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Diffstat (limited to 'Documentation/device-mapper/cache.txt')
| -rw-r--r-- | Documentation/device-mapper/cache.txt | 121 |
1 files changed, 87 insertions, 34 deletions
diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt index 33d45ee0b73..68c0f517c60 100644 --- a/Documentation/device-mapper/cache.txt +++ b/Documentation/device-mapper/cache.txt @@ -68,10 +68,11 @@ So large block sizes are bad because they waste cache space. And small block sizes are bad because they increase the amount of metadata (both in core and on disk). -Writeback/writethrough ----------------------- +Cache operating modes +--------------------- -The cache has two modes, writeback and writethrough. +The cache has three operating modes: writeback, writethrough and +passthrough. If writeback, the default, is selected then a write to a block that is cached will go only to the cache and the block will be marked dirty in @@ -81,8 +82,31 @@ If writethrough is selected then a write to a cached block will not complete until it has hit both the origin and cache devices. Clean blocks should remain clean. +If passthrough is selected, useful when the cache contents are not known +to be coherent with the origin device, then all reads are served from +the origin device (all reads miss the cache) and all writes are +forwarded to the origin device; additionally, write hits cause cache +block invalidates. To enable passthrough mode the cache must be clean. +Passthrough mode allows a cache device to be activated without having to +worry about coherency. Coherency that exists is maintained, although +the cache will gradually cool as writes take place. If the coherency of +the cache can later be verified, or established through use of the +"invalidate_cblocks" message, the cache device can be transitioned to +writethrough or writeback mode while still warm. Otherwise, the cache +contents can be discarded prior to transitioning to the desired +operating mode. + A simple cleaner policy is provided, which will clean (write back) all -dirty blocks in a cache. Useful for decommissioning a cache. +dirty blocks in a cache. Useful for decommissioning a cache or when +shrinking a cache. Shrinking the cache's fast device requires all cache +blocks, in the area of the cache being removed, to be clean. If the +area being removed from the cache still contains dirty blocks the resize +will fail. Care must be taken to never reduce the volume used for the +cache's fast device until the cache is clean. This is of particular +importance if writeback mode is used. Writethrough and passthrough +modes already maintain a clean cache. Future support to partially clean +the cache, above a specified threshold, will allow for keeping the cache +warm and in writeback mode during resize. Migration throttling -------------------- @@ -100,12 +124,11 @@ the default being 204800 sectors (or 100MB). Updating on-disk metadata ------------------------- -On-disk metadata is committed every time a REQ_SYNC or REQ_FUA bio is -written. If no such requests are made then commits will occur every -second. This means the cache behaves like a physical disk that has a -write cache (the same is true of the thin-provisioning target). If -power is lost you may lose some recent writes. The metadata should -always be consistent in spite of any crash. +On-disk metadata is committed every time a FLUSH or FUA bio is written. +If no such requests are made then commits will occur every second. This +means the cache behaves like a physical disk that has a volatile write +cache. If power is lost you may lose some recent writes. The metadata +should always be consistent in spite of any crash. The 'dirty' state for a cache block changes far too frequently for us to keep updating it on the fly. So we treat it as a hint. In normal @@ -161,7 +184,7 @@ Constructor block size : cache unit size in sectors #feature args : number of feature arguments passed - feature args : writethrough. (The default is writeback.) + feature args : writethrough or passthrough (The default is writeback.) policy : the replacement policy to use #policy args : an even number of arguments corresponding to @@ -177,6 +200,13 @@ Optional feature arguments are: back cache block contents later for performance reasons, so they may differ from the corresponding origin blocks. + passthrough : a degraded mode useful for various cache coherency + situations (e.g., rolling back snapshots of + underlying storage). Reads and writes always go to + the origin. If a write goes to a cached origin + block, then the cache block is invalidated. + To enable passthrough mode the cache must be clean. + A policy called 'default' is always registered. This is an alias for the policy we currently think is giving best all round performance. @@ -186,36 +216,43 @@ the characteristics of a specific policy, always request it by name. Status ------ -<#used metadata blocks>/<#total metadata blocks> <#read hits> <#read misses> -<#write hits> <#write misses> <#demotions> <#promotions> <#blocks in cache> -<#dirty> <#features> <features>* <#core args> <core args>* <#policy args> -<policy args>* - -#used metadata blocks : Number of metadata blocks used -#total metadata blocks : Total number of metadata blocks -#read hits : Number of times a READ bio has been mapped +<metadata block size> <#used metadata blocks>/<#total metadata blocks> +<cache block size> <#used cache blocks>/<#total cache blocks> +<#read hits> <#read misses> <#write hits> <#write misses> +<#demotions> <#promotions> <#dirty> <#features> <features>* +<#core args> <core args>* <policy name> <#policy args> <policy args>* + +metadata block size : Fixed block size for each metadata block in + sectors +#used metadata blocks : Number of metadata blocks used +#total metadata blocks : Total number of metadata blocks +cache block size : Configurable block size for the cache device + in sectors +#used cache blocks : Number of blocks resident in the cache +#total cache blocks : Total number of cache blocks +#read hits : Number of times a READ bio has been mapped to the cache -#read misses : Number of times a READ bio has been mapped +#read misses : Number of times a READ bio has been mapped to the origin -#write hits : Number of times a WRITE bio has been mapped +#write hits : Number of times a WRITE bio has been mapped to the cache -#write misses : Number of times a WRITE bio has been +#write misses : Number of times a WRITE bio has been mapped to the origin -#demotions : Number of times a block has been removed +#demotions : Number of times a block has been removed from the cache -#promotions : Number of times a block has been moved to +#promotions : Number of times a block has been moved to the cache -#blocks in cache : Number of blocks resident in the cache -#dirty : Number of blocks in the cache that differ +#dirty : Number of blocks in the cache that differ from the origin -#feature args : Number of feature args to follow -feature args : 'writethrough' (optional) -#core args : Number of core arguments (must be even) -core args : Key/value pairs for tuning the core +#feature args : Number of feature args to follow +feature args : 'writethrough' (optional) +#core args : Number of core arguments (must be even) +core args : Key/value pairs for tuning the core e.g. migration_threshold -#policy args : Number of policy arguments to follow (must be even) -policy args : Key/value pairs - e.g. 'sequential_threshold 1024 +policy name : Name of the policy +#policy args : Number of policy arguments to follow (must be even) +policy args : Key/value pairs + e.g. sequential_threshold Messages -------- @@ -231,12 +268,28 @@ The message format is: E.g. dmsetup message my_cache 0 sequential_threshold 1024 + +Invalidation is removing an entry from the cache without writing it +back. Cache blocks can be invalidated via the invalidate_cblocks +message, which takes an arbitrary number of cblock ranges. Each cblock +range's end value is "one past the end", meaning 5-10 expresses a range +of values from 5 to 9. Each cblock must be expressed as a decimal +value, in the future a variant message that takes cblock ranges +expressed in hexidecimal may be needed to better support efficient +invalidation of larger caches. The cache must be in passthrough mode +when invalidate_cblocks is used. + + invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]* + +E.g. + dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789 + Examples ======== The test suite can be found here: -https://github.com/jthornber/thinp-test-suite +https://github.com/jthornber/device-mapper-test-suite dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \ /dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0' |