.TH "LVMCACHE" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\"" .SH NAME lvmcache \(em 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)