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Balancer

Manage Sharded Cluster Balancer

On this page

  • Check the Balancer State
  • Check if Balancer is Running
  • Configure Default Range Size
  • Schedule the Balancing Window
  • Remove a Balancing Window Schedule
  • Disable the Balancer
  • Enable the Balancer
  • Disable Balancing During Backups
  • Disable Balancing on a Collection
  • Enable Balancing on a Collection
  • Confirm Balancing is Enabled or Disabled
  • Change Replication Behavior for Chunk Migration
  • Change the Maximum Storage Size for a Given Shard

Changed in version 6.1.

This page describes common administrative procedures related to balancing. For an introduction to balancing, see Sharded Cluster Balancer. For lower level information on balancing, see Balancer Internals.

The balancer process has moved from the mongos instances to the primary member of the config server replica set.

Check the Balancer State

sh.getBalancerState() checks if the balancer is enabled (i.e. that the balancer is permitted to run). sh.getBalancerState() does not check if the balancer is actively migrating data.

To see if the balancer is enabled in your sharded cluster, issue the following command, which returns a boolean:

sh.getBalancerState()

You can also see if the balancer is enabled using sh.status(). The currently-enabled field indicates whether the balancer is enabled, while the currently-running field indicates if the balancer is currently running.

Check if Balancer is Running

To see if the balancer process is active in your cluster:

  1. Connect to any mongos in the cluster using the mongosh shell.

  2. Use the following operation to determine if the balancer is running:

    sh.isBalancerRunning()

Configure Default Range Size

The default range size for a sharded cluster is 128 megabytes. In most situations, the default size is appropriate for splitting and migrating chunks. For information on how range size affects deployments, see details, see Range Size.

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

Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.

To configure default range size, see Modify Range Size in a Sharded Cluster.

Schedule the Balancing Window

In some situations, particularly when your data set grows slowly and a migration can impact performance, it is useful to ensure that the balancer is active only at certain times. The following procedure specifies the activeWindow, which is the timeframe during which the balancer will be able to migrate chunks:

1

Connect to mongos using mongosh.

You can connect to any mongos in the cluster.

2

Switch to the Config Database.

Issue the following command to switch to the config database.

use config
3

Ensure that the balancer is not stopped.

The balancer will not activate in the stopped state. To ensure that the balancer is not stopped, use sh.startBalancer(), as in the following:

sh.startBalancer()

The balancer will not start if you are outside of the activeWindow timeframe.

Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.

In MongoDB versions earlier than 6.1, sh.startBalancer() also enables auto-splitting for the sharded cluster.

4

Modify the balancer's window.

Set the activeWindow using updateOne(), as in the following:

db.settings.updateOne(
   { _id: "balancer" },
   { $set: { activeWindow : { start : "<start-time>", stop : "<stop-time>" } } },
   { upsert: true }
)

Replace <start-time> and <end-time> with time values using two digit hour and minute values (i.e. HH:MM) that specify the beginning and end boundaries of the balancing window.

  • For HH values, use hour values ranging from 00 - 23.

  • For MM value, use minute values ranging from 00 - 59.

For on-premises or self-managed sharded clusters, MongoDB evaluates the start and stop times relative to the time zone of the primary member in the config server replica set.

For Atlas clusters, MongoDB evaluates the start and stop times relative to the UTC timezone.

Note

The balancer window must be sufficient to complete the migration of all data inserted during the day.

As data insert rates can change based on activity and usage patterns, it is important to ensure that the balancing window you select will be sufficient to support the needs of your deployment.

Remove a Balancing Window Schedule

If you have set the balancing window and wish to remove the schedule so that the balancer is always running, use $unset to clear the activeWindow, as in the following:

use config
db.settings.updateOne( { _id : "balancer" }, { $unset : { activeWindow : true } } )

Disable the Balancer

By default, the balancer may run at any time and only moves chunks as needed. To disable the balancer for a short period of time and prevent all migration, use the following procedure:

  1. Connect to any mongos in the cluster using the mongosh shell.

  2. Issue the following operation to disable the balancer:

    sh.stopBalancer()

    If a migration is in progress, the system will complete the in-progress migration before stopping.

    Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.

    In MongoDB versions earlier than 6.1, sh.stopBalancer() also disables auto-splitting for the sharded cluster.

  3. To verify that the balancer will not start, issue the following command, which returns false if the balancer is disabled:

    sh.getBalancerState()

    Optionally, to verify no migrations are in progress after disabling, issue the following operation in the mongosh shell:

    use config
    while( sh.isBalancerRunning() ) {
              print("waiting...");
              sleep(1000);
    }

Note

To disable the balancer from a driver, use the balancerStop command against the admin database, as in the following:

db.adminCommand( { balancerStop: 1 } )

Enable the Balancer

Use this procedure if you have disabled the balancer and are ready to re-enable it:

  1. Connect to any mongos in the cluster using the mongosh shell.

  2. Issue one of the following operations to enable the balancer:

    From the mongosh shell, issue:

    sh.startBalancer()

    Note

    To enable the balancer from a driver, use the balancerStart command against the admin database, as in the following:

    db.adminCommand( { balancerStart: 1 } )

    Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.

    In MongoDB versions earlier than 6.1, sh.startBalancer() also enables auto-splitting for the sharded cluster.

Disable Balancing During Backups

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:

If MongoDB migrates a chunk during a backup, you can end with an inconsistent snapshot of your sharded cluster. Never run a backup while the balancer is active. To ensure that the balancer is inactive during your backup operation:

If you turn the balancer off while it is in the middle of a balancing round, the shut down is not instantaneous. The balancer completes the chunk move in-progress and then ceases all further balancing rounds.

Before starting a backup operation, confirm that the balancer is not active. You can use the following command to determine if the balancer is active:

!sh.getBalancerState() && !sh.isBalancerRunning()

When the backup procedure is complete you can reactivate the balancer process.

Disable Balancing on a Collection

You can disable balancing for a specific collection with the sh.disableBalancing() method. You may want to disable the balancer for a specific collection to support maintenance operations or atypical workloads, for example, during data ingestions or data exports.

When you disable balancing on a collection, MongoDB will not interrupt in progress migrations.

To disable balancing on a collection, connect to a mongos with the mongosh shell and call the sh.disableBalancing() method.

For example:

sh.disableBalancing("students.grades")

The sh.disableBalancing() method accepts as its parameter the full namespace of the collection.

Enable Balancing on a Collection

You can enable balancing for a specific collection with the sh.enableBalancing() method.

When you enable balancing for a collection, MongoDB will not immediately begin balancing data. However, if the data in your sharded collection is not balanced, MongoDB will be able to begin distributing the data more evenly.

To enable balancing on a collection, connect to a mongos with the mongosh shell and call the sh.enableBalancing() method.

For example:

sh.enableBalancing("students.grades")

The sh.enableBalancing() method accepts as its parameter the full namespace of the collection.

Confirm Balancing is Enabled or Disabled

To confirm whether balancing for a collection is enabled or disabled, query the collections collection in the config database for the collection namespace and check the noBalance field. For example:

db.getSiblingDB("config").collections.findOne({_id : "students.grades"}).noBalance;

This operation will return a null error, true, false, or no output:

  • A null error indicates the collection namespace is incorrect.

  • If the result is true, balancing is disabled.

  • If the result is false, balancing is enabled currently but has been disabled in the past for the collection. Balancing of this collection will begin the next time the balancer runs.

  • If the operation returns no output, balancing is enabled currently and has never been disabled in the past for this collection. Balancing of this collection will begin the next time the balancer runs.

You can also see if the balancer is enabled using sh.status(). The currently-enabled field indicates if the balancer is enabled.

Change Replication Behavior for Chunk Migration

Secondary Throttle

During chunk migration, the _secondaryThrottle value determines when the migration proceeds with next document in the chunk.

In the config.settings collection:

  • If the _secondaryThrottle setting for the balancer is set to a write concern, each document move during chunk migration must receive the requested acknowledgement before proceeding with the next document.

  • If the _secondaryThrottle setting for the balancer is set to true, each document move during chunk migration must receive acknowledgement from at least one secondary before the migration proceeds with the next document in the chunk. This is equivalent to a write concern of { w: 2 }.

  • If the _secondaryThrottle setting is unset, the migration process does not wait for replication to a secondary and instead continues with the next document.

    This is the default behavior for WiredTiger.

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

use config
db.settings.updateOne(
   { "_id" : "balancer" },
   { $set : { "_secondaryThrottle" : { "w": "majority" }  } },
   { upsert : true }
)

The effects of changing the _secondaryThrottle setting may not be immediate. To ensure an immediate effect, stop and restart the balancer to enable the selected value of _secondaryThrottle.

For more information on the replication behavior during various steps of chunk migration, see Range Migration and Replication.

  • Use the moveRange command's secondaryThrottle and writeConcern options to specify the behavior during the command.

  • Use the moveChunk command's _secondaryThrottle and writeConcern options to specify the behavior during the command.

For details, see moveRange and moveChunk.

Wait for Delete

The _waitForDelete setting of the balancer and the moveChunk command affects how the balancer migrates multiple chunks from a shard. Similarly, the _waitForDelete setting of the balancer and the moveRange command also affect how the balancer migrates multiple chunks from a shard. By default, the balancer does not wait for the on-going migration's delete phase to complete before starting the next chunk migration. To have the delete phase block the start of the next chunk migration, you can set the _waitForDelete to true.

For details on chunk migration, see Range Migration. For details on the chunk migration queuing behavior, see Asynchronous Range Migration Cleanup.

The _waitForDelete is generally for internal testing purposes. To change the balancer's _waitForDelete value:

  1. Connect to a mongos instance.

  2. Update the _waitForDelete value in the settings collection of the config database. For example:

    use config
    db.settings.updateOne(
       { "_id" : "balancer" },
       { $set : { "_waitForDelete" : true } },
       { upsert : true }
    )

Once set to true, to revert to the default behavior:

  1. Connect to a mongos instance.

  2. Update or unset the _waitForDelete field in the settings collection of the config database:

    use config
    db.settings.updateOne(
       { "_id" : "balancer", "_waitForDelete": true },
       { $unset : { "_waitForDelete" : "" } }
    )

Balance Ranges that Exceed Size Limit

By default, MongoDB cannot move a range if the number of documents in the range is greater than 2 times the result of dividing the configured range size by the average document size.

Starting in MongoDB 4.4, by specifying the balancer setting attemptToBalanceJumboChunks to true, the balancer can migrate these large ranges as long as they have not been labeled as jumbo.

To set the balancer's attemptToBalanceJumboChunks setting, connect to a mongos instance and directly update the config.settings collection. For example, from a mongosh shell connected to a mongos instance, issue the following command:

db.getSiblingDB("config").settings.updateOne(
   { _id: "balancer" },
   { $set: { attemptToBalanceJumboChunks : true } },
   { upsert: true }
)

If the range you want to move is labeled jumbo, you can manually clear the jumbo flag to have the balancer attempt to migrate the range.

You can also manually migrate ranges that exceed the size limit (with or without the jumbo label) using either:

However, when you run moveRange or moveChunk with forceJumbo: true, write operations to the collection may block for a long period of time during the migration.

Change the Maximum Storage Size for a Given Shard

Changing a shard's maximum storage size is deprecated. Starting in version 4.2, MongoDB uses the WiredTiger Storage Engine to manage how data is stored. WiredTiger compresses data to minimize storage use. To learn more, see Compression.

For MongoDB versions 6.1 and earlier, you can still set a maximum storage size for a given shard in the sharded cluster by using the maxSize field. You can find configuration data related to shards in the config.shards collection.

Starting in version 6.2, MongoDB removes the maxSize field from the addShard command. As a result:

  • Running addShard with the maxSize field returns an InvalidOptions error.

  • New documents in the shards collection no longer include the maxSize field.

  • Any pre-existing maxSize field entries are ignored.

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