Back Up a Self-Managed Sharded Cluster with a Database Dump

Find a Backup Window

To find a good time to perform a backup, monitor your application and database usage to find a time when chunk migrations, resharding, and schema transformation operations are unlikely to occur.

For more information, see Schedule Backup Window for a Self-Managed Sharded Cluster.

Stop the Balancer

To prevent chunk migrations from disrupting the backup, connect to mongos and use the sh.stopBalancer() method to stop the balancer:

sh.stopBalancer()

If a balancing round is in progress, the operation waits for balancing to complete.

To verify that the balancer is stopped, use the sh.getBalancerState() method:

use config
while( sh.isBalancerRunning().mode != "off" ) {
   print( "Waiting for Balancer to stop..." );
   sleep( 1000 );
}

Lock the Cluster

The sharded cluster must remain locked during the backup process to protect the database from writes, which may cause inconsistencies in the backup.

To lock a sharded cluster, connect to mongos and use the db.fsyncLock() method:

db.getSiblingDB("admin").fsyncLock()

To confirm the lock, on mongos and the primary mongod of the config servers, run the following aggregation pipeline and ensure that all the shards are locked:

db.getSiblingDB("admin").aggregate( [
   {  $currentOp: { } },
   {  $facet: {
      "locked": [
         { $match: { $and: [
            { fsyncLock: { $exists: true } }
         ] } }],
       "unlocked": [
         { $match: { fsyncLock: { $exists: false } } }
       ]
   } },
   {  $project: {
      "fsyncLocked": { $gt: [ { $size: "$locked" }, 0 ] },
      "fsyncUnlocked": { $gt: [ { $size: "$unlocked" }, 0 ] }
   } }
 ] )

[ { fsyncLocked: true }, { fsyncUnlocked: false } ]

Take Backup

To back up the sharded cluster, use mongodump to connect to mongos and perform the backup:

mongodump \
   --host mongos.example.net \
   --port 27017 \
   --username user \
   --password "passwd" \
   --out /opt/backups/example-cluster-1

Unlock the Cluster

After the backup completes, you can unlock the cluster to allow writes to resume.

To unlock the cluster, use the db.fsyncUnlock() method:

db.getSibling("admin").fsyncUnlock()

To confirm the unlock, on mongos and the primary mongod of the config servers, run the following aggregation pipeline and ensure that all shards are unlocked:

db.getSiblingDB("admin").aggregate( [
   {  $currentOp: { } },
   {  $facet: {
      "locked": [
         { $match: { $and: [
            { fsyncLock: { $exists: true } }
         ] } }],
       "unlocked": [
         { $match: { fsyncLock: { $exists: false } } }
       ]
   } },
   { $project: {
      "fsyncLocked": { $gt: [ { $size: "$locked" }, 0 ] },
      "fsyncUnlocked": { $gt: [ { $size: "$unlocked" }, 0 ] }
   } }
 ] )

[ { fsyncLocked: false }, { fsyncUnlocked: true } ]

Restart the Balancer

To restart the balancer, use the sh.startBalancer() method:

sh.startBalancer()

To confirm that the balancer is running, use the sh.getBalancerState() method:

sh.getBalancerState()

true

The command returns true when the balancer is running.