diff options
Diffstat (limited to 'Documentation/device-mapper')
| -rw-r--r-- | Documentation/device-mapper/cache.txt | 19 |
1 files changed, 16 insertions, 3 deletions
diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt index 33d45ee0b737..ff6639f72536 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 @@ -81,6 +82,18 @@ 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. 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, the cache device can 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. |