Things to consider before you back up

Here are two important considerations you should address before performing a backup:

• Performance impact: When you perform a backup, the amount of data being processed by the local server increases geometrically. Inevitably, this will have a negative impact on performance. You might want to consider scheduling the backup for a time when the least amount of database activity is expected.
• Replica sets: If the server you are backing up is part of a replica set, backing up a secondary can cause problems as the data being backed up might not be current. You can use the --oplog option to cause the backup to include the operations log, which can be used to identify the point in time when the backup of the primary server in a replica set occurred. 
  It is highly recommended when you back up a replica set member to only back up the primary. This is easily accomplished by adding the following option:

--host=<replica_set_name>/<primary_host_address...