to 32GiB:

      --config 'allocation/vdo_slab_size_mb=32768'

      If you use the default slab size of 2GiB on a physical size larger than 16TiB, the lvcreate command fails with the following error:

      ERROR - vdoformat: formatVDO failed on '/dev/device': VDO Status: Exceeds maximum number of slabs supported

      Example 3.1. Creating a VDO LV for container storage

      For example, to create a VDO LV for container storage on a 1TB VDO pool LV, you can use:

      # lvcreate --type vdo \
                 --name vdo1
                 --size 1T
                 --virtualsize 10T \
                 vg-name
      Important

      If a failure occurs when creating the VDO volume, remove the volume to clean up.

  3. Create a file system on the VDO LV:

    • For the XFS file system:

      # mkfs.xfs -K /dev/vg-name/vdo-name
    • For the ext4 file system:

      # mkfs.ext4 -E nodiscard /dev/vg-name/vdo-name

Additional resources

  • lvmvdo(7) man page

3.6. Mounting an LVM-VDO volume

This procedure mounts a file system on an LVM-VDO volume, either manually or persistently.

Prerequisites

Procedure

  • To mount the file system on the LVM-VDO volume manually, use:

    # mount /dev/vg-name/vdo-name mount-point
  • To configure the file system to mount automatically at boot, add a line to the /etc/fstab file:

    • For the XFS file system:

      /dev/vg-name/vdo-name mount-point xfs defaults 0 0
    • For the ext4 file system:

      /dev/vg-name/vdo-name mount-point ext4 defaults 0 0

    If the LVM-VDO volume is located on a block device that requires network, such as iSCSI, add the _netdev mount option. For iSCSI and other block devices requiring network, see the systemd.mount(5) man page for information on the _netdev mount option.

Additional resources

  • systemd.mount(5) man page

3.7. Changing the compression and deduplication settings on an LVM-VDO volume

This procedure enables or disables the compression and deduplication of a VDO pool logical volume (LV).

Note

Compression and deduplication are enabled by default.

Prerequisites

  • An LVM-VDO volume exists on your system.

Procedure

  1. To find out if the compression and deduplication is enabled or disabled on your logical volumes:

    # lvs -o+vdo_compression,vdo_deduplication
  2. Find out status of the compression and status of the deduplication index of your running active VDOPoolLV:

    # lvs -o+vdo_compression_state,vdo_index_state

    The vdo_index_state can show as error, close, opening, closing, online, and offline.

  3. To enable or disable the compression for VDOPoolLV:

    # lvchange --compression y|n  vg-name/vdopoolname
  4. To enable or disable the deduplication for VDOPoolLV:

    # lvchange --deduplication y|n vg-name/vdopoolname

Additional resources

  • lvmvdo(7) man page

Chapter 4. Trim options on an LVM-VDO volume

You can mount your file system with the discard option, which informs the VDO volume of the unused space. Another option is to use the fstrim application, which is an on-demand discarding, or mount -o discard command for immediate discarding.

When using the fstrim application, the admin needs to schedule and monitor an additional process, while using mount -o discard command allows for immediate recovery of space when possible.

Note that it is currently recommended to use fstrim application to discard unused blocks rather than the discard mount option because the performance impact of this option can be quite severe. For this reason, nodiscard is the default.

4.1. Enabling discard mount option on VDO

This procedure enables the discard option on your VDO volume.

Prerequisites

  • An LVM-VDO volume exists on your system.

Procedure

  • Enable the discard on your volume:

    # mount -o discard /dev/vg-name/vdo-name mount-point

Additional resources

  • xfs(5), mount(8), and lvmvdo(7) man pages

4.2. Setting up periodic TRIM operation

This procedure enables a scheduled TRIM operation on your system.

Prerequisites

  • An LVM-VDO volume exists on your system.

Procedure

  • Enable and start the timer:

    # systemctl enable --now fstrim.timer

Verification

  • Verify that the timer is enabled:

    # systemctl list-timers fstrim.timer

    Example 4.1. Possible output of the verification procedure

    # systemctl list-timers fstrim.timer
    NEXT                         LEFT         LAST  PASSED  UNIT         ACTIVATES
    Mon 2021-05-10 00:00:00 EDT  5 days left  n/a   n/a     fstrim.timer fstrim.service
Note

You will not see any reference to a VDO volume, because the fstrim.timer runs across all mounted filesystems.

Additional resources

  • fstrim(8) man page

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