Chapter 12. Enabling caching to improve logical volume performance

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

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

12.1. Caching methods in LVM

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

dm-cache

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

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

dm-writecache

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

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

Additional resources

  • lvmcache(7) man page

12.2. LVM caching components

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

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

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

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

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

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

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

    The dm-writecache method is not compatible with cachepool.

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

Additional resources

12.3. Enabling dm-cache caching for a logical volume

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

Prerequisites

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

Procedure

  1. Create a cachevol volume on the fast device: 

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

    Replace the following values:

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

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

    Example 12.1. Creating a cachevol volume

    # lvcreate --size 5G --name fastvol vg /dev/sdf
Logical volume "fastvol" created.

  2. Attach the cachevol volume to the main logical volume to begin caching: 

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

    Replace the following values:

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

    The name of the slow logical volume

    Example 12.2. Attaching the cachevol volume to the main LV

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

Verification steps

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

    # lvs --all --options +devices <vg>

LV              Pool           Type   Devices
main-lv         [fastvol_cvol] cache  main-lv_corig(0)
[fastvol_cvol]                 linear /dev/fast-pv
[main-lv_corig]                linear /dev/slow-pv

Additional resources

  • lvmcache(7) man page

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

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

Prerequisites

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

Procedure

  1. Create a cachepool volume on the fast device: 

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

    Replace the following values:

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

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

    Note

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

    Example 12.3. Creating a cachepool volume

    # lvcreate --type cache-pool --size 5G --name fastpool vg /dev/sde
Logical volume "fastpool" created.

  2. Attach the cachepool to the main logical volume to begin caching: 

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

    Replace the following values:

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

    The name of the slow logical volume

    Example 12.4. Attaching the cachepool to the main LV

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

Verification steps

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

    # lvs --all --options +devices <vg>

LV                      Pool               Type        Devices
[fastpool_cpool]                           cache-pool  fastpool_pool_cdata(0)
[fastpool_cpool_cdata]                     linear      /dev/sdf1(4)
[fastpool_cpool_cmeta]                     linear      /dev/sdf1(2)
[lvol0_pmspare]                            linear      /dev/sdf1(0)
main                    [fastpoool_cpool]  cache       main_corig(0)
[main_corig]                               linear      /dev/sdf1(O)

Additional resources

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

12.5. Enabling dm-writecache caching for a logical volume

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

Prerequisites

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

Procedure

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

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

    Replace the following values:

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

  2. Create a deactivated cachevol volume on the fast device: 

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

    Replace the following values:

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

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

    Example 12.5. Creating a deactivated cachevol volume

    # lvcreate --activate n --size 5G --name fastvol vg /dev/sdf
WARNING: Logical volume vg/fastvol not zeroed.
Logical volume "fastvol" created.

  3. Attach the cachevol volume to the main logical volume to begin caching: 

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

    Replace the following values:

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

    The name of the slow logical volume

    Example 12.6. Attaching the cachevol volume to the main LV

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

  4. Activate the resulting logical volume: 

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

    Replace the following values:

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

Verification steps

  • Examine the newly created devices: 

    # lvs --all --options +devices vg

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

Additional resources

  • lvmcache(7) man page

12.6. Disabling caching for a logical volume

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

Prerequisites

  • Caching is enabled on a logical volume.

Procedure

  1. Deactivate the logical volume: 

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

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

  2. Detach the cachevol or cachepool volume: 

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

    Replace the following values:

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

    Example 12.7. Detaching the cachevol or cachepool volume

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

Verification steps

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

    # lvs --all --options +devices <vg>

LV      Attr       Type   Devices
fastvol -wi------- linear /dev/fast-pv
main-lv -wi------- linear /dev/slow-pv

Additional resources

  • The lvmcache(7) man page