DOCSP-44657-mongosync-latency-performance

source/reference/mongosync/mongosync-behavior.txt

@@ -369,6 +369,31 @@ progress.
 For more information on DDL operations, see
 :ref:`txn-prod-considerations-ddl`. 
 
+Network Latency
+~~~~~~~~~~~~~~~
+
+Network latency or long physical distances between migration components
+can negatively affect sync speed.
+
+Latency between ``mongosync`` and destination shards
+  For each operation on the source cluster, ``mongosync`` does two 
+  roundtrips to the destination server. The larger the latency, the 
+  slower the sync.
+
+Latency between destination shards
+  ``mongosync`` runs operations and updates its own metadata in batches 
+  in a transaction on the destination cluster. This can result in 
+  cross-shard transactions, which may be more costly if the shards are 
+  far apart.
+
+Latency between the nodes of any replica set on the source or destination cluster
+  ``mongosync`` uses :writeconcern:`"majority"` writes and 
+  :readconcern:`"majority"` reads, which require acknowledgement from 
+  multiple nodes in a replica set, including shard-backing replica 
+  sets. If the majority of these nodes aren't in the same region, there 
+  will be negative performance implications.
+
+
 Learn More
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