doc/osd: Fixes the introduction for writeback mode of cache tier.

Now, the statement of writeback is incorrect, it says the clients
write data to cache tier firstly, then flush to base tier. But
is not in fact, that will make users confused. Reads too.

Signed-off-by: Mingyuan Liang <liangmingyuan@baidu.com>

diff --git a/doc/rados/operations/cache-tiering.rst b/doc/rados/operations/cache-tiering.rst
index 8dec0ced6544329c1d1c1524f2318b14d83accc2..00a3f37f93715b9775158c3bcafd158895792184 100644 (file)
--- a/doc/rados/operations/cache-tiering.rst
+++ b/doc/rados/operations/cache-tiering.rst
@@ -46,15 +46,17 @@ configure how this migration takes place by setting the ``cache-mode``. There ar
 two main scenarios:
 
 - **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 
-  overlaid "in front" of the backing storage tier. When a Ceph client needs 
-  data that resides in the storage tier, the cache tiering agent migrates the
-  data to the cache tier on read, then it is sent to the Ceph client. 
-  Thereafter, the Ceph client can perform I/O using the cache tier, until the 
-  data becomes inactive. This is ideal for mutable data (e.g., photo/video 
-  editing, transactional data, etc.).
+  clients write data to the base tier and receive an ACK from it. Then the cache
+  tiering agent compares ``osd_tier_default_cache_min_write_recency_for_promote``,
+  that means if the data is written over given times at a interval, it will
+  be promoted to the cache tier. When Ceph clients need data that resides in
+  the base tier, the cache tier will proxy read the data from the base tier
+  and return to the client. Meanwhile, as same as write does, the cache tiering
+  agent will decide whether migrating the data to the cache tier based on
+  ``osd_tier_default_cache_min_read_recency_for_promote``. After the data
+  is promoted to from the base tier, the Ceph client can perform I/O using
+  the cache tier, until the data becomes inactive. This is ideal for mutable
+  data (e.g., photo/video editing, transactional data, etc.).
 
 - **readproxy** mode: This mode will use any objects that already
   exist in the cache tier, but if an object is not present in the