Provided by: lvm2_2.03.11-2.1ubuntu3_amd64 bug


       lvmcache — LVM caching


       lvm(8)  includes  two  kinds  of  caching that can be used to improve the performance of a
       Logical Volume (LV). When caching, varying subsets of an LV's data are temporarily  stored
       on a smaller, faster device (e.g. an SSD) to improve the performance of the LV.

       To  do  this  with  lvm, a new special LV is first created from the faster device. This LV
       will hold the cache. Then, the new fast LV is attached  to  the  main  LV  by  way  of  an
       lvconvert  command.  lvconvert  inserts  one of the device mapper caching targets into the
       main LV's i/o path. The device mapper target combines the main  LV  and  fast  LV  into  a
       hybrid  device  that looks like the main LV, but has better performance. While the main LV
       is being used, portions of its data will be temporarily and transparently  stored  on  the
       special fast LV.

       The two kinds of caching are:

       · A  read  and  write hot-spot cache, using the dm-cache kernel module.  This cache tracks
         access patterns and adjusts its content deliberately so that commonly used parts of  the
         main LV are likely to be found on the fast storage. LVM refers to this using the LV type

       · A write cache, using the dm-writecache kernel module.  This cache can be used  with  SSD
         or PMEM devices to speed up all writes to the main LV. Data read from the main LV is not
         stored in the cache, only newly written data.  LVM refers to  this  using  the  LV  type


       1. Identify main LV that needs caching

       The  main LV may already exist, and is located on larger, slower devices.  A main LV would
       be created with a command like:

         $ lvcreate -n main -L Size vg /dev/slow_hhd

       2. Identify fast LV to use as the cache

       A fast LV is created using one or more fast devices, like an SSD.  This special LV will be
       used to hold the cache:

         $ lvcreate -n fast -L Size vg /dev/fast_ssd

         $ lvs -a
         LV   Attr       Type   Devices
         fast -wi------- linear /dev/fast_ssd
         main -wi------- linear /dev/slow_hhd

       3. Start caching the main LV

       To start caching the main LV, convert the main LV to the desired caching type, and specify
       the fast LV to use as the cache:

       using dm-cache:

         $ lvconvert --type cache --cachevol fast vg/main

       using dm-writecache:

         $ lvconvert --type writecache --cachevol fast vg/main

       using dm-cache (with cachepool):

         $ lvconvert --type cache --cachepool fast vg/main

       4. Display LVs

       Once the fast LV has been attached to the main LV, lvm reports the main LV type as  either
       cache  or  writecache  depending on the type used.  While attached, the fast LV is hidden,
       and renamed with a _cvol or _cpool suffix.  It is displayed by  lvs  -a.   The  _corig  or
       _wcorig LV represents the original LV without the cache.

       using dm-cache:

         $ lvs -a
         LV           Pool        Type   Devices
         main         [fast_cvol] cache  main_corig(0)
         [fast_cvol]              linear /dev/fast_ssd
         [main_corig]             linear /dev/slow_hhd

       using dm-writecache:

         $ lvs -a
         LV            Pool        Type       Devices
         main          [fast_cvol] writecache main_wcorig(0)
         [fast_cvol]               linear     /dev/fast_ssd
         [main_wcorig]             linear     /dev/slow_hhd

       using dm-cache (with cachepool):

         $ lvs -a
         LV                 Pool         Type       Devices
         main               [fast_cpool] cache      main_corig(0)
         [fast_cpool]                    cache-pool fast_pool_cdata(0)
         [fast_cpool_cdata]              linear     /dev/fast_ssd
         [fast_cpool_cmeta]              linear     /dev/fast_ssd
         [main_corig]                    linear     /dev/slow_hhd

       5. Use the main LV

       Use the LV until the cache is no longer wanted, or needs to be changed.

       6. Stop caching

       To stop caching the main LV, separate the fast LV from the main LV.  This changes the type
       of the main LV back to what it was before the cache was attached.

         $ lvconvert --splitcache vg/main

         $ lvs -a
         LV   VG Attr       Type   Devices
         fast vg -wi------- linear /dev/fast_ssd
         main vg -wi------- linear /dev/slow_hhd

       To stop caching the main LV and also remove unneeded cache pool,
       use the --uncache:

         $ lvconvert --uncache vg/main

         $ lvs -a
         LV   VG Attr       Type   Devices
         main vg -wi------- linear /dev/slow_hhd

   Create a new LV with caching.
       A new LV can be created with caching attached at the time of creation using the  following

       $ lvcreate --type cache|writecache -n Name -L Size
            --cachedevice /dev/fast_ssd vg /dev/slow_hhd

       The  main LV is created with the specified Name and Size from the slow_hhd.  A hidden fast
       LV is created on the fast_ssd and is then attached to the new main LV.  If the fast_ssd is
       unused, the entire disk will be used as the cache unless the --cachesize option is used to
       specify a size for the fast LV.  The --cachedevice option can be repeated to use  multiple
       disks for the fast LV.


   option args

       --cachevol LV

       Pass  this option a fast LV that should be used to hold the cache.  With a cachevol, cache
       data and metadata are stored in different parts of the same fast LV.  This option  can  be
       used with dm-writecache or dm-cache.

       --cachepool CachePoolLV|LV

       Pass  this  option  a  cachepool LV or a standard LV.  When using a cache pool, lvm places
       cache data and cache metadata on different LVs.  The two LVs together are called  a  cache
       pool.   This  has a bit better performance for dm-cache and permits specific placement and
       segment type selection for data and metadata volumes.  A cache pool is  represented  as  a
       special  type  of  LV  that cannot be used directly.  If a standard LV is passed with this
       option, lvm will first convert it to a cache pool by combining it with another LV  to  use
       for metadata.  This option can be used with dm-cache.

       --cachedevice PV

       This  option  can  be  used  in  place  of --cachevol, in which case a cachevol LV will be
       created using the specified device.  This option can be  repeated  to  create  a  cachevol
       using  multiple devices, or a tag name can be specified in which case the cachevol will be
       created using any of the devices with the given tag.  If a named cache device  is  unused,
       the entire device will be used to create the cachevol.  To create a cachevol of a specific
       size from the cache devices, include the --cachesize option.

   dm-cache block size

       A cache pool will have a logical block size of 4096 bytes if it is  created  on  a  device
       with a logical block size of 4096 bytes.

       If  a  main  LV  has  logical  block size 512 (with an existing xfs file system using that
       size), then it cannot use a cache pool with a 4096 logical block size.  If the cache  pool
       is attached, the main LV will likely fail to mount.

       To avoid this problem, use a mkfs option to specify a 4096 block size for the file system,
       or attach the cache pool before running mkfs.

   dm-writecache block size

       The dm-writecache block size can be 4096 bytes (the default), or 512 bytes.   The  default
       4096  has  better  performance  and  should  be  used  except  when  512  is necessary for
       compatibility.   The  dm-writecache  block  size   is   specified   with   --cachesettings
       block_size=4096|512 when caching is started.

       When  a  file system like xfs already exists on the main LV prior to caching, and the file
       system is using a block size of 512, then the writecache block size should be set to  512.
       (The  file  system  will  likely fail to mount if writecache block size of 4096 is used in
       this case.)

       Check the xfs sector size while the fs is mounted:

       $ xfs_info /dev/vg/main
       Look for sectsz=512 or sectsz=4096

       The writecache block size should be chosen to match the xfs sectsz value.

       It is also possible to specify a sector size of 4096 to mkfs.xfs when  creating  the  file
       system.  In this case the writecache block size of 4096 can be used.

   dm-writecache settings

       Tunable   parameters   can  be  passed  to  the  dm-writecache  kernel  module  using  the
       --cachesettings option when caching is started, e.g.

       $ lvconvert --type writecache --cachevol fast \
            --cachesettings 'high_watermark=N writeback_jobs=N' vg/main

       Tunable options are:

       · high_watermark = <percent>

         Start writeback when the writecache usage reaches this percent (0-100).

       · low_watermark = <percent>

         Stop writeback when the writecache usage reaches this percent (0-100).

       · writeback_jobs = <count>

         Limit the number of blocks that are in flight  during  writeback.   Setting  this  value
         reduces writeback throughput, but it may improve latency of read requests.

       · autocommit_blocks = <count>

         When the application writes this amount of blocks without issuing the FLUSH request, the
         blocks are automatically commited.

       · autocommit_time = <milliseconds>

         The data is automatically commited if this time passes and no FLUSH request is received.

       · fua = 0|1

         Use the FUA flag when writing data from persistent memory back to the underlying device.
         Applicable only to persistent memory.

       · nofua = 0|1

         Don't  use  the  FUA  flag when writing back data and send the FLUSH request afterwards.
         Some underlying devices perform better with fua, some with nofua.  Testing is  necessary
         to determine which.  Applicable only to persistent memory.

       · cleaner = 0|1

         Setting cleaner=1 enables the writecache cleaner mode in which data is gradually flushed
         from the cache.  If this is done prior to detaching the writecache, then the  splitcache
         command  will  have  little  or  no  flushing  to  perform.  If not done beforehand, the
         splitcache command enables the cleaner mode and waits for flushing  to  complete  before
         detaching  the  writecache.   Adding  cleaner=0  to the splitcache command will skip the
         cleaner mode, and any required flushing is performed in device suspend.

   dm-cache with separate data and metadata LVs

       When using dm-cache, the cache metadata and cache data can be stored on separate LVs.   To
       do this, a "cache pool" is created, which is a special LV that references two sub LVs, one
       for data and one for metadata.

       To create a cache pool from two separate LVs:

       $ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
       $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
       $ lvconvert --type cache-pool --poolmetadata fastmeta vg/fast

       Then use the cache pool LV to start caching the main LV:

       $ lvconvert --type cache --cachepool fast vg/main

       A variation of the same procedure automatically creates  a  cache  pool  when  caching  is
       started.   To  do  this, use a standard LV as the --cachepool (this will hold cache data),
       and use another standard LV as the --poolmetadata (this will hold  cache  metadata).   LVM
       will  create  a  cache pool LV from the two specified LVs, and use the cache pool to start
       caching the main LV.

       $ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
       $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
       $ lvconvert --type cache --cachepool fast --poolmetadata fastmeta vg/main

   dm-cache cache modes

       The default dm-cache cache mode is "writethrough".  Writethrough  ensures  that  any  data
       written  will  be  stored  both  in  the cache and on the origin LV.  The loss of a device
       associated with the cache in this case would not mean the loss of any data.

       A second cache mode is "writeback".  Writeback delays writing data blocks from  the  cache
       back  to  the  origin  LV.   This  mode will increase performance, but the loss of a cache
       device can result in lost data.

       With the --cachemode option, the cache mode can be set when caching is started, or changed
       on  an  LV  that  is  already  cached.   The  current cache mode can be displayed with the
       cache_mode reporting option:

       lvs -o+cache_mode VG/LV

       lvm.conf(5) allocation/cache_mode
       defines the default cache mode.

       $ lvconvert --type cache --cachevol fast \
            --cachemode writethrough vg/main

   dm-cache chunk size

       The size of data blocks managed by dm-cache can be specified with the  --chunksize  option
       when  caching is started.  The default unit is KiB.  The value must be a multiple of 32KiB
       between 32KiB and 1GiB. Cache chunks bigger then 512KiB shall be only used when necessary.

       Using a chunk size that is too large can result in wasteful use of  the  cache,  in  which
       small  reads  and  writes  cause large sections of an LV to be stored in the cache. It can
       also require increasing migration threshold which defaults to 2048 sectors (1  MiB).  Lvm2
       ensures migration threshold is at least 8 chunks in size. This may in some cases result in
       very high bandwidth load of transfering data between the cache LV and its cache origin LV.
       However,  choosing  a  chunk  size that is too small can result in more overhead trying to
       manage the numerous chunks that become mapped into the cache.  Overhead can  include  both
       excessive CPU time searching for chunks, and excessive memory tracking chunks.

       Command to display the chunk size:
       lvs -o+chunksize VG/LV

       lvm.conf(5) cache_pool_chunk_size
       controls the default chunk size.

       The default value is shown by:
       lvmconfig --type default allocation/cache_pool_chunk_size

       Checking migration threshold (in sectors) of running cached LV:
       lvs -o+kernel_cache_settings VG/LV

   dm-cache migration threshold

       Migrating  data  between  the  origin  and  cache  LV  uses bandwidth.  The user can set a
       throttle to prevent more than a certain amount of migration occurring  at  any  one  time.
       Currently dm-cache is not taking any account of normal io traffic going to the devices.

       User     can     set    migration    threshold    via    cache    policy    settings    as
       "migration_threshold=<#sectors>" to set the maximum number of sectors being migrated,  the
       default being 2048 sectors (1MiB).

       Command to set migration threshold to 2MiB (4096 sectors):
       lvcreate --cachepolicy 'migration_threshold=4096' VG/LV

       Command to display the migration threshold:
       lvs -o+kernel_cache_settings,cache_settings VG/LV
       lvs -o+chunksize VG/LV

   dm-cache cache policy

       The  dm-cache  subsystem  has  additional  per-LV parameters: the cache policy to use, and
       possibly tunable parameters for the cache policy.  Three policies are currently available:
       "smq"  is  the  default  policy, "mq" is an older implementation, and "cleaner" is used to
       force the cache to write back (flush) all cached writes to the origin LV.

       The older "mq" policy has a number of tunable parameters. The defaults are  chosen  to  be
       suitable  for the majority of systems, but in special circumstances, changing the settings
       can improve performance.

       With the --cachepolicy and --cachesettings options, the cache policy and settings  can  be
       set when caching is started, or changed on an existing cached LV (both options can be used
       together).  The current cache policy and settings can be displayed with  the  cache_policy
       and cache_settings reporting options:

       lvs -o+cache_policy,cache_settings VG/LV

       Change the cache policy and settings of an existing LV.

       $ lvchange --cachepolicy mq --cachesettings \
            'migration_threshold=2048 random_threshold=4' vg/main

       lvm.conf(5) allocation/cache_policy
       defines the default cache policy.

       lvm.conf(5) allocation/cache_settings
       defines the default cache settings.

   dm-cache spare metadata LV

       See  lvmthin(7)  for  a  description of the "pool metadata spare" LV.  The same concept is
       used for cache pools.

   dm-cache metadata formats

       There are two disk formats for dm-cache metadata.  The metadata format  can  be  specified
       with  --cachemetadataformat  when caching is started, and cannot be changed.  Format 2 has
       better performance; it is more compact, and stores dirty bits in a separate  btree,  which
       improves  the  speed  of  shutting down the cache.  With auto, lvm selects the best option
       provided by the current dm-cache kernel module.

   RAID1 cache device

       RAID1 can be used to create the fast LV holding the cache so that it can tolerate a device
       failure.   (When  using  dm-cache with separate data and metadata LVs, each of the sub-LVs
       can use RAID1.)

       $ lvcreate -n main -L Size vg /dev/slow
       $ lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/ssd1 /dev/ssd2
       $ lvconvert --type cache --cachevol fast vg/main

   dm-cache command shortcut

       A single command can be used to create a cache pool and attach that new cache  pool  to  a
       main LV:

       $ lvcreate --type cache --name Name --size Size VG/LV [PV]

       In  this  command,  the specified LV already exists, and is the main LV to be cached.  The
       command creates a new cache pool with the  given  name  and  size,  using  the  optionally
       specified PV (typically an ssd).  Then it attaches the new cache pool to the existing main
       LV to begin caching.

       (Note: ensure that the specified main LV is  a  standard  LV.   If  a  cache  pool  LV  is
       mistakenly specified, then the command does something different.)

       (Note:  the type option is interpreted differently by this command than by normal lvcreate
       commands in which --type specifies the type of the newly created LV.  In this case, an  LV
       with type cache-pool is being created, and the existing main LV is being converted to type


       lvm.conf(5),   lvchange(8),   lvcreate(8),   lvdisplay(8),    lvextend(8),    lvremove(8),
       lvrename(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8), vgreduce(8), vgsplit(8)