Sharded Cluster Balancer¶

Adding and Removing Shards from the Cluster¶

Adding a shard to a cluster creates an imbalance, since the new shard has no chunks. While MongoDB begins migrating data to the new shard immediately, it can take some time before the cluster balances. See the Add Shards to a Cluster tutorial for instructions on adding a shard to a cluster.

Removing a shard from a cluster creates a similar imbalance, since chunks residing on that shard must be redistributed throughout the cluster. While MongoDB begins draining a removed shard immediately, it can take some time before the cluster balances. Do not shutdown the servers associated to the removed shard during this process.

When you remove a shard in a cluster with an uneven chunk distribution, the balancer first removes the chunks from the draining shard and then balances the remaining uneven chunk distribution.

See the Remove Shards from an Existing Sharded Cluster tutorial for instructions on safely removing a shard from a cluster.

Migration Thresholds¶

To minimize the impact of balancing on the cluster, the balancer only begins balancing after the distribution of chunks for a sharded collection has reached certain thresholds. The thresholds apply to the difference in number of chunks between the shard with the most chunks for the collection and the shard with the fewest chunks for that collection. The balancer has the following thresholds:

The balancer stops running on the target collection when the difference between the number of chunks on any two shards for that collection is less than two, or a chunk migration fails.

Asynchronous Chunk Migration Cleanup¶

To migrate multiple chunks from a shard, the balancer migrates the chunks one at a time. However, the balancer does not wait for the current migration’s delete phase to complete before starting the next chunk migration. See Chunk Migration for the chunk migration process and the delete phase.

This queuing behavior allows shards to unload chunks more quickly in cases of heavily imbalanced cluster, such as when performing initial data loads without pre-splitting and when adding new shards.

This behavior also affects the moveChunk command, and migration scripts that use the moveChunk command may proceed more quickly.

In some cases, the delete phases may persist longer. Starting in MongoDB 4.4, chunk migrations are enhanced to be more resilient in the event of a failover during the delete phase. Orphaned documents are cleaned up even if a replica set’s primary crashes or restarts during this phase.

The _waitForDelete, available as a setting for the balancer as well as the moveChunk command, can alter the behavior so that the delete phase of the current migration blocks the start of the next chunk migration. The _waitForDelete is generally for internal testing purposes. For more information, see Wait for Delete.

Chunk Migration and Replication¶

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.

To update the _secondaryThrottle parameter for the balancer, see Secondary Throttle for an example.

Independent of any _secondaryThrottle setting, certain phases of the chunk migration have the following replication policy:

• MongoDB briefly pauses all application reads and writes to the collection being migrated, on the source shard, before updating the config servers with the new location for the chunk, and resumes the application reads and writes after the update. The chunk move requires all writes to be acknowledged by majority of the members of the replica set both before and after committing the chunk move to config servers.
• When an outgoing chunk migration finishes and cleanup occurs, all writes must be replicated to a majority of servers before further cleanup (from other outgoing migrations) or new incoming migrations can proceed.

To update the _secondaryThrottle setting in the config.settings collection, see Secondary Throttle for an example.

Maximum Number of Documents Per Chunk to Migrate¶

By default, MongoDB cannot move a chunk if the number of documents in the chunk is greater than 1.3 times the result of dividing the configured chunk size by the average document size. db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.

For chunks that are too large to migrate, starting in MongoDB 4.4:

• A new balancer setting attemptToBalanceJumboChunks allows the balancer to migrate chunks too large to move as long as the chunks are not labeled jumbo. See Balance Chunks that Exceed Size Limit for details.
• The moveChunk command can specify a new option forceJumbo to allow for the migration of chunks that are too large to move. The chunks may or may not be labeled jumbo.

Range Deletion Performance Tuning¶

You can tune the performance impact of range deletions with rangeDeleterBatchSize and rangeDeleterBatchDelayMS parameters. For example:例如：

• To limit the number of documents deleted per batch, you can set rangeDeleterBatchSize to a small value such as 32.
• To add an additional delay between batch deletions, you can set rangeDeleterBatchDelayMS above the current default of 20 milliseconds.