mirror of
git://sourceware.org/git/lvm2.git
synced 2024-12-22 17:35:59 +03:00
229e63b638
instead of a separate --writecacheblocksize option. writecache block_size is not technically a setting, but it can borrow the option as a special case.
380 lines
11 KiB
Plaintext
380 lines
11 KiB
Plaintext
.TH "LVMCACHE" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\""
|
|
.SH NAME
|
|
lvmcache \(em LVM caching
|
|
|
|
.SH DESCRIPTION
|
|
|
|
\fBlvm\fP(8) includes two kinds of caching that can be used to improve the
|
|
performance of a Logical Volume (LV). Typically, a smaller, faster device
|
|
is used to improve i/o performance of a larger, slower LV. To do this, a
|
|
separate LV is created from the faster device, and then the original LV is
|
|
converted to start using the fast LV.
|
|
|
|
The two kinds of caching are:
|
|
|
|
.IP \[bu] 2
|
|
A read and write hot-spot cache, using the dm-cache kernel module. This
|
|
cache is slow moving, and adjusts the cache content over time so that the
|
|
most used parts of the LV are kept on the faster device. Both reads and
|
|
writes use the cache. LVM refers to this using the LV type \fBcache\fP.
|
|
|
|
.IP \[bu] 2
|
|
A streaming write cache, using the dm-writecache kernel module. This
|
|
cache is intended to be used with SSD or PMEM devices to speed up all
|
|
writes to an LV. Reads do not use this cache. LVM refers to this using
|
|
the LV type \fBwritecache\fP.
|
|
|
|
.SH USAGE
|
|
|
|
Both kinds of caching use similar lvm commands:
|
|
|
|
.B 1. Identify main LV that needs caching
|
|
|
|
A main LV exists on slower devices.
|
|
|
|
.nf
|
|
$ lvcreate -n main -L Size vg /dev/slow
|
|
.fi
|
|
|
|
.B 2. Identify fast LV to use as the cache
|
|
|
|
A fast LV exists on faster devices. This LV will be used to hold the
|
|
cache.
|
|
|
|
.nf
|
|
$ lvcreate -n fast -L Size vg /dev/fast
|
|
|
|
$ lvs vg -o+devices
|
|
LV VG Attr LSize Devices
|
|
fast vg -wi------- xx.00m /dev/fast(0)
|
|
main vg -wi------- yyy.00m /dev/slow(0)
|
|
.fi
|
|
|
|
.B 3. Start caching the main LV
|
|
|
|
To start caching the main LV using the fast LV, convert the main LV to the
|
|
desired caching type, and specify the fast LV to use:
|
|
|
|
.nf
|
|
using dm-cache:
|
|
|
|
$ lvconvert --type cache --cachepool fast vg/main
|
|
|
|
or dm-writecache:
|
|
|
|
$ lvconvert --type writecache --cachepool fast vg/main
|
|
.fi
|
|
|
|
.B 4. Display LVs
|
|
|
|
Once the fast LV has been attached to the main LV, lvm reports the main LV
|
|
type as either \fBcache\fP or \fBwritecache\fP depending on the type used.
|
|
While attached, the fast LV is hidden, and only displayed when lvs is
|
|
given -a. The _corig or _wcorig LV represents the original LV without the
|
|
cache.
|
|
|
|
.nf
|
|
using dm-cache:
|
|
|
|
$ lvs -a -o name,vgname,lvattr,origin,segtype,devices vg
|
|
LV VG Attr Origin Type Devices
|
|
[fast] vg Cwi-aoC--- linear /dev/fast(xx)
|
|
main vg Cwi-a-C--- [main_corig] cache main_corig(0)
|
|
[main_corig] vg owi-aoC--- linear /dev/slow(0)
|
|
|
|
or dm-writecache:
|
|
|
|
$ lvs -a -o name,vgname,lvattr,origin,segtype,devices vg
|
|
LV VG Attr Origin Type Devices
|
|
[fast] vg -wi-ao---- linear /dev/fast(xx)
|
|
main vg Cwi-a----- [main_wcorig] writecache main_wcorig(0)
|
|
[main_wcorig] vg -wi-ao---- linear /dev/slow(0)
|
|
.fi
|
|
|
|
.B 5. Use the main LV
|
|
|
|
Use the LV until the cache is no longer wanted, or needs to be changed.
|
|
|
|
.B 6. Stop caching
|
|
|
|
To stop caching the main LV, separate the fast LV from the main LV. This
|
|
changes the type of the main LV back to what it was before the cache was
|
|
attached.
|
|
|
|
.nf
|
|
$ lvconvert --splitcache vg/main
|
|
.fi
|
|
|
|
|
|
.SH OPTIONS
|
|
|
|
\&
|
|
|
|
.SS dm-writecache block size
|
|
|
|
\&
|
|
|
|
The dm-writecache block size can be 4096 bytes (the default), or 512
|
|
bytes. The default 4096 has better performance and should be used except
|
|
when 512 is necessary for compatibility. The dm-writecache block size is
|
|
specified with --cachesettings block_size=4096|512 when caching is started.
|
|
|
|
When a file system like xfs already exists on the main LV prior to
|
|
caching, and the file system is using a block size of 512, then the
|
|
writecache block size should be set to 512. (The file system will likely
|
|
fail to mount if writecache block size of 4096 is used in this case.)
|
|
|
|
Check the xfs sector size while the fs is mounted:
|
|
|
|
.nf
|
|
$ xfs_info /dev/vg/main
|
|
Look for sectsz=512 or sectsz=4096
|
|
.fi
|
|
|
|
The writecache block size should be chosen to match the xfs sectsz value.
|
|
|
|
It is also possible to specify a sector size of 4096 to mkfs.xfs when
|
|
creating the file system. In this case the writecache block size of 4096
|
|
can be used.
|
|
|
|
.SS dm-writecache settings
|
|
|
|
\&
|
|
|
|
Tunable parameters can be passed to the dm-writecache kernel module using
|
|
the --cachesettings option when caching is started, e.g.
|
|
|
|
.nf
|
|
$ lvconvert --type writecache --cachepool fast \\
|
|
--cachesettings 'high_watermark=N writeback_jobs=N' vg/main
|
|
.fi
|
|
|
|
Tunable options are:
|
|
|
|
.IP \[bu] 2
|
|
high_watermark = <count>
|
|
|
|
Start writeback when the number of used blocks reach this watermark
|
|
|
|
.IP \[bu] 2
|
|
low_watermark = <count>
|
|
|
|
Stop writeback when the number of used blocks drops below this watermark
|
|
|
|
.IP \[bu] 2
|
|
writeback_jobs = <count>
|
|
|
|
Limit the number of blocks that are in flight during writeback. Setting
|
|
this value reduces writeback throughput, but it may improve latency of
|
|
read requests.
|
|
|
|
.IP \[bu] 2
|
|
autocommit_blocks = <count>
|
|
|
|
When the application writes this amount of blocks without issuing the
|
|
FLUSH request, the blocks are automatically commited.
|
|
|
|
.IP \[bu] 2
|
|
autocommit_time = <milliseconds>
|
|
|
|
The data is automatically commited if this time passes and no FLUSH
|
|
request is received.
|
|
|
|
.IP \[bu] 2
|
|
fua = 0|1
|
|
|
|
Use the FUA flag when writing data from persistent memory back to the
|
|
underlying device.
|
|
Applicable only to persistent memory.
|
|
|
|
.IP \[bu] 2
|
|
nofua = 0|1
|
|
|
|
Don't use the FUA flag when writing back data and send the FLUSH request
|
|
afterwards. Some underlying devices perform better with fua, some with
|
|
nofua. Testing is necessary to determine which.
|
|
Applicable only to persistent memory.
|
|
|
|
|
|
.SS dm-cache with separate data and metadata LVs
|
|
|
|
\&
|
|
|
|
When using dm-cache, the cache metadata and cache data can be stored on
|
|
separate LVs. To do this, a "cache-pool LV" is created, which is a
|
|
special LV that references two sub LVs, one for data and one for metadata.
|
|
|
|
To create a cache-pool LV from two separate LVs:
|
|
|
|
.nf
|
|
$ lvcreate -n fastpool -L DataSize vg /dev/fast1
|
|
$ lvcreate -n fastpoolmeta -L MetadataSize vg /dev/fast2
|
|
$ lvconvert --type cache-pool --poolmetadata fastpoolmeta vg/fastpool
|
|
.fi
|
|
|
|
Then use the cache-pool LV to start caching the main LV:
|
|
|
|
.nf
|
|
$ lvconvert --type cache --cachepool fastpool vg/main
|
|
.fi
|
|
|
|
A variation of the same procedure automatically creates a cache-pool when
|
|
caching is started. To do this, use a standard LV as the --cachepool
|
|
(this will hold cache data), and use another standard LV as the
|
|
--poolmetadata (this will hold cache metadata). LVM will create a
|
|
cache-pool LV from the two specified LVs, and use the cache-pool to start
|
|
caching the main LV.
|
|
|
|
.nf
|
|
$ lvcreate -n fastpool -L DataSize vg /dev/fast1
|
|
$ lvcreate -n fastpoolmeta -L MetadataSize vg /dev/fast2
|
|
$ lvconvert --type cache --cachepool fastpool \\
|
|
--poolmetadata fastpoolmeta vg/main
|
|
.fi
|
|
|
|
.SS dm-cache cache modes
|
|
|
|
\&
|
|
|
|
The default dm-cache cache mode is "writethrough". Writethrough ensures
|
|
that any data written will be stored both in the cache and on the origin
|
|
LV. The loss of a device associated with the cache in this case would not
|
|
mean the loss of any data.
|
|
|
|
A second cache mode is "writeback". Writeback delays writing data blocks
|
|
from the cache back to the origin LV. This mode will increase
|
|
performance, but the loss of a cache device can result in lost data.
|
|
|
|
With the --cachemode option, the cache mode can be set when caching is
|
|
started, or changed on an LV that is already cached. The current cache
|
|
mode can be displayed with the cache_mode reporting option:
|
|
|
|
.B lvs -o+cache_mode VG/LV
|
|
|
|
.BR lvm.conf (5)
|
|
.B allocation/cache_mode
|
|
.br
|
|
defines the default cache mode.
|
|
|
|
.nf
|
|
$ lvconvert --type cache --cachepool fast \\
|
|
--cachemode writethrough vg/main
|
|
.nf
|
|
|
|
.SS dm-cache chunk size
|
|
|
|
\&
|
|
|
|
The size of data blocks managed by dm-cache can be specified with the
|
|
--chunksize option when caching is started. The default unit is KiB. The
|
|
value must be a multiple of 32KiB between 32KiB and 1GiB.
|
|
|
|
Using a chunk size that is too large can result in wasteful use of the
|
|
cache, in which small reads and writes cause large sections of an LV to be
|
|
stored in the cache. However, choosing a chunk size that is too small
|
|
can result in more overhead trying to manage the numerous chunks that
|
|
become mapped into the cache. Overhead can include both excessive CPU
|
|
time searching for chunks, and excessive memory tracking chunks.
|
|
|
|
Command to display the chunk size:
|
|
.br
|
|
.B lvs -o+chunksize VG/LV
|
|
|
|
.BR lvm.conf (5)
|
|
.B cache_pool_chunk_size
|
|
.br
|
|
controls the default chunk size.
|
|
|
|
The default value is shown by:
|
|
.br
|
|
.B lvmconfig --type default allocation/cache_pool_chunk_size
|
|
|
|
|
|
.SS dm-cache cache policy
|
|
|
|
\&
|
|
|
|
The dm-cache subsystem has additional per-LV parameters: the cache policy
|
|
to use, and possibly tunable parameters for the cache policy. Three
|
|
policies are currently available: "smq" is the default policy, "mq" is an
|
|
older implementation, and "cleaner" is used to force the cache to write
|
|
back (flush) all cached writes to the origin LV.
|
|
|
|
The older "mq" policy has a number of tunable parameters. The defaults are
|
|
chosen to be suitable for the majority of systems, but in special
|
|
circumstances, changing the settings can improve performance.
|
|
|
|
With the --cachepolicy and --cachesettings options, the cache policy and
|
|
settings can be set when caching is started, or changed on an existing
|
|
cached LV (both options can be used together). The current cache policy
|
|
and settings can be displayed with the cache_policy and cache_settings
|
|
reporting options:
|
|
|
|
.B lvs -o+cache_policy,cache_settings VG/LV
|
|
|
|
.nf
|
|
Change the cache policy and settings of an existing LV.
|
|
|
|
$ lvchange --cachepolicy mq --cachesettings \\
|
|
\(aqmigration_threshold=2048 random_threshold=4\(aq vg/main
|
|
.fi
|
|
|
|
.BR lvm.conf (5)
|
|
.B allocation/cache_policy
|
|
.br
|
|
defines the default cache policy.
|
|
|
|
.BR lvm.conf (5)
|
|
.B allocation/cache_settings
|
|
.br
|
|
defines the default cache settings.
|
|
|
|
.SS dm-cache spare metadata LV
|
|
|
|
\&
|
|
|
|
See
|
|
.BR lvmthin (7)
|
|
for a description of the "pool metadata spare" LV.
|
|
The same concept is used for cache pools.
|
|
|
|
.SS dm-cache metadata formats
|
|
|
|
\&
|
|
|
|
There are two disk formats for dm-cache metadata. The metadata format can
|
|
be specified with --cachemetadataformat when caching is started, and
|
|
cannot be changed. Format \fB2\fP has better performance; it is more
|
|
compact, and stores dirty bits in a separate btree, which improves the
|
|
speed of shutting down the cache. With \fBauto\fP, lvm selects the best
|
|
option provided by the current dm-cache kernel module.
|
|
|
|
.SS mirrored cache device
|
|
|
|
\&
|
|
|
|
The fast LV holding the cache can be created as a raid1 mirror so that it
|
|
can tolerate a device failure. (When using dm-cache with separate data
|
|
and metadata LVs, each of the sub-LVs can use raid1.)
|
|
|
|
.nf
|
|
$ lvcreate -n main -L Size vg /dev/slow
|
|
$ lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/fast1 /dev/fast2
|
|
$ lvconvert --type cache --cachepool fast vg/main
|
|
.fi
|
|
|
|
.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)
|