doc/dev/cache-pool: document cache pool management interface

Signed-off-by: Sage Weil <sage@inktank.com>

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+Cache pool
+==========
+
+Purpose
+-------
+
+Use a pool of fast storage devices (probably SSDs) and use it as a
+cache for an existing larger pool.
+
+We should be able to create and add a cache pool to an existing pool
+of data, and later remove it, without disrupting service or migrating
+data around.
+
+Use cases
+---------
+
+Read-write pool, writeback
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We have an existing data pool and put a fast cache pool "in front" of it.  Writes will
+go to the cache pool and immediately ack.  We flush them back to the data pool based on
+some policy.
+
+Read-only pool, weak consistency
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We have an existing data pool and add one or more read-only cache
+pools.  We copy data to the cache pool(s) on read.  Writes are
+forwarded to the original data pool.  Stale data is expired from the
+cache pools based on some as-yet undetermined policy.
+
+This is likely only useful for specific applications with specific
+data access patterns.  It may be a match for rgw, for example.
+
+
+Interface
+---------
+
+Set up a read/write cache pool foo-hot for pool foo::
+
+ ceph osd tier add foo foo-hot
+ ceph osd tier cache-mode foo-hot writeback
+ ceph osd tier cache-target-size foo-hot 10G
+ ceph osd tier cache-target-dirty foo-hot 1G
+
+Direct all traffic for foo to foo-hot::
+
+ ceph osd tier set-overlay foo foo-hot
+
+Drain the cache in preparation for turning it off::
+
+ ceph osd tier cache-mode foo-hot invalidate+forward
+ ceph osd tier cache-target-size foo-hot 0   # do not cache any new items
+
+When cache pool is finally empty, disable it::
+
+ ceph osd tier remove-overlay foo
+ ceph osd tier remove foo foo-hot
+
+Read-only pools with lazy consistency::
+
+ ceph osd tier add foo foo-east
+ ceph osd tier cache-mode foo-east readonly
+ ceph osd tier add foo foo-west
+ ceph osd tier cache-mode foo-west readonly
+
+Set up a cold storage tier::
+
+ ceph osd tier add foo foo-cold
+