Page: http://docs.mongodb.org/manual/tutorial/backup-with-filesystem-snapshots/#create-backups-on-instances-that-do-not-have-journaling-enabled

This page should contain a warning message highlighting following two facts:

• The fsyncLock / fsyncUnlock should happen on the same connection for safety else is susceptible to locking out the mongod instance that may need restart
• An termination of connection once fsyncLock has been done, does not imply automatic fsyncUnlock and can lead to a locked out mongod instance. The application performing fsyncLock over a specific connection should ideally handle any error condition and perform an fsyncUnlock appropriately.