lvmcache(category15-traffic.html) - phpMan

LVMCACHE(7)                                                        LVMCACHE(7)

NAME
       lvmcache -- LVM caching

DESCRIPTION
       An lvm(8) cache Logical Volume (LV) 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 origin 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  meta-
       data  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 LVs.  All of these associated LVs must be  in  the  same  Volume
       Group (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 LV:

   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 /dev/slow

   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 neces-
       sary, 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
       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  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 "mq" policy has a number of tunable parameters.  The  defaults  are
       chosen  to be suitable for the majority of systems, but in special cir-
       cumstances, 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.

   Chunk size

       The size of data blocks managed by a cache pool can be  specified  with
       the  --chunksize option when the cache LV is created.  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,  where  small reads and writes can cause large sections of an LV
       to be mapped into 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 cache pool LV chunk size:
       lvs -o+chunksize VG/CacheLV
       lvm.conf(5) cache_pool_chunk_size
       controls the default chunk size used when creating a cache LV.
       The default value is shown by:
       lvmconfig --type default allocation/cache_pool_chunk_size

   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 combines 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

   Cache metadata formats

       There are two disk formats for cache metadata.  The metadata format can
       be specified when a cache pool is created, and cannot be changed.  For-
       mat 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 target.
       lvconvert --type cache-pool --cachemetadataformat auto|1|2
              VG/CacheDataLV

SEE ALSO
       lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8), lvre-
       move(8),  lvrename(8),  lvresize(8),  lvs(8),  vgchange(8), vgmerge(8),
       vgreduce(8), vgsplit(8)

Red Hat, Inc       LVM TOOLS 2.02.187(2)-RHEL7 (2020-03-24)        LVMCACHE(7)