Provided by: lvm2_2.03.07-1ubuntu1_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).  Typically,  a  smaller,  faster  device  is  used  to  improve  i/o
       performance  of  a larger, slower LV. To do this, a separate LV is created from the faster
       device, and then the original LV is converted to start using the fast LV.

       The two kinds of caching are:

       • A read and write hot-spot cache, using the dm-cache kernel module.  This cache  is  slow
         moving,  and  adjusts  the cache content over time so that the most used parts of the LV
         are kept on the faster device.  Both reads and writes use the cache. LVM refers to  this
         using the LV type cache.

       • A  streaming write cache, using the dm-writecache kernel module.  This cache is intended
         to be used with SSD or PMEM devices to speed up all writes to an LV.  Reads do  not  use
         this cache.  LVM refers to this using the LV type writecache.


       Both kinds of caching use similar lvm commands:

       1. Identify main LV that needs caching

       A main LV exists on slower devices.

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

       2. Identify fast LV to use as the cache

       A fast LV exists on faster devices.  This LV will be used to hold the cache.

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

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

       3. Start caching the main LV

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

       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
         [main_corig]             linear /dev/slow

       using dm-writecache:

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

       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
         [fast_cpool_cmeta]              linear     /dev/fast
         [main_corig]                    linear     /dev/slow

       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
         main vg -wi------- linear /dev/slow


   option args

       --cachevol LV

       Pass this option a standard LV.  With a cachevol, cache data and  metadata  are  contained
       within the single LV.  This is used with dm-writecache or dm-cache.

       --cachepool CachePoolLV|LV

       Pass  this option a cache pool object.  With a cache pool, lvm places cache data and cache
       metadata on different LVs.  The two LVs together are called a cache  pool.   This  permits
       specific placement of data and metadata.  A cache pool is represented as a special type of
       LV that cannot be used directly.  (If a standard LV is passed to  this  option,  lvm  will
       first  convert  it  to  a cache pool by combining it with another LV to use for metadata.)
       This can be used with dm-cache.

   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 = <count>

         Start writeback when the number of used blocks reach this watermark

       • low_watermark = <count>

         Stop writeback when the number of used blocks drops below this watermark

       • 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.

   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/fast1
       $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast2
       $ 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/fast1
       $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast2
       $ 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.

       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.   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

   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.

   mirrored cache device

       The fast LV holding the cache can be created as a raid1 mirror 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/fast1 /dev/fast2
       $ 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)