1LVMCACHE(7) LVMCACHE(7)
2
6 lvmcache — LVM caching
7
10 lvm(8) includes two kinds of caching that can be used to improve the
11 performance of a Logical Volume (LV). When caching, varying subsets of
12 an LV's data are temporarily stored on a smaller, faster device (e.g.
13 an SSD) to improve the performance of the LV.
14 To do this with lvm, a new special LV is first created from the faster
16 device. This LV will hold the cache. Then, the new fast LV is attached
17 to the main LV by way of an lvconvert command. lvconvert inserts one of
18 the device mapper caching targets into the main LV's i/o path. The
19 device mapper target combines the main LV and fast LV into a hybrid
20 device that looks like the main LV, but has better performance. While
21 the main LV is being used, portions of its data will be temporarily and
22 transparently stored on the special fast LV.
23 The two kinds of caching are:
25 · A read and write hot-spot cache, using the dm-cache kernel module.
28 This cache tracks access patterns and adjusts its content deliber‐
29 ately so that commonly used parts of the main LV are likely to be
30 found on the fast storage. LVM refers to this using the LV type
31 cache.
32 · A write cache, using the dm-writecache kernel module. This cache can
35 be used with SSD or PMEM devices to speed up all writes to the main
36 LV. Data read from the main LV is not stored in the cache, only newly
37 written data. LVM refers to this using the LV type writecache.
38
41 1. Identify main LV that needs caching
42 The main LV may already exist, and is located on larger, slower
44 devices. A main LV would be created with a command like:
45 $ lvcreate -n main -L Size vg /dev/slow_hhd
47 2. Identify fast LV to use as the cache
49 A fast LV is created using one or more fast devices, like an SSD. This
51 special LV will be used to hold the cache:
52 $ lvcreate -n fast -L Size vg /dev/fast_ssd
54 $ lvs -a
56 LV Attr Type Devices
57 fast -wi------- linear /dev/fast_ssd
58 main -wi------- linear /dev/slow_hhd
59 3. Start caching the main LV
61 To start caching the main LV, convert the main LV to the desired
63 caching type, and specify the fast LV to use as the cache:
64 using dm-cache:
66 $ lvconvert --type cache --cachevol fast vg/main
68 using dm-writecache:
70 $ lvconvert --type writecache --cachevol fast vg/main
72 using dm-cache (with cachepool):
74 $ lvconvert --type cache --cachepool fast vg/main
76 4. Display LVs
78 Once the fast LV has been attached to the main LV, lvm reports the main
80 LV type as either cache or writecache depending on the type used.
81 While attached, the fast LV is hidden, and renamed with a _cvol or
82 _cpool suffix. It is displayed by lvs -a. The _corig or _wcorig LV
83 represents the original LV without the cache.
84 using dm-cache:
86 $ lvs -a
88 LV Pool Type Devices
89 main [fast_cvol] cache main_corig(0)
90 [fast_cvol] linear /dev/fast_ssd
91 [main_corig] linear /dev/slow_hhd
92 using dm-writecache:
94 $ lvs -a
96 LV Pool Type Devices
97 main [fast_cvol] writecache main_wcorig(0)
98 [fast_cvol] linear /dev/fast_ssd
99 [main_wcorig] linear /dev/slow_hhd
100 using dm-cache (with cachepool):
102 $ lvs -a
104 LV Pool Type Devices
105 main [fast_cpool] cache main_corig(0)
106 [fast_cpool] cache-pool fast_pool_cdata(0)
107 [fast_cpool_cdata] linear /dev/fast_ssd
108 [fast_cpool_cmeta] linear /dev/fast_ssd
109 [main_corig] linear /dev/slow_hhd
110 5. Use the main LV
112 Use the LV until the cache is no longer wanted, or needs to be changed.
114 6. Stop caching
116 To stop caching the main LV, separate the fast LV from the main LV.
118 This changes the type of the main LV back to what it was before the
119 cache was attached.
120 $ lvconvert --splitcache vg/main
122 $ lvs -a
124 LV VG Attr Type Devices
125 fast vg -wi------- linear /dev/fast_ssd
126 main vg -wi------- linear /dev/slow_hhd
127 To stop caching the main LV and also remove unneeded cache pool,
129 use the --uncache:
130 $ lvconvert --uncache vg/main
132 $ lvs -a
134 LV VG Attr Type Devices
135 main vg -wi------- linear /dev/slow_hhd
136 Create a new LV with caching.
140 A new LV can be created with caching attached at the time of creation
141 using the following command:
142 $ lvcreate --type cache|writecache -n Name -L Size
144 --cachedevice /dev/fast_ssd vg /dev/slow_hhd
145 The main LV is created with the specified Name and Size from the
147 slow_hhd. A hidden fast LV is created on the fast_ssd and is then
148 attached to the new main LV. If the fast_ssd is unused, the entire
149 disk will be used as the cache unless the --cachesize option is used to
150 specify a size for the fast LV. The --cachedevice option can be
151 repeated to use multiple disks for the fast LV.
152
155 option args
159 --cachevol LV
162 Pass this option a fast LV that should be used to hold the cache. With
164 a cachevol, cache data and metadata are stored in different parts of
165 the same fast LV. This option can be used with dm-writecache or dm-
166 cache.
167 --cachepool CachePoolLV|LV
169 Pass this option a cachepool LV or a standard LV. When using a cache
171 pool, lvm places cache data and cache metadata on different LVs. The
172 two LVs together are called a cache pool. This has a bit better per‐
173 formance for dm-cache and permits specific placement and segment type
174 selection for data and metadata volumes. A cache pool is represented
175 as a special type of LV that cannot be used directly. If a standard LV
176 is passed with this option, lvm will first convert it to a cache pool
177 by combining it with another LV to use for metadata. This option can
178 be used with dm-cache.
179 --cachedevice PV
181 This option can be used in place of --cachevol, in which case a
183 cachevol LV will be created using the specified device. This option
184 can be repeated to create a cachevol using multiple devices, or a tag
185 name can be specified in which case the cachevol will be created using
186 any of the devices with the given tag. If a named cache device is
187 unused, the entire device will be used to create the cachevol. To cre‐
188 ate a cachevol of a specific size from the cache devices, include the
189 --cachesize option.
190 dm-cache block size
195 A cache pool will have a logical block size of 4096 bytes if it is cre‐
198 ated on a device with a logical block size of 4096 bytes.
199 If a main LV has logical block size 512 (with an existing xfs file sys‐
201 tem using that size), then it cannot use a cache pool with a 4096 logi‐
202 cal block size. If the cache pool is attached, the main LV will likely
203 fail to mount.
204 To avoid this problem, use a mkfs option to specify a 4096 block size
206 for the file system, or attach the cache pool before running mkfs.
207 dm-writecache block size
210 The dm-writecache block size can be 4096 bytes (the default), or 512
213 bytes. The default 4096 has better performance and should be used
214 except when 512 is necessary for compatibility. The dm-writecache
215 block size is specified with --cachesettings block_size=4096|512 when
216 caching is started.
217 When a file system like xfs already exists on the main LV prior to
219 caching, and the file system is using a block size of 512, then the
220 writecache block size should be set to 512. (The file system will
221 likely fail to mount if writecache block size of 4096 is used in this
222 case.)
223 Check the xfs sector size while the fs is mounted:
225 $ xfs_info /dev/vg/main
227 Look for sectsz=512 or sectsz=4096
228 The writecache block size should be chosen to match the xfs sectsz
230 value.
231 It is also possible to specify a sector size of 4096 to mkfs.xfs when
233 creating the file system. In this case the writecache block size of
234 4096 can be used.
235 dm-writecache settings
238 Tunable parameters can be passed to the dm-writecache kernel module
241 using the --cachesettings option when caching is started, e.g.
242 $ lvconvert --type writecache --cachevol fast \
244 --cachesettings 'high_watermark=N writeback_jobs=N' vg/main
245 Tunable options are:
247 · high_watermark = <percent>
250 Start writeback when the writecache usage reaches this percent
252 (0-100).
253 · low_watermark = <percent>
256 Stop writeback when the writecache usage reaches this percent
258 (0-100).
259 · writeback_jobs = <count>
262 Limit the number of blocks that are in flight during writeback. Set‐
264 ting this value reduces writeback throughput, but it may improve
265 latency of read requests.
266 · autocommit_blocks = <count>
269 When the application writes this amount of blocks without issuing the
271 FLUSH request, the blocks are automatically commited.
272 · autocommit_time = <milliseconds>
275 The data is automatically commited if this time passes and no FLUSH
277 request is received.
278 · fua = 0|1
281 Use the FUA flag when writing data from persistent memory back to the
283 underlying device. Applicable only to persistent memory.
284 · nofua = 0|1
287 Don't use the FUA flag when writing back data and send the FLUSH
289 request afterwards. Some underlying devices perform better with fua,
290 some with nofua. Testing is necessary to determine which. Applica‐
291 ble only to persistent memory.
292 dm-cache with separate data and metadata LVs
296 When using dm-cache, the cache metadata and cache data can be stored on
299 separate LVs. To do this, a "cache pool" is created, which is a spe‐
300 cial LV that references two sub LVs, one for data and one for metadata.
301 To create a cache pool from two separate LVs:
303 $ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
305 $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
306 $ lvconvert --type cache-pool --poolmetadata fastmeta vg/fast
307 Then use the cache pool LV to start caching the main LV:
309 $ lvconvert --type cache --cachepool fast vg/main
311 A variation of the same procedure automatically creates a cache pool
313 when caching is started. To do this, use a standard LV as the
314 --cachepool (this will hold cache data), and use another standard LV as
315 the --poolmetadata (this will hold cache metadata). LVM will create a
316 cache pool LV from the two specified LVs, and use the cache pool to
317 start caching the main LV.
318 $ lvcreate -n fast -L DataSize vg /dev/fast_ssd1
320 $ lvcreate -n fastmeta -L MetadataSize vg /dev/fast_ssd2
321 $ lvconvert --type cache --cachepool fast --poolmetadata fastmeta vg/main
322 dm-cache cache modes
325 The default dm-cache cache mode is "writethrough". Writethrough
328 ensures that any data written will be stored both in the cache and on
329 the origin LV. The loss of a device associated with the cache in this
330 case would not mean the loss of any data.
331 A second cache mode is "writeback". Writeback delays writing data
333 blocks from the cache back to the origin LV. This mode will increase
334 performance, but the loss of a cache device can result in lost data.
335 With the --cachemode option, the cache mode can be set when caching is
337 started, or changed on an LV that is already cached. The current cache
338 mode can be displayed with the cache_mode reporting option:
339 lvs -o+cache_mode VG/LV
341 lvm.conf(5) allocation/cache_mode
343 defines the default cache mode.
344 $ lvconvert --type cache --cachevol fast \
346 --cachemode writethrough vg/main
347 dm-cache chunk size
350 The size of data blocks managed by dm-cache can be specified with the
353 --chunksize option when caching is started. The default unit is KiB.
354 The value must be a multiple of 32KiB between 32KiB and 1GiB. Cache
355 chunks bigger then 512KiB shall be only used when necessary.
356 Using a chunk size that is too large can result in wasteful use of the
358 cache, in which small reads and writes cause large sections of an LV to
359 be stored in the cache. It can also require increasing migration
360 threshold which defaults to 2048 sectors (1 MiB). Lvm2 ensures migra‐
361 tion threshold is at least 8 chunks in size. This may in some cases
362 result in very high bandwidth load of transfering data between the
363 cache LV and its cache origin LV. However, choosing a chunk size that
364 is too small can result in more overhead trying to manage the numerous
365 chunks that become mapped into the cache. Overhead can include both
366 excessive CPU time searching for chunks, and excessive memory tracking
367 chunks.
368 Command to display the chunk size:
370 lvs -o+chunksize VG/LV
371 lvm.conf(5) cache_pool_chunk_size
373 controls the default chunk size.
374 The default value is shown by:
376 lvmconfig --type default allocation/cache_pool_chunk_size
377 Checking migration threshold (in sectors) of running cached LV:
379 lvs -o+kernel_cache_settings VG/LV
380 dm-cache migration threshold
384 Migrating data between the origin and cache LV uses bandwidth. The
387 user can set a throttle to prevent more than a certain amount of migra‐
388 tion occurring at any one time. Currently dm-cache is not taking any
389 account of normal io traffic going to the devices.
390 User can set migration threshold via cache policy settings as "migra‐
392 tion_threshold=<#sectors>" to set the maximum number of sectors being
393 migrated, the default being 2048 sectors (1MiB).
394 Command to set migration threshold to 2MiB (4096 sectors):
396 lvcreate --cachepolicy 'migration_threshold=4096' VG/LV
397 Command to display the migration threshold:
400 lvs -o+kernel_cache_settings,cache_settings VG/LV
401 lvs -o+chunksize VG/LV
402 dm-cache cache policy
406 The dm-cache subsystem has additional per-LV parameters: the cache pol‐
409 icy to use, and possibly tunable parameters for the cache policy.
410 Three policies are currently available: "smq" is the default policy,
411 "mq" is an older implementation, and "cleaner" is used to force the
412 cache to write back (flush) all cached writes to the origin LV.
413 The older "mq" policy has a number of tunable parameters. The defaults
415 are chosen to be suitable for the majority of systems, but in special
416 circumstances, changing the settings can improve performance.
417 With the --cachepolicy and --cachesettings options, the cache policy
419 and settings can be set when caching is started, or changed on an
420 existing cached LV (both options can be used together). The current
421 cache policy and settings can be displayed with the cache_policy and
422 cache_settings reporting options:
423 lvs -o+cache_policy,cache_settings VG/LV
425 Change the cache policy and settings of an existing LV.
427 $ lvchange --cachepolicy mq --cachesettings \
429 'migration_threshold=2048 random_threshold=4' vg/main
430 lvm.conf(5) allocation/cache_policy
432 defines the default cache policy.
433 lvm.conf(5) allocation/cache_settings
435 defines the default cache settings.
436 dm-cache spare metadata LV
439 See lvmthin(7) for a description of the "pool metadata spare" LV. The
442 same concept is used for cache pools.
443 dm-cache metadata formats
446 There are two disk formats for dm-cache metadata. The metadata format
449 can be specified with --cachemetadataformat when caching is started,
450 and cannot be changed. Format 2 has better performance; it is more
451 compact, and stores dirty bits in a separate btree, which improves the
452 speed of shutting down the cache. With auto, lvm selects the best
453 option provided by the current dm-cache kernel module.
454 RAID1 cache device
457 RAID1 can be used to create the fast LV holding the cache so that it
460 can tolerate a device failure. (When using dm-cache with separate data
461 and metadata LVs, each of the sub-LVs can use RAID1.)
462 $ lvcreate -n main -L Size vg /dev/slow
464 $ lvcreate --type raid1 -m 1 -n fast -L Size vg /dev/ssd1 /dev/ssd2
465 $ lvconvert --type cache --cachevol fast vg/main
466 dm-cache command shortcut
469 A single command can be used to create a cache pool and attach that new
472 cache pool to a main LV:
473 $ lvcreate --type cache --name Name --size Size VG/LV [PV]
475 In this command, the specified LV already exists, and is the main LV to
477 be cached. The command creates a new cache pool with the given name
478 and size, using the optionally specified PV (typically an ssd). Then
479 it attaches the new cache pool to the existing main LV to begin
480 caching.
481 (Note: ensure that the specified main LV is a standard LV. If a cache
483 pool LV is mistakenly specified, then the command does something dif‐
484 ferent.)
485 (Note: the type option is interpreted differently by this command than
487 by normal lvcreate commands in which --type specifies the type of the
488 newly created LV. In this case, an LV with type cache-pool is being
489 created, and the existing main LV is being converted to type cache.)
490
494 lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8), lvre‐
495 move(8), lvrename(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8),
496 vgreduce(8), vgsplit(8)
497Red Hat, Inc LVM TOOLS 2.03.10(2) (2020-08-09) LVMCACHE(7)