| |
lvmcache(7) - phpMan
LVMCACHE(7) LVMCACHE(7)
NAME
lvmcache — LVM caching
DESCRIPTION
The cache 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 cache pool LV. The large slow LV is called the ori‐
gin LV. Due to requirements from dm-cache (the kernel driver), LVM further splits the
cache pool LV into two devices - the cache data LV and cache metadata LV. 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.
Cache Terms
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
Cache Usage
The primary method for using a cache type logical volume:
0. create OriginLV
Create an LV or identify an existing LV to be the origin LV.
lvcreate -n OriginLV -L LargeSize VG SlowPVs
Example
# lvcreate -n lvol0 -L 100G vg
1. create CacheDataLV
Create the cache data LV. This LV will hold data blocks from the OriginLV. The size of
this LV is the size of the cache and will be reported as the size of the cache pool LV.
lvcreate -n CacheDataLV -L CacheSize VG FastPVs
Example
# lvcreate -n cache0 -L 10G vg /dev/fast
2. create CacheMetaLV
Create the cache metadata LV. This LV will hold cache pool metadata. The size of this LV
should be 1000 times smaller than the cache data LV, with a minimum size of 8MiB.
lvcreate -n CacheMetaLV -L MetaSize VG FastPVs
Example
# lvcreate -n cache0meta -L 12M vg /dev/fast
# lvs -a vg
LV VG Attr LSize Pool Origin
cache0 vg -wi-a----- 10.00g
cache0meta vg -wi-a----- 12.00m
lvol0 vg -wi-a----- 100.00g
3. create CachePoolLV
Combine the data and metadata LVs into a cache pool LV. The behavior of the cache pool LV
can be set in this step.
CachePoolLV takes the name of CacheDataLV.
CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
lvconvert --cachepool VG/CacheDataLV --poolmetadata VG/CacheMetaLV
Example
# lvconvert --cachepool vg/cache0 --poolmetadata vg/cache0meta
# lvs -a vg
LV VG Attr LSize Pool Origin
cache0 vg Cwi---C--- 10.00g
[cache0_cdata] vg Cwi------- 10.00g
[cache0_cmeta] vg ewi------- 12.00m
lvol0 vg -wi-a----- 100.00g
4. create CacheLV
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. This can be done while the origin LV is in use.
CacheLV takes the name of OriginLV.
OriginLV is renamed OriginLV_corig and becomes hidden.
lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
Example
# lvconvert --type cache --cachepool vg/cache0 vg/lvol0
# lvs -a vg
LV VG Attr LSize Pool Origin
cache0 vg Cwi---C--- 10.00g
[cache0_cdata] vg Cwi-ao---- 10.00g
[cache0_cmeta] vg ewi-ao---- 12.00m
lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
[lvol0_corig] vg -wi-ao---- 100.00g
Cache Removal
Removing a cache pool LV without removing its linked origin LV
This writes back data from the cache pool to the origin LV when necessary, then removes
the cache pool LV, leaving the un-cached origin LV.
lvremove VG/CachePoolLV
Example
# lvs vg
LV VG Attr LSize Pool Origin
cache0 vg Cwi---C--- 10.00g
lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
# lvremove vg/cache0
# lvs vg
LV VG Attr LSize Pool Origin
lvol0 vg -wi-a----- 100.00g
Removing an origin LV without removing its linked cache pool LV
This removes the origin LV, leaving the cache pool LV which can be linked to another ori‐
gin LV.
lvremove VG/CacheLV
Example
# lvs vg
LV VG Attr LSize Pool Origin
cache0 vg Cwi---C--- 10.00g
lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
# lvremove vg/lvol0
# lvs vg
LV VG Attr LSize Pool Origin
cache0 vg Cwi---C--- 10.00g
Cache Topics
Tolerate device failures in a 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.
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 --cachepool vg/lvx_cache --poolmetadata vg/lvx_cache_meta
4. Create a cached LV by combining the cache pool LV and origin LV
# lvconvert --type cache --cachepool vg/lvx_cache vg/lvx
Writethough caching
Writethrough caching ensures 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, writethrough vs. writeback.
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 --cachepool vg/lvx_cache --poolmetadata vg/lvx_cache_meta \
--cachemode writethrough
4. Create a cache LV by combining the cache pool LV and origin LV
# lvconvert --type cache --cachepool vg/lvx_cache vg/lvx
Spare metadata LV
See lvmthin(7) for a description of the "pool metadata spare" LV. The same concept is
used for cache pools.
Automatic pool metadata LV
A cache data LV can be converted to cache pool LV without specifying a cache pool metadata
LV. LVM will automatically create a metadata LV from the same VG.
lvcreate -n CacheDataLV -L CacheSize VG
lvconvert --cachepool VG/CacheDataLV
SEE ALSO
lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8), lvremove(8), lvre‐
name(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8), vgreduce(8), vgsplit(8)
Red Hat, Inc LVM TOOLS 2.02.111(2) (2014-09-01) LVMCACHE(7)
|
