dm cache: add passthrough mode

"Passthrough" is a dm-cache operating mode (like writethrough or
writeback) which is intended to be used when the cache contents are not
known to be coherent with the origin device.  It behaves as follows:

* All reads are served from the origin device (all reads miss the cache)
* All writes are forwarded to the origin device; additionally, write
  hits cause cache block invalidates

This mode decouples cache coherency checks from cache device creation,
largely to avoid having to perform coherency checks while booting.  Boot
scripts can create cache devices in passthrough mode and put them into
service (mount cached filesystems, for example) without having to worry
about coherency.  Coherency that exists is maintained, although the
cache will gradually cool as writes take place.

Later, applications can perform coherency checks, the nature of which
will depend on the type of the underlying storage.  If coherency can be
verified, 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.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Morgan Mears <Morgan.Mears@netapp.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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.