Not atomic1

source/reference/command/mapReduce.txt

@@ -123,6 +123,18 @@ mapReduce
                    enabled, the map-reduce operation will shard the
                    results collection according to the ``_id`` field.
 
+   :param nonAtomic: 
+
+          .. versionadded:: 2.1
+
+          Optional. Specify output operation as non-atomic such that
+          the output behaves like a normal ``multi`` :method:`update()
+          <db.collection.update()>`. If ``true``, post processing step
+          will not be executed while locking the database, and partial
+          results will be visible as they are processed. The option is
+          valid only for ``merge`` and ``reduce`` output operations for
+          which post processing can be a long operation.
+
    .. seealso:: ":method:`mapReduce()`" and ":term:`map-reduce`."
 
       Also, the ":wiki:`MapReduce` page, provides a greater overview

source/reference/method/db.collection.mapReduce.txt

@@ -153,6 +153,18 @@ db.collection.mapReduce()
                    shard the results collection according to the
                    ``_id`` field.
 
+   :param nonAtomic: 
+
+          .. versionadded:: 2.1
+
+          Optional. Specify output operation as non-atomic such that
+          the output behaves like a normal ``multi`` :method:`update()
+          <db.collection.update()>`. If ``true``, post processing step
+          will not be executed while locking the database, and partial
+          results will be visible as they are processed. The option is
+          valid only for ``merge`` and ``reduce`` output operations for
+          which post processing can be a long operation.
+
    .. seealso:: :term:`map-reduce`, provides a greater overview
       of MongoDB's map-reduce functionality.