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381 lines
11 KiB
Groff
381 lines
11 KiB
Groff
.TH "LVMCACHE" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\""
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.SH NAME
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lvmcache \(em LVM caching
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.SH DESCRIPTION
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The \fBcache\fP logical volume type uses a small and fast LV to improve
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the performance of a large and slow LV. It does this by storing the
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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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logical volumes.
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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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.fi
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.SH Cache Usage
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The primary method for using a cache type logical volume:
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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
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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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.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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.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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\&
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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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\&
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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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.B lvremove VG/CachePoolLV
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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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.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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.fi
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.SS Removing a cache LV: both origin LV and the cache pool LV
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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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.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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.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_devs
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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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.fi
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.SS Cache mode
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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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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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The cache mode can be specified with the --cachemode option when a cache
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pool LV is created.
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.BR lvm.conf (5)
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.B cache_pool_cachemode
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.br
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defines the default cache mode.
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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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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 specifying cache mode "writethrough"
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# lvconvert \-\-type cache\-pool \-\-poolmetadata vg/cache1meta \\
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\-\-cachemode writethrough vg/cache1
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4. Create a cache 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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.fi
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The cache mode can be changed on an existing LV with the command:
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.B lvconvert --cachemode writethrough|writeback VG/CacheLV
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.SS Cache policy & policy settings
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\&
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The cache subsystem has an additional per-LV parameter, namely the cache policy
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to use, and possibly the tunable parameters of the said cache policy. In the
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current implementation, two policies are available, "mq" which is the default
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policy and "cleaner" which is used to force the cache to write back (flush) all
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cached writes to the origin LV. Moreover, the "mq" policy has a number of
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tunable parameters: the defaults are chosen to be suitable for the vast
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majority of systems. However, under special circumstances, changing the tunable
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settings of the cache policy can improve performance.
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On an existing cache LV, the policy can be set (to "mq") and the cache settings
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can be changed using commands like these:
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.I Example
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.nf
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# lvchange \-\-cachepolicy mq vg/lv1
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# lvchange \-\-cachesettings \(aqmigration_threshold=2048 random_threshold=4\(aq \\
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vg/lv1
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.fi
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Both commands can be combined, setting both cache policy and its settings
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together. Moreover, when creating a cache LV for the first time (using
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lvcreate), the \-\-cachepolicy and \-\-cachesettings parameters can be used as
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well. The current policy and the policy settings can be listed using the lvs
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command, using 'cache_policy' and 'cache_settings' fields:
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# lvs -o +cache_policy,cache_settings
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.SS Spare metadata LV
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\&
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See
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.BR lvmthin (7)
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for a description of the "pool metadata spare" LV.
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The same concept is used for cache pools.
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.SS Automatic pool metadata LV
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\&
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A cache data LV can be converted to cache pool LV without specifying a
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cache pool metadata LV. LVM will automatically create a metadata LV from
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the same VG.
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.B lvcreate -n CacheDataLV -L CacheSize VG
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.br
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.B lvconvert --type cache\-pool VG/CacheDataLV
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.SS Create a new cache LV without an existing origin LV
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\&
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A cache LV can be created using an existing cache pool without an existing
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origin LV. A new origin LV is created and linked to the cache pool in a
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single step.
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.B lvcreate \-\-type cache \-L LargeSize \-n CacheLV
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.RS
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.B \-\-cachepool VG/CachePoolLV VG SlowPVs
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.RE
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.SS Single step cache pool LV creation
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\&
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A cache pool LV can be created with a single lvcreate command, rather than
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using lvconvert on existing LVs. This one command creates a cache data
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LV, a cache metadata LV, and combines the two into a cache pool LV.
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.B lvcreate \-\-type cache\-pool \-L CacheSize \-n CachePoolLV VG FastPVs
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.SS Convert existing LVs to cache types
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\&
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When an existing origin LV is converted to a cache LV, the specified cache
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pool may be a normal LV, rather than a cache pool LV. In this case, lvm
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will first convert the normal LV to a cache pool LV. A pool metadata LV
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may optionally be specified.
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.B lvcreate -n OriginLV -L LargeSize VG
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.br
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.B lvcreate -n CacheDataLV -L CacheSize VG
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.br
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.B lvconvert --type cache --cachepool VG/CataDataLV VG/OriginLV
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This is equivalent to:
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.B lvcreate -n OriginLV -L LargeSize VG
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.br
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.B lvcreate -n CacheDataLV -L CacheSize VG
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.br
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.B lvconvert --type cache-pool VG/CacheDataLV
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.br
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.B lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
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.SH SEE ALSO
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.BR lvm.conf (5),
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.BR lvchange (8),
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.BR lvcreate (8),
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.BR lvdisplay (8),
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.BR lvextend (8),
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.BR lvremove (8),
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.BR lvrename (8),
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.BR lvresize (8),
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.BR lvs (8),
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.BR vgchange (8),
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.BR vgmerge (8),
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.BR vgreduce (8),
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.BR vgsplit (8)
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