1LVMCACHE(7) LVMCACHE(7)
2
6 lvmcache — LVM caching
7
10 An lvm(8) cache Logical Volume (LV) uses a small and fast LV to improve
11 the performance of a large and slow LV. It does this by storing the
12 frequently used blocks on the faster LV. LVM refers to the small fast
13 LV as a cache pool LV. The large slow LV is called the origin LV. Due
14 to requirements from dm-cache (the kernel driver), LVM further splits
15 the cache pool LV into two devices - the cache data LV and cache meta‐
16 data LV. The cache data LV is where copies of data blocks are kept
17 from the origin LV to increase speed. The cache metadata LV holds the
18 accounting information that specifies where data blocks are stored
19 (e.g. on the origin LV or on the cache data LV). Users should be
20 familiar with these LVs if they wish to create the best and most robust
21 cached LVs. All of these associated LVs must be in the same Volume
22 Group (VG).
23
26 origin LV OriginLV large slow LV
27 cache data LV CacheDataLV small fast LV for cache pool data
28 cache metadata LV CacheMetaLV small fast LV for cache pool metadata
29 cache pool LV CachePoolLV CacheDataLV + CacheMetaLV
30 cache LV CacheLV OriginLV + CachePoolLV
31
34 The primary method for using a cache type LV:
35 0. create OriginLV
39 Create an LV or identify an existing LV to be the origin LV.
40 lvcreate -n OriginLV -L LargeSize VG SlowPVs
42 Example
44 # lvcreate -n lvol0 -L 100G vg /dev/slow
45 1. create CacheDataLV
49 Create the cache data LV. This LV will hold data blocks from the
50 OriginLV. The size of this LV is the size of the cache and will be
51 reported as the size of the cache pool LV.
52 lvcreate -n CacheDataLV -L CacheSize VG FastPVs
54 Example
56 # lvcreate -n cache0 -L 10G vg /dev/fast
57 2. create CacheMetaLV
61 Create the cache metadata LV. This LV will hold cache pool metadata.
62 The size of this LV should be 1000 times smaller than the cache data
63 LV, with a minimum size of 8MiB.
64 lvcreate -n CacheMetaLV -L MetaSize VG FastPVs
66 Example
68 # lvcreate -n cache0meta -L 12M vg /dev/fast
69 # lvs -a vg
71 LV VG Attr LSize Pool Origin
72 cache0 vg -wi-a----- 10.00g
73 cache0meta vg -wi-a----- 12.00m
74 lvol0 vg -wi-a----- 100.00g
75 3. create CachePoolLV
79 Combine the data and metadata LVs into a cache pool LV. The behavior
80 of the cache pool LV can be set in this step.
81 CachePoolLV takes the name of CacheDataLV.
82 CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
83 CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
84 lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV
86 VG/CacheDataLV
87 Example
89 # lvconvert --type cache-pool --poolmetadata vg/cache0meta vg/cache0
90 # lvs -a vg
92 LV VG Attr LSize Pool Origin
93 cache0 vg Cwi---C--- 10.00g
94 [cache0_cdata] vg Cwi------- 10.00g
95 [cache0_cmeta] vg ewi------- 12.00m
96 lvol0 vg -wi-a----- 100.00g
97 4. create CacheLV
101 Create a cache LV by linking the cache pool LV to the origin LV. The
102 user accessible cache LV takes the name of the origin LV, while the
103 origin LV becomes a hidden LV with the name OriginLV_corig. This can
104 be done while the origin LV is in use.
105 CacheLV takes the name of OriginLV.
106 OriginLV is renamed OriginLV_corig and becomes hidden.
107 lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
109 Example
111 # lvconvert --type cache --cachepool vg/cache0 vg/lvol0
112 # lvs -a vg
114 LV VG Attr LSize Pool Origin
115 cache0 vg Cwi---C--- 10.00g
116 [cache0_cdata] vg Cwi-ao---- 10.00g
117 [cache0_cmeta] vg ewi-ao---- 12.00m
118 lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
119 [lvol0_corig] vg -wi-ao---- 100.00g
120
124 Split a cache pool LV off of a cache LV
125 A cache pool LV can be disconnected from a cache LV, leaving an unused
128 cache pool LV, and an uncached origin LV. This command writes back
129 data from the cache pool to the origin LV when necessary.
130 lvconvert --splitcache VG/CacheLV
132 Removing a cache pool LV without removing its linked origin LV
135 This writes back data from the cache pool to the origin LV when neces‐
138 sary, then removes the cache pool LV, leaving the uncached origin LV.
139 lvremove VG/CachePoolLV
141 An alternative command that also disconnects the cache pool from the
143 cache LV, and deletes the cache pool:
144 lvconvert --uncache VG/CacheLV
146 Example
148 # lvs vg
149 LV VG Attr LSize Pool Origin
150 cache0 vg Cwi---C--- 10.00g
151 lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
152 # lvremove vg/cache0
154 # lvs vg
156 LV VG Attr LSize Pool Origin
157 lvol0 vg -wi-a----- 100.00g
158 Removing a cache LV: both origin LV and the cache pool LV
161 Removing a cache LV removes both the origin LV and the linked cache
164 pool LV.
165 lvremove VG/CacheLV
167
171 Tolerate device failures in a cache pool LV
172 Users who are concerned about the possibility of failures in their fast
175 devices that could lead to data loss might consider making their cache
176 pool sub-LVs redundant.
177 Example
179 0. Create an origin LV we wish to cache
180 # lvcreate -L 10G -n lv1 vg /dev/slow
181 1. Create a 2-way RAID1 cache data LV
183 # lvcreate --type raid1 -m 1 -L 1G -n cache1 vg \
184 /dev/fast1 /dev/fast2
185 2. Create a 2-way RAID1 cache metadata LV
187 # lvcreate --type raid1 -m 1 -L 8M -n cache1meta vg \
188 /dev/fast1 /dev/fast2
189 3. Create a cache pool LV combining cache data LV and cache metadata LV
191 # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
192 4. Create a cached LV by combining the cache pool LV and origin LV
194 # lvconvert --type cache --cachepool vg/cache1 vg/lv1
195 Cache mode
198 The default cache mode is "writethrough". Writethrough ensures that
201 any data written will be stored both in the cache pool LV and on the
202 origin LV. The loss of a device associated with the cache pool LV in
203 this case would not mean the loss of any data.
204 A second cache mode is "writeback". Writeback delays writing data
206 blocks from the cache pool back to the origin LV. This mode will
207 increase performance, but the loss of a device associated with the
208 cache pool LV can result in lost data.
209 With the --cachemode option, the cache mode can be set when creating a
211 cache LV, or changed on an existing cache LV. The current cache mode
212 of a cache LV can be displayed with the cache_mode reporting option:
213 lvs -o+cache_mode VG/CacheLV
215 lvm.conf(5) allocation/cache_mode
217 defines the default cache mode.
218 Example
220 0. Create an origin LV we wish to cache (yours may already exist)
221 # lvcreate -L 10G -n lv1 vg /dev/slow
222 1. Create a cache data LV
224 # lvcreate -L 1G -n cache1 vg /dev/fast
225 2. Create a cache metadata LV
227 # lvcreate -L 8M -n cache1meta vg /dev/fast
228 3. Create a cache pool LV
230 # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
231 4. Create a cache LV by combining the cache pool LV and origin LV,
233 and use the writethrough cache mode.
234 # lvconvert --type cache --cachepool vg/cache1 \
235 --cachemode writethrough vg/lv1
236 Cache policy
240 The cache subsystem has additional per-LV parameters: the cache policy
243 to use, and possibly tunable parameters for the cache policy. Three
244 policies are currently available: "smq" is the default policy, "mq" is
245 an older implementation, and "cleaner" is used to force the cache to
246 write back (flush) all cached writes to the origin LV.
247 The "mq" policy has a number of tunable parameters. The defaults are
249 chosen to be suitable for the majority of systems, but in special cir‐
250 cumstances, changing the settings can improve performance.
251 With the --cachepolicy and --cachesettings options, the cache policy
253 and settings can be set when creating a cache LV, or changed on an
254 existing cache LV (both options can be used together). The current
255 cache policy and settings of a cache LV can be displayed with the
256 cache_policy and cache_settings reporting options:
257 lvs -o+cache_policy,cache_settings VG/CacheLV
259 Example
261 Change the cache policy and settings of an existing cache LV.
262 # lvchange --cachepolicy mq --cachesettings \
263 'migration_threshold=2048 random_threshold=4' vg/lv1
264 lvm.conf(5) allocation/cache_policy
266 defines the default cache policy.
267 lvm.conf(5) allocation/cache_settings
269 defines the default cache settings.
270 Chunk size
274 The size of data blocks managed by a cache pool can be specified with
277 the --chunksize option when the cache LV is created. The default unit
278 is KiB. The value must be a multiple of 32KiB between 32KiB and 1GiB.
279 Using a chunk size that is too large can result in wasteful use of the
281 cache, where small reads and writes can cause large sections of an LV
282 to be mapped into the cache. However, choosing a chunk size that is
283 too small can result in more overhead trying to manage the numerous
284 chunks that become mapped into the cache. Overhead can include both
285 excessive CPU time searching for chunks, and excessive memory tracking
286 chunks.
287 Command to display the cache pool LV chunk size:
289 lvs -o+chunksize VG/CacheLV
290 lvm.conf(5) cache_pool_chunk_size
292 controls the default chunk size used when creating a cache LV.
293 The default value is shown by:
295 lvmconfig --type default allocation/cache_pool_chunk_size
296 Spare metadata LV
300 See lvmthin(7) for a description of the "pool metadata spare" LV. The
303 same concept is used for cache pools.
304 Automatic pool metadata LV
307 A cache data LV can be converted to cache pool LV without specifying a
310 cache pool metadata LV. LVM will automatically create a metadata LV
311 from the same VG.
312 lvcreate -n CacheDataLV -L CacheSize VG
314 lvconvert --type cache-pool VG/CacheDataLV
315 Create a new cache LV without an existing origin LV
319 A cache LV can be created using an existing cache pool without an
322 existing origin LV. A new origin LV is created and linked to the cache
323 pool in a single step.
324 lvcreate --type cache -L LargeSize -n CacheLV
326 --cachepool VG/CachePoolLV VG SlowPVs
327 Single step cache pool LV creation
331 A cache pool LV can be created with a single lvcreate command, rather
334 than using lvconvert on existing LVs. This one command creates a cache
335 data LV, a cache metadata LV, and combines the two into a cache pool
336 LV.
337 lvcreate --type cache-pool -L CacheSize -n CachePoolLV VG FastPVs
339 Convert existing LVs to cache types
343 When an existing origin LV is converted to a cache LV, the specified
346 cache pool may be a normal LV, rather than a cache pool LV. In this
347 case, lvm will first convert the normal LV to a cache pool LV. A pool
348 metadata LV may optionally be specified.
349 lvcreate -n OriginLV -L LargeSize VG
351 lvcreate -n CacheDataLV -L CacheSize VG
352 lvconvert --type cache --cachepool VG/CataDataLV VG/OriginLV
353 This is equivalent to:
355 lvcreate -n OriginLV -L LargeSize VG
357 lvcreate -n CacheDataLV -L CacheSize VG
358 lvconvert --type cache-pool VG/CacheDataLV
359 lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
360 Cache metadata formats
364 There are two disk formats for cache metadata. The metadata format can
367 be specified when a cache pool is created, and cannot be changed. For‐
368 mat 2 has better performance; it is more compact, and stores dirty bits
369 in a separate btree, which improves the speed of shutting down the
370 cache. With auto, lvm selects the best option provided by the current
371 dm-cache kernel target.
372 lvconvert --type cache-pool --cachemetadataformat auto|1|2
374 VG/CacheDataLV
375
379 lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8), lvre‐
380 move(8), lvrename(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8),
381 vgreduce(8), vgsplit(8)
382Red Hat, Inc LVM TOOLS 2.02.180(2)-RHEL7 (2018-07-20) LVMCACHE(7)