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Backup a Sharded Cluster with Database Dumps

Overview

This document describes a procedure for taking a backup of all components of a sharded cluster. This procedure uses mongodump to create dumps of the mongod instance. An alternate procedure uses file system snapshots to capture the backup data, and may be more efficient in some situations if your system configuration allows file system backups. See Backup a Sharded Cluster with Filesystem Snapshots.

See MongoDB Backup Methods and Backup and Restore Sharded Clusters for a complete information on backups in MongoDB and backups of sharded clusters in particular.

Important

To capture a point-in-time backup from a sharded cluster you must stop all writes to the cluster. On a running production system, you can only capture an approximation of point-in-time snapshot.

Procedure

In this procedure, you will stop the cluster balancer and take a backup up of the config database, and then take backups of each shard in the cluster using mongodump to capture the backup data. If you need an exact moment-in-time snapshot of the system, you will need to stop all application writes before taking the filesystem snapshots; otherwise the snapshot will only approximate a moment of time.

For approximate point-in-time snapshots, you can improve the quality 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.

  1. Disable the balancer process that equalizes the distribution of data among the shards. To disable the balancer, use the sh.stopBalancer() method in the mongo shell. For example:

    use config
    sh.setBalancerState(false)
    

    For more information, see the Disable the Balancer procedure.

    Warning

    It is essential that you stop the balancer before creating backups. If the balancer remains active, your resulting backups could have duplicate data or miss some data, as chunks migrate while recording backups.

  2. Lock one 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 mongod instances in as short of an interval as possible.

    To lock or freeze a sharded cluster, you shut down one member of each replica set. Ensure that the oplog has sufficient capacity to allow these secondaries to catch up to the state of the primaries after finishing the backup procedure. See Oplog Size for more information.

  3. Use mongodump with the --oplog option to backup one of the config servers. This backs up the cluster’s metadata. You only need to back up one config server as they all hold the same data.

    Run mongodump against the config server mongod instance; the config server mongod instance must be version 2.4 or later and must run with the --configsvr option.

    mongodump --oplog
    
  4. Back up the replica set members of the shards that shut down using mongodump and specifying the --dbpath option. You may back up the shards in parallel. Consider the following invocation:

    mongodump --journal --dbpath /data/db/ --out /data/backup/
    

    You must run this command on the system where the mongod ran. This operation will use journaling and create a dump of the entire mongod instance with data files stored in /data/db/. mongodump will write the output of this dump to the /data/backup/ directory.

  5. Restart all stopped replica set members of each shard as normal and allow them to catch up with the state of the primary.

  6. Re-enable the balancer with the sh.setBalancerState() method.

    Use the following command sequence when connected to the mongos with the mongo shell:

    use config
    sh.setBalancerState(true)