(DOCS-14449): maxCatchUpPercentageBeforeBlockingWrites server parameter

Author: jeff-allen-mongo

source/reference/command/moveChunk.txt

@@ -188,5 +188,14 @@ while :dbcommand:`moveChunk` is running, you may see this
 error. You may retry the :dbcommand:`moveChunk` operation without
 side effects.
 
+``maxCatchUpPercentageBeforeBlockingWrites`` Server Parameter
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 4.2.15 (and 4.0.26), you can set the
+:parameter:`maxCatchUpPercentageBeforeBlockingWrites` to specify the
+maximum allowed percentage of data not yet migrated
+during a :dbcommand:`moveChunk` operation when compared to the
+total size (in MBs) of the chunk being transferred.
+
 
 .. admin-only

source/reference/parameters.txt

@@ -2073,6 +2073,40 @@ Sharding Parameters
       - :serverstatus:`shardedIndexConsistency` metrics returned by the
         :dbcommand:`serverStatus` command.
 
+.. parameter:: maxCatchUpPercentageBeforeBlockingWrites
+
+   .. versionadded:: 4.2.15 (*Also available starting in 4.0.26*)
+
+   *Type*: integer
+
+   *Default*: 10
+
+   |mongod-only|
+
+   For :dbcommand:`moveChunk` operations, specifies the maximum
+   percentage of untrasferred data allowed by the migration protocol
+   (expressed in percentage of the total chunk size) to
+   transition from the ``catchup`` phase to the ``commit`` phase.
+
+   Setting a higher catchup percentage can decrease the amount of time
+   it takes for the migration to complete at the cost of increased
+   latency during concurrent :method:`upsert <db.collection.updateOne>`
+   and :method:`delete <db.collection.deleteOne>` operations.
+
+   For example, to set the maximum percentage to 20, you can issue the
+   followingduring startup:
+
+   .. code-block:: bash
+
+      mongod --setParameter maxCatchUpPercentageBeforeBlockingWrites=20
+
+   You cannot change
+   :parameter:`maxCatchUpPercentageBeforeBlockingWrites` during runtime.
+
+   .. seealso::
+
+      `Live Migration Protocol <https://github.com/mongodb/mongo/blob/master/src/mongo/db/s/README.md#the-live-migration-protocol>`__
+
 .. parameter:: shardedIndexConsistencyCheckIntervalMS
 
    .. versionadded:: 4.2.6

source/tutorial/migrate-chunks-in-sharded-cluster.txt

@@ -1,3 +1,5 @@
+.. _migrate-chunks-sharded-cluster:
+
 ===================================
 Migrate Chunks in a Sharded Cluster
 ===================================