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man: updates to lvmcache
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David Teigland
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@ -5,24 +5,32 @@ lvmcache \(em LVM caching
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.SH DESCRIPTION
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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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performance of a Logical Volume (LV). When caching, varying subsets of an
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LV's data are temporarily stored on a smaller, faster device (e.g. an SSD)
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to improve the performance of the LV.
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To do this with lvm, a new special LV is first created from the faster
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device. This LV will hold the cache. Then, the new fast LV is attached to
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the main LV by way of an lvconvert command. lvconvert inserts one of the
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device mapper caching targets into the main LV's i/o path. The device
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mapper target combines the main LV and fast LV into a hybrid device that looks
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like the main LV, but has better performance. While the main LV is being
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used, portions of its data will be temporarily and transparently stored on
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the special 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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A read and write hot-spot cache, using the dm-cache kernel module.
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This cache tracks access patterns and adjusts its content deliberately so
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that commonly used parts of the main LV are likely to be found on the fast
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storage. 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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A write cache, using the dm-writecache kernel module. This cache can be
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used with SSD or PMEM devices to speed up all writes to the main LV. Data
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read from the main LV is not stored in the cache, only newly written data.
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LVM refers to this using the LV type \fBwritecache\fP.
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.SH USAGE
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@ -30,30 +38,31 @@ 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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The main LV may already exist, and is located on larger, slower devices.
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A main LV would be created with a command like:
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.nf
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$ lvcreate -n main -L Size vg /dev/slow
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$ lvcreate -n main -L Size vg /dev/slow_hhd
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.fi
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.B 2. Identify fast LV to use as the cache
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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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A fast LV is created using one or more fast devices, like an SSD. This
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special LV will be used to hold the cache:
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.nf
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$ lvcreate -n fast -L Size vg /dev/fast
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$ lvcreate -n fast -L Size vg /dev/fast_ssd
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$ lvs -a
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LV Attr Type Devices
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fast -wi------- linear /dev/fast
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main -wi------- linear /dev/slow
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fast -wi------- linear /dev/fast_ssd
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main -wi------- linear /dev/slow_hhd
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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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To start caching the main LV, convert the main LV to the desired caching
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type, and specify the fast LV to use as the cache:
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.nf
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using dm-cache:
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@ -83,16 +92,16 @@ using dm-cache:
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$ lvs -a
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LV Pool Type Devices
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main [fast_cvol] cache main_corig(0)
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[fast_cvol] linear /dev/fast
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[main_corig] linear /dev/slow
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[fast_cvol] linear /dev/fast_ssd
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[main_corig] linear /dev/slow_hhd
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using dm-writecache:
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$ lvs -a
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LV Pool Type Devices
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main [fast_cvol] writecache main_wcorig(0)
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[fast_cvol] linear /dev/fast
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[main_wcorig] linear /dev/slow
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[fast_cvol] linear /dev/fast_ssd
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[main_wcorig] linear /dev/slow_hhd
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using dm-cache (with cachepool):
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@ -100,9 +109,9 @@ using dm-cache (with cachepool):
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LV Pool Type Devices
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main [fast_cpool] cache main_corig(0)
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[fast_cpool] cache-pool fast_pool_cdata(0)
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[fast_cpool_cdata] linear /dev/fast
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[fast_cpool_cmeta] linear /dev/fast
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[main_corig] linear /dev/slow
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[fast_cpool_cdata] linear /dev/fast_ssd
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[fast_cpool_cmeta] linear /dev/fast_ssd
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[main_corig] linear /dev/slow_hhd
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.fi
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.B 5. Use the main LV
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@ -120,8 +129,8 @@ attached.
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$ lvs -a
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LV VG Attr Type Devices
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fast vg -wi------- linear /dev/fast
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main vg -wi------- linear /dev/slow
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fast vg -wi------- linear /dev/fast_ssd
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main vg -wi------- linear /dev/slow_hhd
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.fi
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@ -137,21 +146,21 @@ attached.
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.I LV
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.br
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Pass this option a standard LV. With a cachevol, cache data and metadata
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are contained within the single LV. This is used with dm-writecache or
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dm-cache.
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Pass this option a fast LV that should be used to hold the cache. With a
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cachevol, cache data and metadata are stored in different parts of the
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same fast LV. This option can be used with dm-writecache or dm-cache.
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.B --cachepool
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.IR CachePoolLV | LV
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.br
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Pass this option a cache pool object. With a cache pool, lvm places cache
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data and cache metadata on different LVs. The two LVs together are called
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a cache pool. This permits specific placement of data and metadata. A
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cache pool is represented as a special type of LV that cannot be used
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directly. (If a standard LV is passed to this option, lvm will first
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convert it to a cache pool by combining it with another LV to use for
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metadata.) This can be used with dm-cache.
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Pass this option a cachepool LV or a standard LV. When using a cache
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pool, lvm places cache data and cache metadata on different LVs. The two
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LVs together are called a cache pool. This permits specific placement of
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data and metadata. A cache pool is represented as a special type of LV
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that cannot be used directly. If a standard LV is passed with this
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option, lvm will first convert it to a cache pool by combining it with
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another LV to use for metadata. This option can be used with dm-cache.
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\&
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@ -267,8 +276,8 @@ LV that references two sub LVs, one for data and one for metadata.
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To create a cache pool from two separate LVs:
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.nf
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$ lvcreate -n fast -L DataSize vg /dev/fast1
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$ lvcreate -n fastmeta -L MetadataSize vg /dev/fast2
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$ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
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$ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
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$ lvconvert --type cache-pool --poolmetadata fastmeta vg/fast
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.fi
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@ -286,8 +295,8 @@ 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 fast -L DataSize vg /dev/fast1
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$ lvcreate -n fastmeta -L MetadataSize vg /dev/fast2
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$ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
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$ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
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$ lvconvert --type cache --cachepool fast --poolmetadata fastmeta vg/main
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.fi
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@ -418,7 +427,7 @@ and metadata LVs, each of the sub-LVs can use raid1.)
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.nf
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$ lvcreate -n main -L Size vg /dev/slow
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$ lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/fast1 /dev/fast2
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$ lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/ssd1 /dev/ssd2
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$ lvconvert --type cache --cachevol fast vg/main
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.fi
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