DOCS-2324: fsync lock recomendation in sharded cluster backup

Author: Sam Kleinman

source/includes/steps-backup-sharded-cluster-with-snapshots.yaml

@@ -15,14 +15,21 @@ post: |
   For more information, see the
   :ref:`sharding-balancing-disable-temporarily` procedure.
 ---
-title: Lock one secondary member of each replica set in each shard.
+title: If necessary, lock one secondary member of each replica set in each shard.
 stepnum: 2
 ref: lock
 content: |
-  Lock one secondary member of each replica set in each shard so that your
+  If your :program:`mongod` does not have journaling enabled *or* your
+  journal and data files are on different volumes, you **must** lock
+  your :program:`mongod` before capturing a back up.
+
+  If your :program:`mongod` has journaling enabled and your journal
+  and data files are on the same volume, you may skip this step.
+
+  If you need to lock the :program:`monogd`, attempt to lock one
+  secondary member of each replica set in each shard so that your
   backups reflect the state of your database at the nearest possible
-  approximation of a single moment in time. Lock these :program:`mongod`
-  instances in as short of an interval as possible.
+  approximation of a single moment in time.
 
   To lock a secondary, connect through the :program:`mongo` shell to the
   secondary member's :program:`mongod` instance and issue the
@@ -69,6 +76,9 @@ title: Unlock locked replica set members.
 stepnum: 5
 ref: unlock
 content: |
+  If you locked any :program:`mongod` instances to capture the backup,
+  unlock them now.
+
   Unlock all locked replica set members of each shard using the
   :method:`db.fsyncUnlock()` method in the :program:`mongo` shell.
 ---

source/tutorial/backup-sharded-cluster-with-filesystem-snapshots.txt

@@ -50,6 +50,16 @@ of the backup while minimizing impact on the cluster by taking the
 backup from a secondary member of the replica set that provides each
 shard.
 
+Consistency
+~~~~~~~~~~~
+
+If the journal and data files are on the same logical volume, you can
+use a single point-in-time snapshot to capture a valid copy of the data.
+
+If the journal and data files are on different file systems, you must
+use :method:`db.fsyncLock()` and :method:`db.fsyncUnLock()` to capture
+a valid copy of your data.
+
 Procedure
 ---------