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Diffstat (limited to 'Documentation/device-mapper/cache.txt')
| -rw-r--r-- | Documentation/device-mapper/cache.txt | 57 |
1 files changed, 6 insertions, 51 deletions
diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt index 274752f..33d45ee 100644 --- a/Documentation/device-mapper/cache.txt +++ b/Documentation/device-mapper/cache.txt @@ -68,11 +68,10 @@ 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). -Cache operating modes ---------------------- +Writeback/writethrough +---------------------- -The cache has three operating modes: writeback, writethrough and -passthrough. +The cache has two modes, writeback and writethrough. 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 @@ -82,31 +81,8 @@ 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 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. +dirty blocks in a cache. Useful for decommissioning a cache. Migration throttling -------------------- @@ -185,7 +161,7 @@ Constructor block size : cache unit size in sectors #feature args : number of feature arguments passed - feature args : writethrough or passthrough (The default is writeback.) + feature args : writethrough. (The default is writeback.) policy : the replacement policy to use #policy args : an even number of arguments corresponding to @@ -201,13 +177,6 @@ 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. @@ -262,26 +231,12 @@ 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 -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/device-mapper-test-suite +https://github.com/jthornber/thinp-test-suite dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \ /dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0' |