[lvm-devel] master - man: lvmcache
Zdenek Kabelac
zkabelac at fedoraproject.org
Sun May 18 18:10:58 UTC 2014
Gitweb: http://git.fedorahosted.org/git/?p=lvm2.git;a=commitdiff;h=477351fc4d2f2d0a55089fb6b6bc1f26e2605f7b
Commit: 477351fc4d2f2d0a55089fb6b6bc1f26e2605f7b
Parent: 91284bd9b913f74a0071082d542c3c09d11524f2
Author: Zdenek Kabelac <zkabelac at redhat.com>
AuthorDate: Sun May 18 20:09:47 2014 +0200
Committer: Zdenek Kabelac <zkabelac at redhat.com>
CommitterDate: Sun May 18 20:09:47 2014 +0200
man: lvmcache
separate man page for lvm cache
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+.TH "LVMCACHE" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\""
+.SH NAME
+lvmcache - LVM caching
+
+.SH DESCRIPTION
+
+The \fIcache\fP logical volume type uses a small and fast LV to improve
+the performance of a large and slow LV. It does this by storing the
+frequently used blocks on the faster LV.
+LVM refers to the small fast LV as a \fBcache pool LV\fP. The large
+slow LV is called the \fBorigin LV\fP. Due to requirements from dm-cache
+(the kernel driver), LVM further splits the cache pool LV into two
+devices - the \fBcache data LV\fP and \fBcache metadata LV\fP. The cache
+data LV is where copies of data blocks are kept from the
+origin LV to increase speed. The cache metadata LV holds the
+accounting information that specifies where data blocks are stored (e.g.
+on the origin LV or on the cache data LV). Users should be familiar with
+these LVs if they wish to create the best and most robust cached
+logical volumes.
+
+.SS Cache Terms
+.nf
+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
+.fi
+
+.SS Cache Steps
+The steps to create a logical volume of \fIcache\fP type are as follows:
+.TP
+0.
+Create an LV or identify an existing LV to be the origin LV.
+.TP
+1.
+Create the cache data LV. The size of this LV is the size of the cache
+and will be reported as the size of the cache pool LV.
+.TP
+2.
+Create the cache metadata LV.
+The size of this LV should be 1000 times smaller than the cache data LV
+with a minimum size of 8MiB.
+.TP
+3.
+Create the cache pool LV by combining the cache data LV (from step 1)
+and cache metadata LV (from step 2). When performing this step,
+behavioral characteristics of the cache pool LV can be set.
+The name of the cache pool LV takes the name of the cache data LV and
+the cache data LV and cache metadata LV are renamed
+to CachePoolLV_cdata and CachePoolLV_cmeta.
+.TP
+4.
+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. Users can perform this step while the origin LV
+is in use.
+
+.P
+The steps above represent the best way to create a cache LV.
+They provide the most options and have the ability to create the
+most robust logical volumes. The examples below illustrate how these
+steps might be used in practice.
+
+.SS Cache Commands
+.nf
+0. create OriginLV
+lvcreate -L LargeSize -n OriginLV VG SlowPVs
+
+1. create CacheDataLV
+lvcreate -L CacheSize -n CacheDataLV VG FastPVs
+
+2. create CacheMetaLV
+lvcreate -L MetaSize -n CacheMetaLV VG FastPVs
+
+3. create CachePoolLV
+lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV VG/CacheDataLV
+CachePoolLV takes the name of CacheDataLV.
+CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
+CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
+
+4. create CacheLV
+lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
+CacheLV takes the name of OriginLV.
+OriginLV is renamed OriginLV_corig and becomes hidden.
+.fi
+
+.SS Cache Examples
+
+.B Example 1:
+Creating a simple cache LV.
+.br
+
+.nf
+0. Create the origin LV
+# lvcreate -L 10G -n lvx vg /dev/slow_dev
+
+1. Create a cache data LV
+# lvcreate -L 1G -n lvx_cache vg /dev/fast_dev
+
+2. Create a cache metadata LV (~1/1000th size of CacheDataLV or 8MiB)
+# lvcreate -L 8M -n lvx_cache_meta vg /dev/fast_dev
+
+3. Create a cache pool LV, combining cache data LV and cache metadata LV
+# lvconvert --type cache-pool --poolmetadata vg/lvx_cache_meta \\
+ vg/lvx_cache
+
+4. Create a cached LV by combining the cache pool LV and origin LV
+# lvconvert --type cache --cachepool vg/lvx_cache vg/lvx
+.fi
+
+.B Example 2:
+Creating a cache LV with a fault tolerant 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 2 illustrates how to do that. Note that only steps
+1 & 2 change.
+
+.nf
+0. Create an origin LV we wish to cache
+# lvcreate -L 10G -n lvx vg /dev/slow_devs
+
+1. Create a 2-way RAID1 cache data LV
+# lvcreate --type raid1 -m 1 -L 1G -n lvx_cache vg \\
+ /dev/fast1 /dev/fast2
+
+2. Create a 2-way RAID1 cache metadata LV
+# lvcreate --type raid1 -m 1 -L 8M -n lvx_cache_meta 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/lvx_cache_meta \\
+ vg/lvx_cache
+
+4. Create a cached LV by combining the cache pool LV and origin LV
+# lvconvert --type cache --cachepool vg/lvx_cache vg/lvx
+.fi
+
+.B Example 3:
+Creating a simple cache LV with \fIwritethough\fP caching.
+
+Some users wish to ensure 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. When
+combining the cache data LV and the cache metadata LV to form the cache pool
+LV, properties of the cache can be specified - in this case,
+\fIwritethrough\fP vs. \fIwriteback\fP. Note that only step 3 is affected
+in this case.
+
+.nf
+0. Create an origin LV we wish to cache (yours may already exist)
+# lvcreate -L 10G -n lvx vg /dev/slow
+
+1. Create a cache data LV
+# lvcreate -L 1G -n lvx_cache vg /dev/fast
+
+2. Create a cache metadata LV
+# lvcreate -L 8M -n lvx_cache_meta vg /dev/fast
+
+3. Create a cache pool LV specifying cache mode "writethrough"
+# lvconvert --type cache-pool --poolmetadata vg/lvx_cache_meta \\
+ --cachemode writethrough vg/lvx_cache
+
+4. Create a cache LV by combining the cache pool LV and origin LV
+# lvconvert --type cache --cachepool vg/lvx_cache vg/lvx
+.fi
+
+.SS Removing Cache Logical Volumes
+If you wish to remove all logical volumes associated with a cache
+LV, you must remove both top-level, user-visible devices.
+The cache metadata LV and cache data LV cannot be removed
+directly. If only the cache pool LV is specfied for removal, any cached
+blocks not yet on the origin LV will be flush, the cache pool LV will be
+removed, and the now un-cached origin LV will remain. If the user
+specifies a cache LV for removal, then the origin LV is
+removed and only the cache pool LV will remain. The cache pool LV can then
+be used to create another cache LV with a different origin LV if desired.
+
+When users intend to remove all logical volumes associated with a
+cache LV, it is generally better to start with the origin LV and then
+remove the cache pool LV. If the operations are performed in the
+reverse order, the user will have to wait for the contents of the
+cache pool LV to be flushed before the origin LV is removed. This
+could take some time.
+
+.SH SEE ALSO
+.BR lvm.conf (5),
+.BR lvchange (8),
+.BR lvcreate (8),
+.BR lvdisplay (8),
+.BR lvextend (8),
+.BR lvremove (8),
+.BR lvrename (8),
+.BR lvresize (8),
+.BR lvs (8),
+.BR vgchange (8),
+.BR vgmerge (8),
+.BR vgreduce (8),
+.BR vgsplit (8)
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