DOCSP-44120-clarify-mongosync-sharding-behavior (#487) (#497)

* DOCSP-44120-mongosync-sharding-behavior

* DOCSP-44120-mongosync-sharding-behavior

* DOCSP-44120-mongosync-sharding-behavior

* DOCSP-44120-FAQ

* DOCSP-44120 tech edit

* DOCSP-44120 tech edit

(cherry picked from commit aba042e)

Author: ltran-mdb2

source/faq.txt

@@ -187,3 +187,11 @@ Does ``mongosync`` Support TTL Indexes?
 {+c2c-product-name+} supports syncing :ref:`index-feature-ttl` from the source
 to the destination cluster.
 
+
+Can I customize chunk distributions when syncing into a sharded cluster?
+------------------------------------------------------------------------
+
+No, you can't configure ``mongosync`` to customize chunk distributions
+on a destination sharded cluster.  ``mongosync`` samples each collection
+during initialization to determine how to distribute documents
+efficiently across the destination cluster’s shards after migration.

source/reference/mongosync/mongosync-behavior.txt

@@ -74,6 +74,19 @@ for sharded collections on the destination cluster. For each sharded collection,
 ``mongosync`` creates twice as many chunks as there are shards in the 
 destination cluster. 
 
+Chunk Distribution
+''''''''''''''''''
+
+``mongosync`` does not preserve chunk distribution from the source to
+the destination, even with multiple ``mongosync`` instances. It is not
+possible to reproduce a particular pre-split of chunks from a source
+cluster on the destination cluster. 
+
+The only sharding configuration that ``mongosync`` preserves from the
+source cluster to the destination cluster is the sharding key. Once the
+migration finishes, you can enable the destination cluster's balancer which
+distributes documents independently of the source cluster's distribution. 
+
 Primary Shards
 ''''''''''''''