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man: lvmcache update
for cache changes including writecache
This commit is contained in:
David Teigland
parent
499923f0f6
commit
0eafb76772
@ -4,291 +4,319 @@ lvmcache \(em LVM caching
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.SH DESCRIPTION
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An \fBlvm\fP(8) \fBcache\fP Logical Volume (LV) uses a small and
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fast LV to improve the performance of a large and slow LV. It does this
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by storing the frequently used blocks on the faster LV.
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LVM refers to the small fast LV as a \fBcache pool LV\fP. The large
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slow LV is called the \fBorigin LV\fP. Due to requirements from dm-cache
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(the kernel driver), LVM further splits the cache pool LV into two
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devices - the \fBcache data LV\fP and \fBcache metadata LV\fP. The cache
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data LV is where copies of data blocks are kept from the
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origin LV to increase speed. The cache metadata LV holds the
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accounting information that specifies where data blocks are stored (e.g.
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on the origin LV or on the cache data LV). Users should be familiar with
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these LVs if they wish to create the best and most robust cached
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LVs. All of these associated LVs must be in the same Volume
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Group (VG).
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\fBlvm\fP(8) includes two kinds of caching that can be used to improve the
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performance of a Logical Volume (LV). Typically, a smaller, faster device
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is used to improve i/o performance of a larger, slower LV. To do this, a
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separate LV is created from the faster device, and then the original LV is
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converted to start using the fast LV.
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The two kinds of caching are:
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.IP \[bu] 2
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A read and write hot-spot cache, using the dm-cache kernel module. This
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cache is slow moving, and adjusts the cache content over time so that the
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most used parts of the LV are kept on the faster device. Both reads and
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writes use the cache. LVM refers to this using the LV type \fBcache\fP.
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.IP \[bu] 2
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A streaming write cache, using the dm-writecache kernel module. This
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cache is intended to be used with SSD or PMEM devices to speed up all
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writes to an LV. Reads do not use this cache. LVM refers to this using
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the LV type \fBwritecache\fP.
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.SH USAGE
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Both kinds of caching use similar lvm commands:
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.B 1. Identify main LV that needs caching
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A main LV exists on slower devices.
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.SH Cache Terms
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.nf
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origin LV OriginLV large slow LV
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cache data LV CacheDataLV small fast LV for cache pool data
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cache metadata LV CacheMetaLV small fast LV for cache pool metadata
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cache pool LV CachePoolLV CacheDataLV + CacheMetaLV
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cache LV CacheLV OriginLV + CachePoolLV
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$ lvcreate -n main -L Size vg /dev/slow
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.fi
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.SH Cache Usage
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.B 2. Identify fast LV to use as the cache
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The primary method for using a cache type LV:
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.SS 0. create OriginLV
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Create an LV or identify an existing LV to be the origin LV.
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.B lvcreate -n OriginLV -L LargeSize VG SlowPVs
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.I Example
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.br
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# lvcreate -n lvol0 -L 100G vg /dev/slow
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.SS 1. create CacheDataLV
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Create the cache data LV. This LV will hold data blocks from the
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OriginLV. The size of this LV is the size of the cache and will be
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reported as the size of the cache pool LV.
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.B lvcreate -n CacheDataLV -L CacheSize VG FastPVs
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.I Example
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.br
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# lvcreate -n cache0 -L 10G vg /dev/fast
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.SS 2. create CacheMetaLV
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Create the cache metadata LV. This LV will hold cache pool metadata. The
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size of this LV should be 1000 times smaller than the cache data LV, with
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a minimum size of 8MiB.
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.B lvcreate -n CacheMetaLV -L MetaSize VG FastPVs
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.I Example
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.br
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# lvcreate -n cache0meta -L 12M vg /dev/fast
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A fast LV exists on faster devices. This LV will be used to hold the
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cache.
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.nf
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# lvs -a vg
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LV VG Attr LSize Pool Origin
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cache0 vg -wi-a----- 10.00g
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cache0meta vg -wi-a----- 12.00m
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lvol0 vg -wi-a----- 100.00g
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$ lvcreate -n fast -L Size vg /dev/fast
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$ lvs vg -o+devices
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LV VG Attr LSize Devices
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fast vg -wi------- xx.00m /dev/fast(0)
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main vg -wi------- yyy.00m /dev/slow(0)
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.fi
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.B 3. Start caching the main LV
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To start caching the main LV using the fast LV, convert the main LV to the
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desired caching type, and specify the fast LV to use:
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.nf
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using dm-cache:
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$ lvconvert --type cache --cachepool fast vg/main
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or dm-writecache:
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$ lvconvert --type writecache --cachepool fast vg/main
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.fi
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.B 4. Display LVs
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Once the fast LV has been attached to the main LV, lvm reports the main LV
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type as either \fBcache\fP or \fBwritecache\fP depending on the type used.
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While attached, the fast LV is hidden, and only displayed when lvs is
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given -a. The _corig or _wcorig LV represents the original LV without the
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cache.
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.nf
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using dm-cache:
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$ lvs -a -o name,vgname,lvattr,origin,segtype,devices vg
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LV VG Attr Origin Type Devices
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[fast] vg Cwi-aoC--- linear /dev/fast(xx)
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main vg Cwi-a-C--- [main_corig] cache main_corig(0)
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[main_corig] vg owi-aoC--- linear /dev/slow(0)
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or dm-writecache:
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$ lvs -a -o name,vgname,lvattr,origin,segtype,devices vg
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LV VG Attr Origin Type Devices
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[fast] vg -wi-ao---- linear /dev/fast(xx)
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main vg Cwi-a----- [main_wcorig] writecache main_wcorig(0)
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[main_wcorig] vg -wi-ao---- linear /dev/slow(0)
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.fi
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.B 5. Use the main LV
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Use the LV until the cache is no longer wanted, or needs to be changed.
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.B 6. Stop caching
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To stop caching the main LV, separate the fast LV from the main LV. This
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changes the type of the main LV back to what it was before the cache was
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attached.
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.nf
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$ lvconvert --splitcache vg/main
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.fi
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.SS 3. create CachePoolLV
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Combine the data and metadata LVs into a cache pool LV.
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The behavior of the cache pool LV can be set in this step.
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.br
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CachePoolLV takes the name of CacheDataLV.
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.br
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CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
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.br
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CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
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.B lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV
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.RS
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.B VG/CacheDataLV
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.RE
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.I Example
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.br
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# lvconvert --type cache-pool --poolmetadata vg/cache0meta vg/cache0
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.nf
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# lvs -a vg
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LV VG Attr LSize Pool Origin
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cache0 vg Cwi---C--- 10.00g
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[cache0_cdata] vg Cwi------- 10.00g
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[cache0_cmeta] vg ewi------- 12.00m
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lvol0 vg -wi-a----- 100.00g
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.fi
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.SS 4. create CacheLV
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Create a cache LV by linking the cache pool LV to the origin LV.
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The user accessible cache LV takes the name of the origin LV,
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while the origin LV becomes a hidden LV with the name
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OriginLV_corig. This can be done while the origin LV is in use.
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.br
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CacheLV takes the name of OriginLV.
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.br
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OriginLV is renamed OriginLV_corig and becomes hidden.
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.B lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
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.I Example
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.br
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# lvconvert --type cache --cachepool vg/cache0 vg/lvol0
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.nf
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# lvs -a vg
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LV VG Attr LSize Pool Origin
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cache0 vg Cwi---C--- 10.00g
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[cache0_cdata] vg Cwi-ao---- 10.00g
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[cache0_cmeta] vg ewi-ao---- 12.00m
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lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
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[lvol0_corig] vg -wi-ao---- 100.00g
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.fi
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.SH Cache Removal
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.SS Split a cache pool LV off of a cache LV
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.SH OPTIONS
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\&
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A cache pool LV can be disconnected from a cache LV, leaving an
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unused cache pool LV, and an uncached origin LV. This command
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writes back data from the cache pool to the origin LV when necessary.
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.B lvconvert --splitcache VG/CacheLV
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.SS Removing a cache pool LV without removing its linked origin LV
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.SS dm-writecache block size
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\&
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This writes back data from the cache pool to the origin LV when necessary,
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then removes the cache pool LV, leaving the uncached origin LV.
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The dm-writecache block size can be 4096 bytes (the default), or 512
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bytes. The default 4096 has better performance and should be used except
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when 512 is necessary for compatibility. The dm-writecache block size is
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specified with --writecacheblocksize 4096b|512b when caching is started.
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.B lvremove VG/CachePoolLV
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When a file system like xfs already exists on the main LV prior to
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caching, and the file system is using a block size of 512, then the
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writecache block size should be set to 512. (The file system will likely
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fail to mount if writecache block size of 4096 is used in this case.)
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An alternative command that also disconnects the cache pool from the cache
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LV, and deletes the cache pool:
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Check the xfs sector size while the fs is mounted:
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.B lvconvert --uncache VG/CacheLV
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.I Example
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.nf
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# lvs vg
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LV VG Attr LSize Pool Origin
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cache0 vg Cwi---C--- 10.00g
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lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
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# lvremove vg/cache0
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# lvs vg
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LV VG Attr LSize Pool Origin
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lvol0 vg -wi-a----- 100.00g
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$ xfs_info /dev/vg/main
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Look for sectsz=512 or sectsz=4096
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.fi
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.SS Removing a cache LV: both origin LV and the cache pool LV
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The writecache block size should be chosen to match the xfs sectsz value.
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It is also possible to specify a sector size of 4096 to mkfs.xfs when
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creating the file system. In this case the writecache block size of 4096
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can be used.
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.SS dm-writecache settings
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\&
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Removing a cache LV removes both the origin LV and the linked cache pool
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LV.
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Tunable parameters can be passed to the dm-writecache kernel module using
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the --cachesettings option when caching is started, e.g.
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.B lvremove VG/CacheLV
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.SH Cache Topics
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.SS Tolerate device failures in a cache pool LV
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\&
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Users who are concerned about the possibility of failures in their fast
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devices that could lead to data loss might consider making their cache
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pool sub-LVs redundant.
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.I Example
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.nf
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0. Create an origin LV we wish to cache
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# lvcreate -L 10G -n lv1 vg /dev/slow
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1. Create a 2-way RAID1 cache data LV
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# lvcreate --type raid1 -m 1 -L 1G -n cache1 vg \\
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/dev/fast1 /dev/fast2
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2. Create a 2-way RAID1 cache metadata LV
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# lvcreate --type raid1 -m 1 -L 8M -n cache1meta vg \\
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/dev/fast1 /dev/fast2
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3. Create a cache pool LV combining cache data LV and cache metadata LV
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# lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
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4. Create a cached LV by combining the cache pool LV and origin LV
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# lvconvert --type cache --cachepool vg/cache1 vg/lv1
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$ lvconvert --type writecache --cachepool fast \\
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--cachesettings 'high_watermark=N writeback_jobs=N' vg/main
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.fi
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.SS Cache mode
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Tunable options are:
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.IP \[bu] 2
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high_watermark = <count>
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Start writeback when the number of used blocks reach this watermark
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.IP \[bu] 2
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low_watermark = <count>
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Stop writeback when the number of used blocks drops below this watermark
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.IP \[bu] 2
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writeback_jobs = <count>
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Limit the number of blocks that are in flight during writeback. Setting
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this value reduces writeback throughput, but it may improve latency of
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read requests.
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.IP \[bu] 2
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autocommit_blocks = <count>
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When the application writes this amount of blocks without issuing the
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FLUSH request, the blocks are automatically commited.
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.IP \[bu] 2
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autocommit_time = <milliseconds>
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The data is automatically commited if this time passes and no FLUSH
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request is received.
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.IP \[bu] 2
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fua = 0|1
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Use the FUA flag when writing data from persistent memory back to the
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underlying device.
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Applicable only to persistent memory.
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.IP \[bu] 2
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nofua = 0|1
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Don't use the FUA flag when writing back data and send the FLUSH request
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afterwards. Some underlying devices perform better with fua, some with
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nofua. Testing is necessary to determine which.
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Applicable only to persistent memory.
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.SS dm-cache with separate data and metadata LVs
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\&
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The default cache mode is "writethrough". Writethrough ensures that any
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data written will be stored both in the cache pool LV and on the origin
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LV. The loss of a device associated with the cache pool LV in this case
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would not mean the loss of any data.
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When using dm-cache, the cache metadata and cache data can be stored on
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separate LVs. To do this, a "cache-pool LV" is created, which is a
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special LV that references two sub LVs, one for data and one for metadata.
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To create a cache-pool LV from two separate LVs:
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.nf
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$ lvcreate -n fastpool -L DataSize vg /dev/fast1
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$ lvcreate -n fastpoolmeta -L MetadataSize vg /dev/fast2
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$ lvconvert --type cache-pool --poolmetadata fastpoolmeta vg/fastpool
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.fi
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Then use the cache-pool LV to start caching the main LV:
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.nf
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$ lvconvert --type cache --cachepool fastpool vg/main
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.fi
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A variation of the same procedure automatically creates a cache-pool when
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caching is started. To do this, use a standard LV as the --cachepool
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(this will hold cache data), and use another standard LV as the
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--poolmetadata (this will hold cache metadata). LVM will create a
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cache-pool LV from the two specified LVs, and use the cache-pool to start
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caching the main LV.
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.nf
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$ lvcreate -n fastpool -L DataSize vg /dev/fast1
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$ lvcreate -n fastpoolmeta -L MetadataSize vg /dev/fast2
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$ lvconvert --type cache --cachepool fastpool \\
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--poolmetadata fastpoolmeta vg/main
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.fi
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.SS dm-cache cache modes
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\&
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The default dm-cache cache mode is "writethrough". Writethrough ensures
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that any data written will be stored both in the cache and on the origin
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LV. The loss of a device associated with the cache in this case would not
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mean the loss of any data.
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A second cache mode is "writeback". Writeback delays writing data blocks
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from the cache pool back to the origin LV. This mode will increase
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performance, but the loss of a device associated with the cache pool LV
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can result in lost data.
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from the cache back to the origin LV. This mode will increase
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performance, but the loss of a cache device can result in lost data.
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With the --cachemode option, the cache mode can be set when creating a
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cache LV, or changed on an existing cache LV. The current cache mode of a
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cache LV can be displayed with the cache_mode reporting option:
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With the --cachemode option, the cache mode can be set when caching is
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started, or changed on an LV that is already cached. The current cache
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mode can be displayed with the cache_mode reporting option:
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.B lvs -o+cache_mode VG/CacheLV
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.B lvs -o+cache_mode VG/LV
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.BR lvm.conf (5)
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.B allocation/cache_mode
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.br
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defines the default cache mode.
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.I Example
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.nf
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0. Create an origin LV we wish to cache (yours may already exist)
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# lvcreate -L 10G -n lv1 vg /dev/slow
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$ lvconvert --type cache --cachepool fast \\
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--cachemode writethrough vg/main
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.nf
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1. Create a cache data LV
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# lvcreate -L 1G -n cache1 vg /dev/fast
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2. Create a cache metadata LV
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# lvcreate -L 8M -n cache1meta vg /dev/fast
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3. Create a cache pool LV
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# lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
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4. Create a cache LV by combining the cache pool LV and origin LV,
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and use the writethrough cache mode.
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# lvconvert --type cache --cachepool vg/cache1 \\
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--cachemode writethrough vg/lv1
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.fi
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.SS Cache policy
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.SS dm-cache chunk size
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\&
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The cache subsystem has additional per-LV parameters: the cache policy to
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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 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.
|
||||
|
||||
The "mq" policy has a number of tunable parameters. The defaults are
|
||||
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:
|
||||
.br
|
||||
.B lvs -o+chunksize VG/LV
|
||||
|
||||
.BR lvm.conf (5)
|
||||
.B cache_pool_chunk_size
|
||||
.br
|
||||
controls the default chunk size.
|
||||
|
||||
The default value is shown by:
|
||||
.br
|
||||
.B lvmconfig --type default allocation/cache_pool_chunk_size
|
||||
|
||||
|
||||
.SS 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 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:
|
||||
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:
|
||||
|
||||
.B lvs -o+cache_policy,cache_settings VG/CacheLV
|
||||
.B lvs -o+cache_policy,cache_settings VG/LV
|
||||
|
||||
.I Example
|
||||
.nf
|
||||
Change the cache policy and settings of an existing cache LV.
|
||||
# lvchange --cachepolicy mq --cachesettings \\
|
||||
\(aqmigration_threshold=2048 random_threshold=4\(aq vg/lv1
|
||||
Change the cache policy and settings of an existing LV.
|
||||
|
||||
$ lvchange --cachepolicy mq --cachesettings \\
|
||||
\(aqmigration_threshold=2048 random_threshold=4\(aq vg/main
|
||||
.fi
|
||||
|
||||
.BR lvm.conf (5)
|
||||
@ -301,37 +329,7 @@ defines the default cache policy.
|
||||
.br
|
||||
defines the default cache settings.
|
||||
|
||||
|
||||
.SS 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:
|
||||
.br
|
||||
.B lvs -o+chunksize VG/CacheLV
|
||||
|
||||
.BR lvm.conf (5)
|
||||
.B cache_pool_chunk_size
|
||||
.br
|
||||
controls the default chunk size used when creating a cache LV.
|
||||
|
||||
The default value is shown by:
|
||||
.br
|
||||
.B lvmconfig --type default allocation/cache_pool_chunk_size
|
||||
|
||||
|
||||
.SS Spare metadata LV
|
||||
.SS dm-cache spare metadata LV
|
||||
|
||||
\&
|
||||
|
||||
@ -340,87 +338,30 @@ See
|
||||
for a description of the "pool metadata spare" LV.
|
||||
The same concept is used for cache pools.
|
||||
|
||||
.SS Automatic pool metadata LV
|
||||
.SS dm-cache metadata formats
|
||||
|
||||
\&
|
||||
|
||||
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.
|
||||
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 \fB2\fP 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 \fBauto\fP, lvm selects the best
|
||||
option provided by the current dm-cache kernel module.
|
||||
|
||||
.B lvcreate -n CacheDataLV -L CacheSize VG
|
||||
.br
|
||||
.B lvconvert --type cache-pool VG/CacheDataLV
|
||||
|
||||
|
||||
.SS Create a new cache LV without an existing origin LV
|
||||
.SS mirrored cache device
|
||||
|
||||
\&
|
||||
|
||||
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.
|
||||
|
||||
.B lvcreate --type cache -L LargeSize -n CacheLV
|
||||
.RS
|
||||
.B --cachepool VG/CachePoolLV VG SlowPVs
|
||||
.RE
|
||||
|
||||
|
||||
.SS 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.
|
||||
|
||||
.B lvcreate --type cache-pool -L CacheSize -n CachePoolLV VG FastPVs
|
||||
|
||||
|
||||
.SS 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.
|
||||
|
||||
.B lvcreate -n OriginLV -L LargeSize VG
|
||||
.br
|
||||
.B lvcreate -n CacheDataLV -L CacheSize VG
|
||||
.br
|
||||
.B lvconvert --type cache --cachepool VG/CataDataLV VG/OriginLV
|
||||
|
||||
This is equivalent to:
|
||||
|
||||
.B lvcreate -n OriginLV -L LargeSize VG
|
||||
.br
|
||||
.B lvcreate -n CacheDataLV -L CacheSize VG
|
||||
.br
|
||||
.B lvconvert --type cache-pool VG/CacheDataLV
|
||||
.br
|
||||
.B lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
|
||||
|
||||
|
||||
.SS 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.
|
||||
Format \fB2\fP 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 \fBauto\fP, lvm selects the best option provided by the current
|
||||
dm-cache kernel target.
|
||||
|
||||
.B lvconvert --type cache-pool --cachemetadataformat auto|1|2
|
||||
.RS
|
||||
.B VG/CacheDataLV
|
||||
.RE
|
||||
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.)
|
||||
|
||||
.nf
|
||||
$ 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 --cachepool fast vg/main
|
||||
.fi
|
||||
|
||||
.SH SEE ALSO
|
||||
.BR lvm.conf (5),
|
||||
|
||||
Loading…
Reference in New Issue
Block a user