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LVMCACHE(7)                                                                           LVMCACHE(7)

NAME
       lvmcache — LVM caching

DESCRIPTION
       The  cache  logical  volume  type uses a small and fast LV to improve the performance of a
       large and slow LV.  It does this by storing the frequently used blocks on the  faster  LV.
       LVM  refers to the small fast LV as a cache pool LV.  The large slow LV is called the ori‐
       gin LV.  Due to requirements from dm-cache (the kernel driver),  LVM  further  splits  the
       cache  pool LV into two devices - the cache data LV and cache metadata LV.  The cache data
       LV is where copies of data blocks are kept from the origin  LV  to  increase  speed.   The
       cache  metadata  LV  holds the accounting information that specifies where data blocks are
       stored (e.g.  on the origin LV or on the cache data LV).  Users should  be  familiar  with
       these  LVs if they wish to create the best and most robust cached logical volumes.  All of
       these associated LVs must be in the same VG.

Cache Terms
       origin LV           OriginLV      large slow LV
       cache data LV       CacheDataLV   small fast LV for cache pool data
       cache metadata LV   CacheMetaLV   small fast LV for cache pool metadata
       cache pool LV       CachePoolLV   CacheDataLV + CacheMetaLV
       cache LV            CacheLV       OriginLV + CachePoolLV

Cache Usage
       The primary method for using a cache type logical volume:

   0. create OriginLV
       Create an LV or identify an existing LV to be the origin LV.

       lvcreate -n OriginLV -L LargeSize VG SlowPVs

       Example
       # lvcreate -n lvol0 -L 100G vg

   1. create CacheDataLV
       Create the cache data LV.  This LV will hold data blocks from the OriginLV.  The  size  of
       this LV is the size of the cache and will be reported as the size of the cache pool LV.

       lvcreate -n CacheDataLV -L CacheSize VG FastPVs

       Example
       # lvcreate -n cache0 -L 10G vg /dev/fast

   2. create CacheMetaLV
       Create the cache metadata LV.  This LV will hold cache pool metadata.  The size of this LV
       should be 1000 times smaller than the cache data LV, with a minimum size of 8MiB.

       lvcreate -n CacheMetaLV -L MetaSize VG FastPVs

       Example
       # lvcreate -n cache0meta -L 12M vg /dev/fast

       # lvs -a vg
         LV         VG   Attr       LSize   Pool Origin
         cache0     vg   -wi-a-----  10.00g
         cache0meta vg   -wi-a-----  12.00m
         lvol0      vg   -wi-a----- 100.00g

   3. create CachePoolLV
       Combine the data and metadata LVs into a cache pool LV.  The behavior of the cache pool LV
       can be set in this step.
       CachePoolLV takes the name of CacheDataLV.
       CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
       CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.

       lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV
              VG/CacheDataLV

       Example
       # lvconvert --type cache-pool --poolmetadata vg/cache0meta vg/cache0

       # lvs -a vg
         LV              VG   Attr       LSize   Pool Origin
         cache0          vg   Cwi---C---  10.00g
         [cache0_cdata]  vg   Cwi-------  10.00g
         [cache0_cmeta]  vg   ewi-------  12.00m
         lvol0           vg   -wi-a----- 100.00g

   4. create CacheLV
       Create  a  cache  LV  by  linking the cache pool LV to the origin LV.  The user accessible
       cache LV takes the name of the origin LV, while the origin LV becomes a hidden LV with the
       name OriginLV_corig.  This can be done while the origin LV is in use.
       CacheLV takes the name of OriginLV.
       OriginLV is renamed OriginLV_corig and becomes hidden.

       lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV

       Example
       # lvconvert --type cache --cachepool vg/cache0 vg/lvol0

       # lvs -a vg
         LV              VG   Attr       LSize   Pool   Origin
         cache0          vg   Cwi---C---  10.00g
         [cache0_cdata]  vg   Cwi-ao----  10.00g
         [cache0_cmeta]  vg   ewi-ao----  12.00m
         lvol0           vg   Cwi-a-C--- 100.00g cache0 [lvol0_corig]
         [lvol0_corig]   vg   -wi-ao---- 100.00g

Cache Removal
   Split a cache pool LV off of a cache LV

       A  cache pool LV can be disconnected from a cache LV, leaving an unused cache pool LV, and
       an uncached origin LV.  This command writes back data from the cache pool to the origin LV
       when necessary.

       lvconvert --splitcache VG/CacheLV

   Removing a cache pool LV without removing its linked origin LV

       This  writes  back  data from the cache pool to the origin LV when necessary, then removes
       the cache pool LV, leaving the uncached origin LV.

       lvremove VG/CachePoolLV

       An alternative command that also disconnects the cache pool from the cache LV, and deletes
       the cache pool:

       lvconvert --uncache VG/CacheLV

       Example
       # lvs vg
         LV     VG   Attr       LSize   Pool   Origin
         cache0 vg   Cwi---C---  10.00g
         lvol0  vg   Cwi-a-C--- 100.00g cache0 [lvol0_corig]

       # lvremove vg/cache0

       # lvs vg
         LV    VG   Attr       LSize   Pool Origin
         lvol0 vg   -wi-a----- 100.00g

   Removing a cache LV: both origin LV and the cache pool LV

       Removing a cache LV removes both the origin LV and the linked cache pool LV.

       lvremove VG/CacheLV

Cache Topics
   Tolerate device failures in a cache pool LV

       Users who are concerned about the possibility of failures in their fast devices that could
       lead to data loss might consider making their cache pool sub-LVs redundant.

       Example
       0. Create an origin LV we wish to cache
       # lvcreate -L 10G -n lv1 vg /dev/slow_devs

       1. Create a 2-way RAID1 cache data LV
       # lvcreate --type raid1 -m 1 -L 1G -n cache1 vg \
            /dev/fast1 /dev/fast2

       2. Create a 2-way RAID1 cache metadata LV
       # lvcreate --type raid1 -m 1 -L 8M -n cache1meta vg \
            /dev/fast1 /dev/fast2

       3. Create a cache pool LV combining cache data LV and cache metadata LV
       # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1

       4. Create a cached LV by combining the cache pool LV and origin LV
       # lvconvert --type cache --cachepool vg/cache1 vg/lv1

   Cache mode

       The default cache mode is "writethrough".  Writethrough ensures that any data written will
       be stored both in the cache pool LV and on the origin LV.  The loss of a device associated
       with the cache pool LV 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
       pool back to the origin LV.  This mode will increase performance, but the loss of a device
       associated with the cache pool LV can result in lost data.

       With the --cachemode option, the cache mode can be  set  when  creating  a  cache  LV,  or
       changed  on  an  existing cache LV.  The current cache mode of a cache LV can be displayed
       with the cache_mode reporting option:

       lvs -o+cache_mode VG/CacheLV

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

       Example
       0. Create an origin LV we wish to cache (yours may already exist)
       # lvcreate -L 10G -n lv1 vg /dev/slow

       1. Create a cache data LV
       # lvcreate -L 1G -n cache1 vg /dev/fast

       2. Create a cache metadata LV
       # lvcreate -L 8M -n cache1meta vg /dev/fast

       3. Create a cache pool LV
       # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1

       4. Create a cache LV by combining the cache pool LV and origin LV,
          and use the writethrough cache mode.
       # lvconvert --type cache --cachepool vg/cache1 \
            --cachemode writethrough vg/lv1

   Cache policy

       The cache subsystem has additional per-LV parameters: the cache policy to use, and  possi‐
       bly  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 "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 creating a cache LV, or changed on an existing cache LV (both options can be used
       together).   The current cache policy and settings of a cache LV can be displayed with the
       cache_policy and cache_settings reporting options:

       lvs -o+cache_policy,cache_settings VG/CacheLV

       Example
       Change the cache policy and settings of an existing cache LV.
       # lvchange --cachepolicy mq --cachesettings \
            'migration_threshold=2048 random_threshold=4' vg/lv1

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

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

   Spare metadata LV

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

   Automatic pool metadata LV

       A cache data LV can be converted to cache pool LV without specifying a cache pool metadata
       LV.  LVM will automatically create a metadata LV from the same VG.

       lvcreate -n CacheDataLV -L CacheSize VG
       lvconvert --type cache-pool VG/CacheDataLV

   Create a new cache LV without an existing origin LV

       A cache LV can be created using an existing cache pool without an existing origin  LV.   A
       new origin LV is created and linked to the cache pool in a single step.

       lvcreate --type cache -L LargeSize -n CacheLV
              --cachepool VG/CachePoolLV VG SlowPVs

   Single step cache pool LV creation

       A cache pool LV can be created with a single lvcreate command, rather than using lvconvert
       on existing LVs.  This one command creates a cache data LV, a cache metadata LV, and  com‐
       bines the two into a cache pool LV.

       lvcreate --type cache-pool -L CacheSize -n CachePoolLV VG FastPVs

   Convert existing LVs to cache types

       When  an  existing origin LV is converted to a cache LV, the specified cache pool may be a
       normal LV, rather than a cache pool LV.  In this case, lvm will first convert  the  normal
       LV to a cache pool LV.  A pool metadata LV may optionally be specified.

       lvcreate -n OriginLV -L LargeSize VG
       lvcreate -n CacheDataLV -L CacheSize VG
       lvconvert --type cache --cachepool VG/CataDataLV VG/OriginLV

       This is equivalent to:

       lvcreate -n OriginLV -L LargeSize VG
       lvcreate -n CacheDataLV -L CacheSize VG
       lvconvert --type cache-pool VG/CacheDataLV
       lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV

SEE ALSO
       lvm.conf(5),  lvchange(8),  lvcreate(8),  lvdisplay(8),  lvextend(8),  lvremove(8),  lvre‐
       name(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8), vgreduce(8), vgsplit(8)

Red Hat, Inc                    LVM TOOLS 2.02.133(2) (2015-10-30)                    LVMCACHE(7)

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