doc: cover more cache modes in rados/operations/cache-tiering.rst

Fixes: http://tracker.ceph.com/issues/14153
Signed-off-by: Nathan Cutler <ncutler@suse.com>

diff --git a/doc/rados/operations/cache-tiering.rst b/doc/rados/operations/cache-tiering.rst
index 0e6ff96b378b0f0108319d3715707852ef9b4c33..c825c22c3035a4ac33595fd2ee87889ac2a687bb 100644 (file)
--- a/doc/rados/operations/cache-tiering.rst
+++ b/doc/rados/operations/cache-tiering.rst
@@ -42,9 +42,10 @@ to Ceph clients.
 
 The cache tiering agent handles the migration of data between the cache tier 
 and the backing storage tier automatically. However, admins have the ability to
-configure how this migration takes place. There are two main scenarios: 
+configure how this migration takes place by setting the ``cache-mode``. There are
+two main scenarios:
 
-- **Writeback Mode:** When admins configure tiers with ``writeback`` mode, Ceph
+- **writeback** mode: When admins configure tiers with ``writeback`` mode, Ceph
   clients write data to the cache tier and receive an ACK from the cache tier.
   In time, the data written to the cache tier migrates to the storage tier
   and gets flushed from the cache tier. Conceptually, the cache tier is 
@@ -55,13 +56,26 @@ configure how this migration takes place. There are two main scenarios:
   data becomes inactive. This is ideal for mutable data (e.g., photo/video 
   editing, transactional data, etc.).
 
-- **Read-proxy Mode:** This mode will use any objects that already
+- **readproxy** mode: This mode will use any objects that already
   exist in the cache tier, but if an object is not present in the
   cache the request will be proxied to the base tier.  This is useful
   for transitioning from ``writeback`` mode to a disabled cache as it
   allows the workload to function properly while the cache is drained,
   without adding any new objects to the cache.
 
+Other cache modes are:
+
+- **readonly** promotes objects to the cache on read operations only; write
+  operations are forwarded to the base tier. This mode is intended for
+  read-only workloads that do not require consistency to be enforced by the
+  storage system. (**Warning**: when objects are updated in the base tier,
+  Ceph makes **no** attempt to sync these updates to the corresponding objects
+  in the cache. Since this mode is considered experimental, a
+  ``--yes-i-really-mean-it`` option must be passed in order to enable it.)
+
+- **none** is used to completely disable caching.
+
+
 A word of caution
 =================