LVMCACHE(7) LVMCACHE(7)
NAME
lvmcache -- LVM caching
DESCRIPTION
lvm(8) includes two kinds of caching that can be used to improve the
performance of a Logical Volume (LV). When caching, varying subsets of
an LV's data are temporarily stored on a smaller, faster device (e.g.
an SSD) to improve the performance of the LV.
To do this with lvm, a new special LV is first created from the faster
device. This LV will hold the cache. Then, the new fast LV is attached
to the main LV by way of an lvconvert command. lvconvert inserts one of
the device mapper caching targets into the main LV's i/o path. The
device mapper target combines the main LV and fast LV into a hybrid
device that looks like the main LV, but has better performance. While
the main LV is being used, portions of its data will be temporarily and
transparently stored on the special fast LV.
The two kinds of caching are:
o A read and write hot-spot cache, using the dm-cache kernel module.
This cache tracks access patterns and adjusts its content deliber-
ately so that commonly used parts of the main LV are likely to be
found on the fast storage. LVM refers to this using the LV type
cache.
o A write cache, using the dm-writecache kernel module. This cache can
be used with SSD or PMEM devices to speed up all writes to the main
LV. Data read from the main LV is not stored in the cache, only newly
written data. LVM refers to this using the LV type writecache.
USAGE
1. Identify main LV that needs caching
The main LV may already exist, and is located on larger, slower
devices. A main LV would be created with a command like:
# lvcreate -n main -L Size vg /dev/slow_hhd
2. Identify fast LV to use as the cache
A fast LV is created using one or more fast devices, like an SSD. This
special LV will be used to hold the cache:
# lvcreate -n fast -L Size vg /dev/fast_ssd
# lvs -a
LV Attr Type Devices
fast -wi------- linear /dev/fast_ssd
main -wi------- linear /dev/slow_hhd
3. Start caching the main LV
To start caching the main LV, convert the main LV to the desired
caching type, and specify the fast LV to use as the cache:
using dm-cache (with cachepool):
# lvconvert --type cache --cachepool fast vg/main
using dm-cache (with cachevol):
# lvconvert --type cache --cachevol fast vg/main
using dm-writecache (with cachevol):
# lvconvert --type writecache --cachevol fast vg/main
For more alternatives see:
dm-cache command shortcut
dm-cache with separate data and metadata LVs
4. Display LVs
Once the fast LV has been attached to the main LV, lvm reports the main
LV type as either cache or writecache depending on the type used.
While attached, the fast LV is hidden, and renamed with a _cvol or
_cpool suffix. It is displayed by lvs -a. The _corig or _wcorig LV
represents the original LV without the cache.
using dm-cache (with cachepool):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cpool] cache main_corig(0)
[fast_cpool] cache-pool fast_pool_cdata(0)
[fast_cpool_cdata] linear /dev/fast_ssd
[fast_cpool_cmeta] linear /dev/fast_ssd
[main_corig] linear /dev/slow_hhd
using dm-cache (with cachevol):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cvol] cache main_corig(0)
[fast_cvol] linear /dev/fast_ssd
[main_corig] linear /dev/slow_hhd
using dm-writecache (with cachevol):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cvol] writecache main_wcorig(0)
[fast_cvol] linear /dev/fast_ssd
[main_wcorig] linear /dev/slow_hhd
5. Use the main LV
Use the LV until the cache is no longer wanted, or needs to be changed.
6. Stop caching
To stop caching the main LV and also remove unneeded cache pool, use
the --uncache:
# lvconvert --uncache vg/main
# lvs -a
LV VG Attr Type Devices
main vg -wi------- linear /dev/slow_hhd
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.
# lvconvert --splitcache vg/main
# lvs -a
LV VG Attr Type Devices
fast vg -wi------- linear /dev/fast_ssd
main vg -wi------- linear /dev/slow_hhd
7. Create a new LV with caching
A new LV can be created with caching attached at the time of creation
using the following command:
# lvcreate --type cache|writecache -n Name -L Size
--cachedevice /dev/fast_ssd vg /dev/slow_hhd
The main LV is created with the specified Name and Size from the
slow_hhd. A hidden fast LV is created on the fast_ssd and is then
attached to the new main LV. If the fast_ssd is unused, the entire
disk will be used as the cache unless the --cachesize option is used to
specify a size for the fast LV. The --cachedevice option can be
repeated to use multiple disks for the fast LV.
OPTIONS
option args
--cachepool CachePoolLV|LV
Pass this option a cachepool LV or a standard LV. When using a cache
pool, lvm places cache data and cache metadata on different LVs. The
two LVs together are called a cache pool. This has a bit better per-
formance for dm-cache and permits specific placement and segment type
selection for data and metadata volumes. A cache pool is represented
as a special type of LV that cannot be used directly. If a standard LV
is passed with this option, lvm will first convert it to a cache pool
by combining it with another LV to use for metadata. This option can
be used with dm-cache.
--cachevol LV
Pass this option a fast LV that should be used to hold the cache. With
a cachevol, cache data and metadata are stored in different parts of
the same fast LV. This option can be used with dm-writecache or dm-
cache.
--cachedevice PV
This option can be used in place of --cachevol, in which case a
cachevol LV will be created using the specified device. This option
can be repeated to create a cachevol using multiple devices, or a tag
name can be specified in which case the cachevol will be created using
any of the devices with the given tag. If a named cache device is
unused, the entire device will be used to create the cachevol. To cre-
ate a cachevol of a specific size from the cache devices, include the
--cachesize option.
dm-cache block size
A cache pool will have a logical block size of 4096 bytes if it is cre-
ated on a device with a logical block size of 4096 bytes.
If a main LV has logical block size 512 (with an existing xfs file sys-
tem using that size), then it cannot use a cache pool with a 4096 logi-
cal block size. If the cache pool is attached, the main LV will likely
fail to mount.
To avoid this problem, use a mkfs option to specify a 4096 block size
for the file system, or attach the cache pool before running mkfs.
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:
# xfs_info /dev/vg/main
Look for sectsz=512 or sectsz=4096
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.
The writecache block size is displayed by the command:
lvs -o writecacheblocksize VG/LV
dm-writecache memory usage
The amount of main system memory used by dm-writecache can be a factor
when selecting the writecache cachevol size and the writecache block
size.
o writecache block size 4096: each 100 GiB of writecache cachevol uses
slightly over 2 GiB of system memory.
o writecache block size 512: each 100 GiB of writecache cachevol uses a
little over 16 GiB of system memory.
dm-writecache settings
Tunable parameters can be passed to the dm-writecache kernel module
using the --cachesettings option when caching is started, e.g.
# lvconvert --type writecache --cachevol fast \
--cachesettings 'high_watermark=N writeback_jobs=N' vg/main
Tunable options are:
high_watermark = <percent>
Start writeback when the writecache usage reaches this percent
(0-100).
low_watermark = <percent>
Stop writeback when the writecache usage reaches this percent
(0-100).
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.
autocommit_blocks = <count>
When the application writes this amount of blocks without issu-
ing the FLUSH request, the blocks are automatically committed.
autocommit_time = <milliseconds>
The data is automatically committed if this time passes and no
FLUSH request is received.
fua = 0|1
Use the FUA flag when writing data from persistent memory back
to the underlying device. Applicable only to persistent memory.
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.
cleaner = 0|1
Setting cleaner=1 enables the writecache cleaner mode in which
data is gradually flushed from the cache. If this is done prior
to detaching the writecache, then the splitcache command will
have little or no flushing to perform. If not done beforehand,
the splitcache command enables the cleaner mode and waits for
flushing to complete before detaching the writecache. Adding
cleaner=0 to the splitcache command will skip the cleaner mode,
and any required flushing is performed in device suspend.
dm-cache with separate data and metadata LVs
Preferred way of using dm-cache is to place the cache metadata and
cache data on separate LVs. To do this, a "cache pool" is created,
which is a special LV that references two sub LVs, one for data and one
for metadata.
To create a cache pool of given data size and let lvm2 calculate appro-
priate metadata size:
# lvcreate --type cache-pool -L DataSize -n fast vg /dev/fast_ssd1
To create a cache pool from separate LV and let lvm2 calculate appro-
priate cache metadata size:
# lvcreate -n fast -L DataSize vg /dev/fast_ssd1
# lvconvert --type cache-pool vg/fast /dev/fast_ssd1
To create a cache pool from two separate LVs:
# lvcreate -n fast -L DataSize vg /dev/fast_ssd1
# lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
# lvconvert --type cache-pool --poolmetadata fastmeta vg/fast
Then use the cache pool LV to start caching the main LV:
# lvconvert --type cache --cachepool fast vg/main
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.
# lvcreate -n fast -L DataSize vg /dev/fast_ssd1
# lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
# lvconvert --type cache --cachepool fast \
--poolmetadata fastmeta vg/main
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:
lvs -o+cache_mode VG/LV
lvm.conf(5) allocation/cache_mode
defines the default cache mode.
# lvconvert --type cache --cachemode writethrough \
--cachepool fast vg/main
# lvconvert --type cache --cachemode writethrough \
--cachevol fast vg/main
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. Cache
chunks bigger then 512KiB shall be only used when necessary.
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. It can also require increasing migration
threshold which defaults to 2048 sectors (1 MiB). Lvm2 ensures migra-
tion threshold is at least 8 chunks in size. This may in some cases
result in very high bandwidth load of transferring data between the
cache LV and its cache origin LV. 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:
lvs -o+chunksize VG/LV
lvm.conf(5) allocation/cache_pool_chunk_size
controls the default chunk size.
The default value is shown by:
lvmconfig --type default allocation/cache_pool_chunk_size
Checking migration threshold (in sectors) of running cached LV:
lvs -o+kernel_cache_settings VG/LV
dm-cache migration threshold
Migrating data between the origin and cache LV uses bandwidth. The
user can set a throttle to prevent more than a certain amount of migra-
tion occurring at any one time. Currently dm-cache is not taking any
account of normal io traffic going to the devices.
User can set migration threshold via cache policy settings as "migra-
tion_threshold=<#sectors>" to set the maximum number of sectors being
migrated, the default being 2048 sectors (1MiB).
Command to set migration threshold to 2MiB (4096 sectors):
lvcreate --cachepolicy 'migration_threshold=4096' VG/LV
Command to display the migration threshold:
lvs -o+kernel_cache_settings,cache_settings VG/LV
lvs -o+chunksize VG/LV
dm-cache cache policy
The dm-cache subsystem has additional per-LV parameters: the cache pol-
icy 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:
lvs -o+cache_policy,cache_settings VG/LV
Change the cache policy and settings of an existing LV.
# lvchange --cachepolicy mq --cachesettings \
'migration_threshold=2048 random_threshold=4' vg/main
lvm.conf(5) allocation/cache_policy
defines the default cache policy.
lvm.conf(5) allocation/cache_settings
defines the default cache settings.
dm-cache using metadata profiles
Cache pools allows to set a variety of options. Lots of these settings
can be specified in lvm.conf or profile settings. You can prepare a
number of different profiles in the /etc/lvm/profile directory and just
specify the metadata profile file name when caching LV or creating
cache-pool. Check the output of lvmconfig --type default --withcom-
ments for a detailed description of all individual cache settings.
Example
# cat <<EOF > /etc/lvm/profile/cache_big_chunk.profile
allocation {
cache_pool_metadata_require_separate_pvs=0
cache_pool_chunk_size=512
cache_metadata_format=2
cache_mode="writethrough"
cache_policy="smq"
cache_settings {
smq {
migration_threshold=8192
random_threshold=4096
}
}
}
EOF
# lvcreate --cache -L10G --metadataprofile cache_big_chunk vg/main \
/dev/fast_ssd
# lvcreate --cache -L10G vg/main --config \
'allocation/cache_pool_chunk_size=512' /dev/fast_ssd
dm-cache spare metadata LV
See lvmthin(7) for a description of the "pool metadata spare" LV. The
same concept is used for cache pools.
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 2 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 auto, lvm selects the best
option provided by the current dm-cache kernel module.
RAID1 cache device
RAID1 can be used to create the fast LV holding the cache 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.)
# lvcreate -n main -L Size vg /dev/slow
# lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/ssd1 /dev/ssd2
# lvconvert --type cache --cachevol fast vg/main
dm-cache command shortcut
A single command can be used to cache main LV with automatic creation
of a cache-pool:
# lvcreate --cache --size CacheDataSize VG/LV [FastPVs]
or the longer variant
# lvcreate --type cache --size CacheDataSize \
--name NameCachePool VG/LV [FastPVs]
In this command, the specified LV already exists, and is the main LV to
be cached. The command creates a new cache pool with size and given
name or the name is automatically selected from a sequence lvolX_cpool,
using the optionally specified fast PV(s) (typically an ssd). Then it
attaches the new cache pool to the existing main LV to begin caching.
(Note: ensure that the specified main LV is a standard LV. If a cache
pool LV is mistakenly specified, then the command does something dif-
ferent.)
(Note: the type option is interpreted differently by this command than
by normal lvcreate commands in which --type specifies the type of the
newly created LV. In this case, an LV with type cache-pool is being
created, and the existing main LV is being converted to type cache.)
SEE ALSO
lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8),
lvremove(8), lvrename(8), lvresize(8), lvs(8),
vgchange(8), vgmerge(8), vgreduce(8), vgsplit(8),
cache_check(8), cache_dump(8), cache_repair(8)
Red Hat, Inc LVM TOOLS 2.03.14(2)-RHEL8 (2021-10-20) LVMCACHE(7)
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