DOCS-831 shard keys and gridfs

source/core/sharding-internals.txt

@@ -534,6 +534,41 @@ a document with a ``msg`` field that holds the string
 If the application is instead connected to a :program:`mongod`, the
 returned document does not include the ``isdbgrid`` string.
 
+Shard GridFS Documents
+----------------------
+
+A common way to shard :term:`GridFS` is to configure the shard as follows:
+
+- Do not shard the ``files`` collection, as the keys in this collection do
+  not easily lend themselves to even distributions.
+
+  Leaving ``files`` unsharded means that all the file metadata documents
+  live on one shard. It is recommended that the shard is a replica set
+  with at least three members, for high availability.
+
+- Shard the ``chunks`` collection using a new ``files_id : 1 , n : 1``
+  index. You must create this index. Do not use the existing
+  ``files_id : 1 , n : 1`` index already created by the drivers.
+
+  The new ``files_id : 1 , n : 1`` index ensures that all chunks of a
+  given file live on the same shard, which is safer and allows FileMD5
+  hashing.
+
+  To shard the ``chunks`` collection by ``files_id : 1 , n : 1``, issue
+  commands similar to the following:
+
+  .. code-block:: javascript
+
+     db.fs.chunks.ensureIndex( { files_id : 1 , n : 1 } )
+
+     db.runCommand( { shardcollection : "test.fs.chunks" , key : { files_id : 1 , n : 1 } } )
+
+  The default ``files_id`` is an :term:`ObjectId`. The ``files_id`` is
+  ascending, and all GridFS chunks are sent to a single sharding chunk.
+  If your write load is too high for a single server to handle, you may
+  want to shard on a different key or use a different value for ``_id``
+  in the ``files`` collection.
+
 .. index:: config database
 .. index:: database, config
 .. _sharding-internals-config-database: