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man: lvmcache
separate man page for lvm cache
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Zdenek Kabelac
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man/lvmcache.7.in
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man/lvmcache.7.in
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.TH "LVMCACHE" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\""
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.SH NAME
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lvmcache - LVM caching
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.SH DESCRIPTION
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The \fIcache\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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.SS 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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.SS Cache Steps
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The steps to create a logical volume of \fIcache\fP type are as follows:
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.TP
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0.
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Create an LV or identify an existing LV to be the origin LV.
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.TP
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1.
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Create the cache data LV. The size of this LV is the size of the cache
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and will be reported as the size of the cache pool LV.
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.TP
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2.
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Create the cache metadata LV.
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The size of this LV should be 1000 times smaller than the cache data LV
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with a minimum size of 8MiB.
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.TP
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3.
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Create the cache pool LV by combining the cache data LV (from step 1)
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and cache metadata LV (from step 2). When performing this step,
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behavioral characteristics of the cache pool LV can be set.
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The name of the cache pool LV takes the name of the cache data LV and
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the cache data LV and cache metadata LV are renamed
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to CachePoolLV_cdata and CachePoolLV_cmeta.
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.TP
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4.
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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. Users can perform this step while the origin LV
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is in use.
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.P
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The steps above represent the best way to create a cache LV.
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They provide the most options and have the ability to create the
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most robust logical volumes. The examples below illustrate how these
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steps might be used in practice.
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.SS Cache Commands
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.nf
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0. create OriginLV
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lvcreate -L LargeSize -n OriginLV VG SlowPVs
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1. create CacheDataLV
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lvcreate -L CacheSize -n CacheDataLV VG FastPVs
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2. create CacheMetaLV
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lvcreate -L MetaSize -n CacheMetaLV VG FastPVs
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3. create CachePoolLV
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lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV VG/CacheDataLV
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CachePoolLV takes the name of CacheDataLV.
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CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
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CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
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4. create CacheLV
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lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
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CacheLV takes the name of OriginLV.
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OriginLV is renamed OriginLV_corig and becomes hidden.
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.fi
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.SS Cache Examples
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.B Example 1:
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Creating a simple cache LV.
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.br
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.nf
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0. Create the origin LV
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# lvcreate -L 10G -n lvx vg /dev/slow_dev
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1. Create a cache data LV
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# lvcreate -L 1G -n lvx_cache vg /dev/fast_dev
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2. Create a cache metadata LV (~1/1000th size of CacheDataLV or 8MiB)
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# lvcreate -L 8M -n lvx_cache_meta vg /dev/fast_dev
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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/lvx_cache_meta \\
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vg/lvx_cache
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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/lvx_cache vg/lvx
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.fi
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.B Example 2:
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Creating a cache LV with a fault tolerant cache pool LV.
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Users who are concerned about the possibility of failures in their fast devices
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that could lead to data loss might consider making their cache pool sub-LVs
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redundant. Example 2 illustrates how to do that. Note that only steps
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1 & 2 change.
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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 lvx 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 lvx_cache 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 lvx_cache_meta 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/lvx_cache_meta \\
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vg/lvx_cache
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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/lvx_cache vg/lvx
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.fi
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.B Example 3:
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Creating a simple cache LV with \fIwritethough\fP caching.
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Some users wish to ensure that any data written will be stored both in the
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cache pool LV and on the origin LV. The loss of a device associated with
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the cache pool LV in this case would not mean the loss of any data. When
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combining the cache data LV and the cache metadata LV to form the cache pool
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LV, properties of the cache can be specified - in this case,
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\fIwritethrough\fP vs. \fIwriteback\fP. Note that only step 3 is affected
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in this case.
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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 lvx vg /dev/slow
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1. Create a cache data LV
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# lvcreate -L 1G -n lvx_cache vg /dev/fast
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2. Create a cache metadata LV
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# lvcreate -L 8M -n lvx_cache_meta 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/lvx_cache_meta \\
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--cachemode writethrough vg/lvx_cache
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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/lvx_cache vg/lvx
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.fi
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.SS Removing Cache Logical Volumes
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If you wish to remove all logical volumes associated with a cache
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LV, you must remove both top-level, user-visible devices.
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The cache metadata LV and cache data LV cannot be removed
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directly. If only the cache pool LV is specfied for removal, any cached
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blocks not yet on the origin LV will be flush, the cache pool LV will be
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removed, and the now un-cached origin LV will remain. If the user
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specifies a cache LV for removal, then the origin LV is
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removed and only the cache pool LV will remain. The cache pool LV can then
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be used to create another cache LV with a different origin LV if desired.
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When users intend to remove all logical volumes associated with a
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cache LV, it is generally better to start with the origin LV and then
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remove the cache pool LV. If the operations are performed in the
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reverse order, the user will have to wait for the contents of the
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cache pool LV to be flushed before the origin LV is removed. This
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could take some time.
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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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