Sharded Cluster Balancer分片群集均衡器

Adding and Removing Shards from the Cluster从群集中添加和删除分片

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. 将分片添加到集群会造成不平衡，因为新分片没有块。虽然MongoDB立即开始将数据迁移到新的shard，但在集群平衡之前可能需要一些时间。See the Add Shards to a Cluster tutorial for instructions on adding a shard to a cluster.有关将分片添加到集群的说明，请参阅将分片添加到集群教程。

Removing a shard from a cluster creates a similar imbalance, since chunks residing on that shard must be redistributed throughout the cluster. 从集群中删除一个分片也会造成类似的不平衡，因为驻留在该分片上的块必须在整个集群中重新分布。While MongoDB begins draining a removed shard immediately, it can take some time before the cluster balances. 虽然MongoDB立即开始排出已移除的分片，但可能需要一段时间才能平衡集群。Do not shutdown the servers associated to the removed shard during this process.在此过程中，请勿关闭与已删除分片关联的服务器。

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.当您在块分布不均匀的集群中移除一个分片时，平衡器首先从排出的分片中移除块，然后平衡剩余的不均匀块分布。

See the Remove Shards from an Existing Sharded Cluster tutorial for instructions on safely removing a shard from a cluster.有关从集群中安全移除分片的说明，请参阅从现有分片集群中移除分片教程。

Migration Thresholds迁移阈值

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. 为了将平衡对集群的影响降至最低，平衡器仅在分片集合的块分布达到特定阈值后才开始平衡。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. 阈值适用于集合中块数最多的分片与集合中块数最少的分片之间的块数差异。The balancer has the following thresholds:平衡器具有以下阈值：

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.当目标集合的任何两个分片上的块数之差小于2时，或者块迁移失败时，均衡器将停止在该集合上运行。

Asynchronous Chunk Migration Cleanup异步块迁移清理

To migrate multiple chunks from a shard, the balancer migrates the chunks one at a time. 要从一个shard迁移多个块，平衡器一次迁移一个块。However, the balancer does not wait for the current migration's delete phase to complete before starting the next chunk migration. 但是，平衡器不会等待当前迁移的删除阶段完成，然后再开始下一个块迁移。See Chunk Migration for the chunk migration process and the delete phase.有关区块迁移过程和删除阶段，请参阅区块迁移。

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.这种排队行为允许分片在集群严重不平衡的情况下更快地卸载块，例如在没有预拆分的情况下执行初始数据加载时以及在添加新分片时。

This behavior also affects the moveChunk command, and migration scripts that use the moveChunk command may proceed more quickly.此行为还影响moveChunk命令，使用moveChunk命令的迁移脚本可能会进行得更快。

In some cases, the delete phases may persist longer. 在某些情况下，删除阶段可能会持续更长时间。Starting in MongoDB 4.4, chunk migrations are enhanced to be more resilient in the event of a failover during the delete phase. 从MongoDB 4.4开始，区块迁移得到了增强，以便在删除阶段发生故障切换时更具弹性。Orphaned documents are cleaned up even if a replica set's primary crashes or restarts during this phase.即使副本集的主副本在此阶段崩溃或重新启动，孤立文档也会被清理。

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. _waitForDelete可以作为平衡器和moveChunk命令的设置，它可以改变行为，以便当前迁移的删除阶段阻止下一个块迁移的开始。The _waitForDelete is generally for internal testing purposes. _waitForDelete通常用于内部测试。For more information, see Wait for Delete.有关详细信息，请参阅等待删除。

Chunk Migration and Replication区块迁移和复制

During chunk migration, the _secondaryThrottle value determines when the migration proceeds with next document in the chunk.在区块迁移期间，_secondaryThrottle值确定何时迁移区块中的下一个文档。

In the config.settings collection:在config.settings集合中：

• 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.如果平衡器的_secondaryThrottle设置设置为写入问题，则区块迁移期间的每个文档移动都必须在继续下一个文档之前收到请求的确认。
• 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. 如果平衡器的_secondaryThrottle设置设置为true，则区块迁移期间的每个文档移动都必须在继续迁移区块中的下一个文档之前收到至少一个辅助文档的确认。This is equivalent to a write concern of { w: 2 }.这相当于{ w: 2 }的写关注点。
• If the _secondaryThrottle setting is unset, the migration process does not wait for replication to a secondary and instead continues with the next document.如果未设置_secondaryThrottle设置，则迁移过程不会等待复制到辅助文档，而是继续下一个文档。

To update the _secondaryThrottle parameter for the balancer, see Secondary Throttle for an example.要更新平衡器的_secondaryThrottle参数，请参阅Secondary Throttle以获取示例。

Independent of any _secondaryThrottle setting, certain phases of the chunk migration have the following replication policy:与任何_secondaryThrottle设置无关，区块迁移的某些阶段具有以下复制策略：

• 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. 在使用区块的新位置更新配置服务器之前，MongoDB会在源分片上短暂暂停对要迁移的集合的所有应用程序读写，并在更新后恢复应用程序读写。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.区块移动要求在将区块移动提交到配置服务器之前和之后，副本集的大多数成员都要确认所有写入操作。
• 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.当传出块迁移完成并进行清理时，必须将所有写入复制到大多数服务器，然后才能进一步清理（来自其他传出迁移）或新的传入迁移。

To update the _secondaryThrottle setting in the config.settings collection, see Secondary Throttle for an example.要更新config.settings集合中的_secondaryThrottle设置，请参阅Secondary Throttle以获取示例。

Maximum Number of Documents Per Chunk to Migrate每个区块要迁移的最大文档数

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. 默认情况下，如果区块中的文档数大于配置的区块大小除以平均文档大小的1.3倍，MongoDB将无法移动区块。db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.db.collection.stats()包括avgObjSize字段，该字段表示集合中的平均文档大小。

For chunks that are too large to migrate, starting in MongoDB 4.4:对于太大而无法迁移的块，从MongoDB 4.4开始：

• A new balancer setting attemptToBalanceJumboChunks allows the balancer to migrate chunks too large to move as long as the chunks are not labeled jumbo. 新的平衡器设置attemptToBalanceJumboChunks允许平衡器迁移太大而无法移动的块，只要这些块没有标记为jumbo。See Balance Chunks that Exceed Size Limit for details.有关详细信息，请参阅超出大小限制的平衡块。
• The moveChunk command can specify a new option forceJumbo to allow for the migration of chunks that are too large to move. moveChunk命令可以指定一个新选项forceJumbo，以允许迁移太大而无法移动的块。The chunks may or may not be labeled jumbo.这些块可以标记为jumbo，也可以不标记为jumbo。

Range Deletion Performance Tuning范围删除性能调整

You can tune the performance impact of range deletions with rangeDeleterBatchSize and rangeDeleterBatchDelayMS parameters. 可以使用rangeDeleterBatchSize和rangeDeleterBatchDelayMS参数调整范围删除对性能的影响。For example:例如：

• To limit the number of documents deleted per batch, you can set rangeDeleterBatchSize to a small value such as 32.要限制每批删除的文档数，可以将rangeDeleterBatchSize设置为较小的值，例如32。
• To add an additional delay between batch deletions, you can set rangeDeleterBatchDelayMS above the current default of 20 milliseconds.要在批删除之间添加额外的延迟，可以将rangeDeleterBatchDelayMS设置为当前默认值20毫秒以上。