doc: Add keyvaluestore config description

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>

diff --git a/doc/rados/configuration/keyvaluestore-config-ref.rst b/doc/rados/configuration/keyvaluestore-config-ref.rst
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+============================
+ KeyValueStore Config Reference
+============================
+
+KeyValueStore is a another OSD backend compared to FileStore. Now it mainly
+use LevelDB as backend.
+
+KeyValueStore doesn't need jounal device, each op will flush into backend
+directly.
+
+``keyvaluestore backend``
+
+:Description: the backend used by keyvaluestore
+:Type: String
+:Required: No
+:Default: ``leveldb``
+
+
+.. index:: keyvaluestore; queue
+
+Queue
+=====
+
+The following settings provide limits on the size of keyvaluestore queue.
+
+``keyvaluestore queue max ops``
+
+:Description: Defines the maximum number of in progress operations the keyvaluestore accepts before blocking on queuing new operations.
+:Type: Integer
+:Required: No. Minimal impact on performance.
+:Default: ``50``
+
+
+``keyvaluestore queue max bytes``
+
+:Description: The maximum number of bytes for an operation.
+:Type: Integer
+:Required: No
+:Default: ``100 << 20``
+
+.. index:: keyvaluestore; thread
+
+Thread
+========
+
+
+``keyvaluestore op threads``
+
+:Description: The number of keyvaluestore operation threads that execute in parallel. 
+:Type: Integer
+:Required: No
+:Default: ``2``
+
+
+``keyvaluestore op thread timeout``
+
+:Description: The timeout for a keyvaluestore operation thread (in seconds).
+:Type: Integer
+:Required: No
+:Default: ``60``
+
+
+``keyvaluestore op thread suicide timeout``
+
+:Description: The timeout for a commit operation before cancelling the commit (in seconds). 
+:Type: Integer
+:Required: No
+:Default: ``180``
+
+
+Misc
+====
+
+
+``keyvaluestore default strip size``
+
+:Description: each object will be split into multi key/value pairs stored into
+              backend.
+              Note: now this option is important to performance for estimable
+              workload
+:Type: Integer
+:Required: No
+:Default: ``4096``
+
+
+``keyvaluestore header cache size``
+
+:Description: the number of header cache, it just like "inode" in local
+              filesystem. The larger cache size will be help for performance
+
+:Type: Integer
+:Required: No
+:Default: ``4096``