DOCS-871 multithreaded replication

Author: Bob Grabar

source/core/replication-internals.txt

@@ -304,6 +304,22 @@ behaviors:
   value, either ``true`` or ``false``, for the
   :data:`members[n].buildIndexes` field.
 
+Multithreaded Replication
+-------------------------
+
+MongoDB supports multithreaded replication by applying write operations
+on secondary nodes in batches. Each batch is divided among many threads.
+The multithreaded writes allow for concurrent CPU usage.
+
+Despite the multithreaded writes, reads on secondaries do not "see"
+operations applied out of order. While the threads are writing, MongoDB
+blocks read operations, even if the write operations yield.
+
+Before writing a batch of oplog operations, MongoDB fetches all the
+memory pages that contain the data and indexes to be written. The
+prefetch stage minimizes the amount of time MongoDB must hold the write
+lock to apply operations.
+
 Pre-Fetching Indexes to Improve Replication Throughput
 ------------------------------------------------------