1LVMCACHE(7) LVMCACHE(7)
2
6 lvmcache — LVM caching
7
10 The cache logical volume type uses a small and fast LV to improve the
11 performance of a large and slow LV. It does this by storing the fre‐
12 quently used blocks on the faster LV. LVM refers to the small fast LV
13 as a cache pool LV. The large slow LV is called the origin LV. Due to
14 requirements from dm-cache (the kernel driver), LVM further splits the
15 cache pool LV into two devices - the cache data LV and cache metadata
16 LV. The cache data LV is where copies of data blocks are kept from the
17 origin LV to increase speed. The cache metadata LV holds the account‐
18 ing information that specifies where data blocks are stored (e.g. on
19 the origin LV or on the cache data LV). Users should be familiar with
20 these LVs if they wish to create the best and most robust cached logi‐
21 cal volumes. All of these associated LVs must be in the same VG.
22
25 origin LV OriginLV large slow LV
26 cache data LV CacheDataLV small fast LV for cache pool data
27 cache metadata LV CacheMetaLV small fast LV for cache pool metadata
28 cache pool LV CachePoolLV CacheDataLV + CacheMetaLV
29 cache LV CacheLV OriginLV + CachePoolLV
30
33 The primary method for using a cache type logical volume:
34 0. create OriginLV
38 Create an LV or identify an existing LV to be the origin LV.
39 lvcreate -n OriginLV -L LargeSize VG SlowPVs
41 Example
43 # lvcreate -n lvol0 -L 100G vg
44 1. create CacheDataLV
48 Create the cache data LV. This LV will hold data blocks from the
49 OriginLV. The size of this LV is the size of the cache and will be
50 reported as the size of the cache pool LV.
51 lvcreate -n CacheDataLV -L CacheSize VG FastPVs
53 Example
55 # lvcreate -n cache0 -L 10G vg /dev/fast
56 2. create CacheMetaLV
60 Create the cache metadata LV. This LV will hold cache pool metadata.
61 The size of this LV should be 1000 times smaller than the cache data
62 LV, with a minimum size of 8MiB.
63 lvcreate -n CacheMetaLV -L MetaSize VG FastPVs
65 Example
67 # lvcreate -n cache0meta -L 12M vg /dev/fast
68 # lvs -a vg
70 LV VG Attr LSize Pool Origin
71 cache0 vg -wi-a----- 10.00g
72 cache0meta vg -wi-a----- 12.00m
73 lvol0 vg -wi-a----- 100.00g
74 3. create CachePoolLV
78 Combine the data and metadata LVs into a cache pool LV. The behavior
79 of the cache pool LV can be set in this step.
80 CachePoolLV takes the name of CacheDataLV.
81 CacheDataLV is renamed CachePoolLV_cdata and becomes hidden.
82 CacheMetaLV is renamed CachePoolLV_cmeta and becomes hidden.
83 lvconvert --type cache-pool --poolmetadata VG/CacheMetaLV
85 VG/CacheDataLV
86 Example
88 # lvconvert --type cache-pool --poolmetadata vg/cache0meta vg/cache0
89 # lvs -a vg
91 LV VG Attr LSize Pool Origin
92 cache0 vg Cwi---C--- 10.00g
93 [cache0_cdata] vg Cwi------- 10.00g
94 [cache0_cmeta] vg ewi------- 12.00m
95 lvol0 vg -wi-a----- 100.00g
96 4. create CacheLV
100 Create a cache LV by linking the cache pool LV to the origin LV. The
101 user accessible cache LV takes the name of the origin LV, while the
102 origin LV becomes a hidden LV with the name OriginLV_corig. This can
103 be done while the origin LV is in use.
104 CacheLV takes the name of OriginLV.
105 OriginLV is renamed OriginLV_corig and becomes hidden.
106 lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
108 Example
110 # lvconvert --type cache --cachepool vg/cache0 vg/lvol0
111 # lvs -a vg
113 LV VG Attr LSize Pool Origin
114 cache0 vg Cwi---C--- 10.00g
115 [cache0_cdata] vg Cwi-ao---- 10.00g
116 [cache0_cmeta] vg ewi-ao---- 12.00m
117 lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
118 [lvol0_corig] vg -wi-ao---- 100.00g
119
123 Split a cache pool LV off of a cache LV
124 A cache pool LV can be disconnected from a cache LV, leaving an unused
127 cache pool LV, and an uncached origin LV. This command writes back
128 data from the cache pool to the origin LV when necessary.
129 lvconvert --splitcache VG/CacheLV
131 Removing a cache pool LV without removing its linked origin LV
134 This writes back data from the cache pool to the origin LV when neces‐
137 sary, then removes the cache pool LV, leaving the uncached origin LV.
138 lvremove VG/CachePoolLV
140 An alternative command that also disconnects the cache pool from the
142 cache LV, and deletes the cache pool:
143 lvconvert --uncache VG/CacheLV
145 Example
147 # lvs vg
148 LV VG Attr LSize Pool Origin
149 cache0 vg Cwi---C--- 10.00g
150 lvol0 vg Cwi-a-C--- 100.00g cache0 [lvol0_corig]
151 # lvremove vg/cache0
153 # lvs vg
155 LV VG Attr LSize Pool Origin
156 lvol0 vg -wi-a----- 100.00g
157 Removing a cache LV: both origin LV and the cache pool LV
160 Removing a cache LV removes both the origin LV and the linked cache
163 pool LV.
164 lvremove VG/CacheLV
166
170 Tolerate device failures in a cache pool LV
171 Users who are concerned about the possibility of failures in their fast
174 devices that could lead to data loss might consider making their cache
175 pool sub-LVs redundant.
176 Example
178 0. Create an origin LV we wish to cache
179 # lvcreate -L 10G -n lv1 vg /dev/slow_devs
180 1. Create a 2-way RAID1 cache data LV
182 # lvcreate --type raid1 -m 1 -L 1G -n cache1 vg \
183 /dev/fast1 /dev/fast2
184 2. Create a 2-way RAID1 cache metadata LV
186 # lvcreate --type raid1 -m 1 -L 8M -n cache1meta vg \
187 /dev/fast1 /dev/fast2
188 3. Create a cache pool LV combining cache data LV and cache metadata LV
190 # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
191 4. Create a cached LV by combining the cache pool LV and origin LV
193 # lvconvert --type cache --cachepool vg/cache1 vg/lv1
194 Cache mode
197 The default cache mode is "writethrough". Writethrough ensures that
200 any data written will be stored both in the cache pool LV and on the
201 origin LV. The loss of a device associated with the cache pool LV in
202 this case would not mean the loss of any data.
203 A second cache mode is "writeback". Writeback delays writing data
205 blocks from the cache pool back to the origin LV. This mode will
206 increase performance, but the loss of a device associated with the
207 cache pool LV can result in lost data.
208 With the --cachemode option, the cache mode can be set when creating a
210 cache LV, or changed on an existing cache LV. The current cache mode
211 of a cache LV can be displayed with the cache_mode reporting option:
212 lvs -o+cache_mode VG/CacheLV
214 lvm.conf(5) allocation/cache_mode
216 defines the default cache mode.
217 Example
219 0. Create an origin LV we wish to cache (yours may already exist)
220 # lvcreate -L 10G -n lv1 vg /dev/slow
221 1. Create a cache data LV
223 # lvcreate -L 1G -n cache1 vg /dev/fast
224 2. Create a cache metadata LV
226 # lvcreate -L 8M -n cache1meta vg /dev/fast
227 3. Create a cache pool LV
229 # lvconvert --type cache-pool --poolmetadata vg/cache1meta vg/cache1
230 4. Create a cache LV by combining the cache pool LV and origin LV,
232 and use the writethrough cache mode.
233 # lvconvert --type cache --cachepool vg/cache1 \
234 --cachemode writethrough vg/lv1
235 Cache policy
239 The cache subsystem has additional per-LV parameters: the cache policy
242 to use, and possibly tunable parameters for the cache policy. Three
243 policies are currently available: "smq" is the default policy, "mq" is
244 an older implementation, and "cleaner" is used to force the cache to
245 write back (flush) all cached writes to the origin LV.
246 The "mq" policy has a number of tunable parameters. The defaults are
248 chosen to be suitable for the majority of systems, but in special cir‐
249 cumstances, changing the settings can improve performance.
250 With the --cachepolicy and --cachesettings options, the cache policy
252 and settings can be set when creating a cache LV, or changed on an
253 existing cache LV (both options can be used together). The current
254 cache policy and settings of a cache LV can be displayed with the
255 cache_policy and cache_settings reporting options:
256 lvs -o+cache_policy,cache_settings VG/CacheLV
258 Example
260 Change the cache policy and settings of an existing cache LV.
261 # lvchange --cachepolicy mq --cachesettings \
262 'migration_threshold=2048 random_threshold=4' vg/lv1
263 lvm.conf(5) allocation/cache_policy
265 defines the default cache policy.
266 lvm.conf(5) allocation/cache_settings
268 defines the default cache settings.
269 Chunk size
273 The size of data blocks managed by a cache pool can be specified with
276 the --chunksize option when the cache LV is created. The default unit
277 is KiB. The value must be a multiple of 32KiB between 32KiB and 1GiB.
278 Using a chunk size that is too large can result in wasteful use of the
280 cache, where small reads and writes can cause large sections of an LV
281 to be mapped into the cache. However, choosing a chunk size that is
282 too small can result in more overhead trying to manage the numerous
283 chunks that become mapped into the cache. Overhead can include both
284 excessive CPU time searching for chunks, and excessive memory tracking
285 chunks.
286 Command to display the cache pool LV chunk size:
288 lvs -o+chunksize VG/CacheLV
289 lvm.conf(5) cache_pool_chunk_size
291 controls the default chunk size used when creating a cache LV.
292 The default value is shown by:
294 lvmconfig --type default allocation/cache_pool_chunk_size
295 Spare metadata LV
299 See lvmthin(7) for a description of the "pool metadata spare" LV. The
302 same concept is used for cache pools.
303 Automatic pool metadata LV
306 A cache data LV can be converted to cache pool LV without specifying a
309 cache pool metadata LV. LVM will automatically create a metadata LV
310 from the same VG.
311 lvcreate -n CacheDataLV -L CacheSize VG
313 lvconvert --type cache-pool VG/CacheDataLV
314 Create a new cache LV without an existing origin LV
318 A cache LV can be created using an existing cache pool without an
321 existing origin LV. A new origin LV is created and linked to the cache
322 pool in a single step.
323 lvcreate --type cache -L LargeSize -n CacheLV
325 --cachepool VG/CachePoolLV VG SlowPVs
326 Single step cache pool LV creation
330 A cache pool LV can be created with a single lvcreate command, rather
333 than using lvconvert on existing LVs. This one command creates a cache
334 data LV, a cache metadata LV, and combines the two into a cache pool
335 LV.
336 lvcreate --type cache-pool -L CacheSize -n CachePoolLV VG FastPVs
338 Convert existing LVs to cache types
342 When an existing origin LV is converted to a cache LV, the specified
345 cache pool may be a normal LV, rather than a cache pool LV. In this
346 case, lvm will first convert the normal LV to a cache pool LV. A pool
347 metadata LV may optionally be specified.
348 lvcreate -n OriginLV -L LargeSize VG
350 lvcreate -n CacheDataLV -L CacheSize VG
351 lvconvert --type cache --cachepool VG/CataDataLV VG/OriginLV
352 This is equivalent to:
354 lvcreate -n OriginLV -L LargeSize VG
356 lvcreate -n CacheDataLV -L CacheSize VG
357 lvconvert --type cache-pool VG/CacheDataLV
358 lvconvert --type cache --cachepool VG/CachePoolLV VG/OriginLV
359
363 lvm.conf(5), lvchange(8), lvcreate(8), lvdisplay(8), lvextend(8), lvre‐
364 move(8), lvrename(8), lvresize(8), lvs(8), vgchange(8), vgmerge(8),
365 vgreduce(8), vgsplit(8)
366Red Hat, Inc LVM TOOLS 2.02.143(2)-RHEL6 (2016-12-13) LVMCACHE(7)