Schedule Backup Window for Sharded Clusters
Overview
In a sharded cluster, the balancer process is responsible for distributing sharded data around the cluster, so that each shard has roughly the same amount of data.
However, when creating backups from a sharded cluster it is important that you disable the balancer while taking backups to ensure that no chunk migrations affect the content of the backup captured by the backup procedure.
Note
Disabling the balancer is only necessary when manually taking backups,
either by calling mongodump or scheduling a task that calls
mongodump at a specific time.
You do not have to disable the balancer when using coordinated backup and restore processes:
Using the procedure outlined in the section Disable the Balancer you can manually stop the balancer process temporarily. As an alternative, you can use the following procedure to define a balancing window so that the balancer is always disabled during your automated backup operation.
Tip
To use mongodump and mongorestore as a backup strategy
for sharded clusters, you must stop the
sharded cluster balancer and use the
fsync command or the db.fsyncLock() method on
mongos to block writes on the cluster during backups.
Sharded clusters can also use one of the following coordinated backup and restore processes, which maintain the atomicity guarantees of transactions across shards:
Procedure
If you have an automated backup schedule, you can disable all balancing operations for a period of time. For instance, consider the following command:
use config db.settings.update( { _id : "balancer" }, { $set : { activeWindow : { start : "06:00", stop : "23:00" } } }, true )
This operation configures the balancer to run between 6:00am and 11:00pm, server time. Schedule your backup operation to run and complete outside of this time. Ensure that the backup can complete outside the window when the balancer is running and that the balancer can effectively balance the collection among the shards in the window allotted to each.