DOCS-387 FAQ entry for shard balancer prioritization

DOCS-387 incorporate Jeremy's comments

DOCS-387 balancer and shard drain

Author: kay-kim

source/faq/sharding.txt

@@ -399,3 +399,45 @@ chunk migration failure independently.
 If you encounter this issue, contact the `MongoDB User Group
 <http://groups.google.com/group/mongodb-user>`_ or 10gen support to
 address this issue.
+
+How does draining a shard affect the balancing of uneven chunk distribution?
+----------------------------------------------------------------------------
+
+Consider the scenario where you have a sharded cluster with an uneven
+chunk distribution and you remove a shard as well as add new shards.
+
+With version 2.2, the balancer first removes the chunks from the
+draining shard and then balances the uneven chunk distribution.
+
+Before version 2.2, the balancer first handles the uneven chunk
+distribution and then removes the chunks from the draining shard.
+Although this balancer prioritization is **not** configurable, you can,
+however, achieve the desired behavior with one of the following manual
+solutions:
+
+- Option 1
+
+  #. Stop the balancer.
+
+  #. Use the :dbcommand:`moveChunk` command to move chunks off of the
+     draining shard.
+
+  #. Restart the balancer.
+
+- Option 2
+
+  #. Set the ``maxSize`` on the new shards to some low value so that they
+     cannot accept more chunks. This should stop the balancing
+     operation and allow the draining to continue with chunks
+     moved to the full shards.
+
+  #. Restore the original ``maxSize`` on the new shards.
+
+  .. note::
+
+     When changing the ``maxSize`` in the :term:`config database`:
+
+     - in versions 2.0+, run the :dbcommand:`flushRouterConfig` command
+       to refresh the :program:`mongos`.
+
+     - in versions pre-2.0, you need to restart :program:`mongos`.