Configure Behavior of Balancer Process in Sharded Clusters

The balancer is a process that runs on one of the mongos instances in a cluster and ensures that chunks are evenly distributed throughout a sharded cluster. In most deployments, the default balancer configuration is sufficient for normal operation. However, administrators might need to modify balancer behavior depending on application or operational requirements. If you encounter a situation where you need to modify the behavior of the balancer, use the procedures described in this document.

For conceptual information about the balancer, see Sharded Collection Balancing and Cluster Balancer.

Schedule a Window of Time for Balancing to Occur

You can schedule a window of time during which the balancer can migrate chunks, as described in the following procedures:

The mongos instances user their own local timezones to when respecting balancer window.

Configure Default Chunk Size

The default chunk size for a sharded cluster is 64 megabytes. In most situations, the default size is appropriate for splitting and migrating chunks. For information on how chunk size affects deployments, see details, see Chunk Size.

Changing the default chunk size affects chunks that are processes during migrations and auto-splits but does not retroactively affect all chunks.

To configure default chunk size, see Modify Chunk Size in a Sharded Cluster.

Change the Maximum Storage Size for a Given Shard

The maxSize field in the shards collection in the config database sets the maximum size for a shard, allowing you to control whether the balancer will migrate chunks to a shard. If mapped size [1] is above a shard’s maxSize, the balancer will not move chunks to the shard. Also, the balancer will not move chunks off an overloaded shard. This must happen manually. The maxSize value only affects the balancer’s selection of destination shards.

By default, maxSize is not specified, allowing shards to consume the total amount of available space on their machines if necessary.

You can set maxSize both when adding a shard and once a shard is running.

To set maxSize when adding a shard, set the addShard command’s maxSize parameter to the maximum size in megabytes. For example, the following command run in the mongo shell adds a shard with a maximum size of 125 megabytes:

db.runCommand( { addshard : "example.net:34008", maxSize : 125 } )

To set maxSize on an existing shard, insert or update the maxSize field in the shards collection in the config database. Set the maxSize in megabytes.


Assume you have the following shard without a maxSize field:

{ "_id" : "shard0000", "host" : "example.net:34001" }

Run the following sequence of commands in the mongo shell to insert a maxSize of 125 megabytes:

use config
db.shards.update( { _id : "shard0000" }, { $set : { maxSize : 125 } } )

To later increase the maxSize setting to 250 megabytes, run the following:

use config
db.shards.update( { _id : "shard0000" }, { $set : { maxSize : 250 } } )
[1]This value includes the mapped size of all data files including the``local`` and admin databases. Account for this when setting maxSize.

Change Replication Behavior for Chunk Migration (Secondary Throttle)

The _secondaryThrottle parameter of the balancer and the moveChunk command affects the replication behavior during chunk migration. By default, _secondaryThrottle is true, which means each document move during chunk migration propagates to at least one secondary before the balancer proceeds with its next operation. For more information on the replication behavior during various steps of chunk migration, see Chunk Migration and Replication.

To change the balancer’s _secondaryThrottle value, connect to a mongos instance and directly update the _secondaryThrottle value in the settings collection of the config database. For example, from a mongo shell connected to a mongos, issue the following command:

use config
   { "_id" : "balancer" },
   { $set : { "_secondaryThrottle" : true } },
   { upsert : true }

The effects of changing the _secondaryThrottle value may not be immediate. To ensure an immediate effect, stop the balancer and restart it with the selected value of _secondaryThrottle. See Manage Sharded Cluster Balancer for details.