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man: rework lvmcache to match lvmthin
Reorganize and rewrite parts to match lvmthin(7).
This commit is contained in:
David Teigland
parent
50961f43d0
commit
864ff3cb18
@ -4,7 +4,7 @@ lvmcache \(em 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 \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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@ -27,59 +27,60 @@ 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 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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.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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.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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.B lvcreate \-n OriginLV \-L LargeSize VG SlowPVs
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.SH Cache Commands
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0. create OriginLV
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.I Example
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.br
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.B lvcreate \-L LargeSize \-n OriginLV VG SlowPVs
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# lvcreate \-n lvol0 \-L 100G vg
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1. create CacheDataLV
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.br
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.B lvcreate \-L CacheSize \-n CacheDataLV VG FastPVs
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2. create CacheMetaLV
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.br
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.B lvcreate \-L MetaSize \-n CacheMetaLV VG FastPVs
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.SS 1. create CacheDataLV
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3. create CachePoolLV
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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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.B lvconvert \-\-cachepool VG/CacheDataLV \-\-poolmetadata VG/CacheMetaLV
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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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@ -87,44 +88,108 @@ 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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4. create CacheLV
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.B lvconvert \-\-cachepool VG/CacheDataLV \-\-poolmetadata VG/CacheMetaLV
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.I Example
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.br
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.B lvconvert \-\-type cache \-\-cachepool VG/CachePoolLV VG/OriginLV
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# lvconvert \-\-cachepool vg/cache0 \-\-poolmetadata vg/cache0meta
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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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.SH Cache Examples
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.SS Example 1
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Create a simple cache LV.
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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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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 \-\-cachepool vg/lvx_cache \-\-poolmetadata vg/lvx_cache_meta
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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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# 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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.SS Example 2
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Create 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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.SH Cache Removal
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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 un-cached origin LV.
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.B lvremove VG/CachePoolLV
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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 an origin LV without removing its linked cache pool LV
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\&
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This removes the origin LV, leaving the cache pool LV which can be linked
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to another origin LV.
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.B lvremove 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/lvol0
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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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.fi
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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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@ -145,16 +210,16 @@ redundant. Example 2 illustrates how to do that. Note that only steps
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# lvconvert \-\-type cache \-\-cachepool vg/lvx_cache vg/lvx
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.fi
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.SS Example 3
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Create a simple cache LV with \fIwritethough\fP caching.
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.SS Writethough 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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\&
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Writethrough caching ensures that any data written will be stored both in
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the cache pool LV and on the origin LV. The loss of a device associated
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with the cache pool LV in this case would not mean the loss of any data.
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When combining the cache data LV and the cache metadata LV to form the
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cache pool LV, properties of the cache can be specified - in this case,
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\fIwritethrough\fP vs. \fIwriteback\fP.
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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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@ -174,35 +239,27 @@ in this case.
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# lvconvert \-\-type cache \-\-cachepool vg/lvx_cache vg/lvx
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.fi
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.SH Cache Removal
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.SS Spare metadata LV
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.SS Removing a cache pool LV without removing an associated origin LV
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\&
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This writes back data from the cache pool to the origin LV, then removes
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the cache pool LV, leaving the un-cached origin LV. (If the origin LV
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will also be removed, the data flushing can be avoided; see "Removing
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both" below.)
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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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.B lvremove VG/CachePoolLV
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.SS Automatic pool metadata LV
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.SS Removing both an origin LV and its associated cache pool LV
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\&
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This removes the cache LV, which includes removing both the origin LV and
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its associated cache pool LV. Data blocks from the cache pool are not
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written back to the origin LV.
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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 lvremove VG/CacheLV
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.B lvcreate -n CacheDataLV -L CacheSize VG
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.br
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.B lvconvert --cachepool VG/CacheDataLV
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./" .SS Separating a cache pool LV from an origin LV
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./" This writes back data from the cache pool to the origin LV, then removes
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./" the association between the cache pool LV and the origin LV.
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./" .br
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./" Not yet implemented.
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./" .SS Removing an origin LV without removing an associated cache pool LV
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./" Separate the cache pool LV from the origin LV, then remove the origin LV.
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./" .br
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./" Not yet implemented.
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.SH SEE ALSO
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.BR lvm.conf (5),
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