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Back Up a Sharded Cluster with Database Dumps

Changed in version 3.2: Starting in MongoDB 3.2, the following procedure can be used with the MMAPv1 and the WiredTiger storage engines. With previous versions of MongoDB, the procedure applied to MMAPv1 only.

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 instances.

You may alternatively use file system snapshots to capture the backup data. File system snapshots may be more efficient in some situations if your system configuration supports them.

For more information on backups in MongoDB and backups of sharded clusters in particular, see MongoDB Backup Methods and Backup and Restore Sharded Clusters.

Prerequisites

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.

Access Control

The backup role provides the required privileges to perform backup on a sharded cluster that has access control enabled.

Changed in version 3.2.1: The backup role provides additional privileges to back up the system.profile collections that exist when running with database profiling. Previously, users required an additional read access on this collection.

Consideration

To create backups of a sharded cluster, you will stop the cluster balancer, take a backup of the config database, and then take backups of each shard in the cluster using mongodump to capture the backup data. To capture a more 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 in time.

For approximate point-in-time snapshots, you can minimize the impact on the cluster by taking the backup from a secondary member of each replica set shard.

Procedure

1

Disable the balancer process.

To disable the balancer, connect the mongo shell to a mongos instance and run sh.stopBalancer() in the config database.

use config
sh.stopBalancer()

For more information, see the Disable the Balancer procedure.

Warning

If you do not stop the balancer, the backup could have duplicate data or omit data as chunks migrate while recording backups.

2

Lock one secondary member of each replica set.

Lock a secondary member of each replica set in the sharded cluster, and, if your sharded cluster uses a replica set for the config servers, one secondary of the config server replica set (CSRS).

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.

Lock shard replica set secondary.

For each shard replica set in the sharded cluster, connect a mongo shell to the secondary member’s mongod instance and run db.fsyncLock().

db.fsyncLock()

When calling db.fsyncLock(), ensure that the connection is kept open to allow a subsequent call to db.fsyncUnlock().

Lock config server replica set secondary.

If locking a secondary of the CSRS, confirm that the member has replicated data up to some control point. To verify, first connect a mongo shell to the CSRS primary and perform a write operation with "majority" write concern on a control collection:

use config
db.BackupControl.findAndModify(
   {
     query: { _id: 'BackupControlDocument' },
     update: { $inc: { counter : 1 } },
     new: true,
     upsert: true,
     writeConcern: { w: 'majority', wtimeout: 15000 }
   }
);

The operation should return either the newly inserted document or the updated document:

{ "_id" : "BackupControlDocument", "counter" : 1 }

Query the CSRS secondary member for the returned control document. Connect a mongo shell to the CSRS secondary to lock and use db.collection.find() to query for the control document:

rs.slaveOk();
use config;

db.BackupControl.find(
   { "_id" : "BackupControlDocument", "counter" : 1 }
).readConcern('majority');

If the secondary member contains the latest control document, it is safe to lock the member. Otherwise, wait until the member contains the document or select a different secondary member that contains the latest control document.

To lock the secondary member, run db.fsyncLock() on the member:

db.fsyncLock()

When calling db.fsyncLock(), ensure that the connection is kept open to allow a subsequent call to db.fsyncUnlock().

3

Backup one config server.

Run mongodump against a config server mongod instance to back up the cluster’s metadata. You only need to back up one config server. If you are using CSRS config servers and locked a config server secondary in the previous step, perform this step against the locked config server.

Use mongodump with the --oplog option to backup one of the config servers.

mongodump --oplog

If your deployment uses CSRS config servers, unlock the config server node before proceeding to the next step. To unlock the CSRS member, use db.fsyncUnlock() method in the mongo shell used to lock the instance.

db.fsyncUnlock()
4

Back up a replica set member for each shard.

Back up the locked replica set members of the shards using mongodump with the --oplog option. You may back up the shards in parallel.

mongodump --oplog
5

Unlock replica set members for each shard.

To unlock the replica set members, use db.fsyncUnlock() method in the mongo shell. For each locked member, use the same mongo shell used to lock the instance.

db.fsyncUnlock()

Allow these members to catch up with the state of the primary.

6

Re-enable the balancer process.

To re-enable to balancer, connect the mongo shell to a mongos instance and run sh.setBalancerState().

use config
sh.setBalancerState(true)

Additional Resources

See also MongoDB Cloud Manager for seamless automation, backup, and monitoring.