Sharded Cluster Balancer分片集群平衡器

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 data. 将分片添加到集群会造成不平衡，因为新的分片没有数据。While MongoDB begins migrating data to the new shard immediately, it can take some time before the cluster balances. 虽然MongoDB立即开始将数据迁移到新的分片，但集群平衡可能需要一些时间。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，请参阅将分片添加至集群教程。

Removing a shard from a cluster creates a similar imbalance, since data residing on that shard must be redistributed throughout the cluster. 从集群中删除分片会产生类似的不平衡，因为驻留在该分片上的数据必须在整个集群中重新分发。While MongoDB begins draining a removed shard immediately, it can take some time before the cluster balances. 虽然MongoDB开始立即排出已删除的分片，但集群平衡可能需要一段时间。Do not shutdown the servers associated to the removed shard during this process.在此过程中，不要关闭与已删除的分片关联的服务器。

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.当您在具有不均匀块分布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配。

See the Remove Shards from a Cluster tutorial for instructions on safely removing a shard from a cluster.有关从集群中安全删除分片的说明，请参阅从集群中删除分片教程。

Migration Thresholds迁移阈值

To minimize the impact of balancing on the cluster, the balancer only begins balancing after the distribution of data for a sharded collection has reached certain thresholds.为了最大限度地减少平衡对集群的影响，平衡器只有在分片集合的数据分布达到特定阈值后才开始进行平衡。

A collection is considered balanced if the difference in data between shards (for that collection) is less than three times the configured range size for the collection. 如果（该集合的）分片之间的数据差异小于该集合的配置范围大小的三倍，则认为该集合是平衡的。For the default range size of 128MB, two shards must have a data size difference for a given collection of at least 384MB for a migration to occur.对于128MB的默认范围大小，对于给定的集合，两个分片的数据大小必须相差至少384MB才能进行迁移。

Asynchronous Range Migration Cleanup异步范围迁移清理

To migrate data from a shard, the balancer migrates the data one range at a time. 要从分片中迁移数据，平衡器一次迁移一个范围的数据。However, the balancer does not wait for the current migration's delete phase to complete before starting the next range migration. 但是，平衡器不会在开始下一个范围迁移之前等待当前迁移的删除阶段完成。See Range Migration for the range migration process and the delete phase.请参阅范围迁移以了解范围迁移过程和删除阶段。

This queuing behavior allows shards to unload data 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严重不平衡集群的情况下更快地卸载数据，例如在不进行预拆分的情况下执行初始数据加载以及添加新分片时。

This behavior also affects the moveRange command, and migration scripts that use the moveRange command may proceed more quickly.此行为还会影响moveRange命令，使用moveRange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。Range migrations are enhanced to be more resilient in the event of a failover during the delete phase. 范围迁移得到了增强，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.孤立文档将被清理，即使复制集的主复制集在此阶段崩溃或重新启动。

The _waitForDelete balancer setting can alter the behavior so that the delete phase of the current migration blocks the start of the next chunk migration. _waitForDelete平衡器设置可以更改行为，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。

Range Migration and Replication范围迁移和复制

During range migration, the _secondaryThrottle value determines when the migration proceeds with next document in the range.在范围迁移过程中，_secondaryThrottle值确定迁移何时继续该范围中的下一个文档。

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 range 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，则范围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 range migration must receive acknowledgement from at least one secondary before the migration proceeds with the next document in the range. 如果平衡器的_secondaryThrottle设置设置为true，则在迁移范围中的下一个文档之前，范围迁移期间的每个文档移动都必须至少收到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 }的写入问题。
• 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，而是继续执行下一个文档。

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

Independent of any _secondaryThrottle setting, certain phases of the range migration have the following replication policy:与任何_secondaryThrottle设置无关，范围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 to on the source shard before updating the config servers with the range location. 在用范围位置更新配置服务器之前，MongoDB会短暂暂停所有应用程序对源分片上迁移到的集合的读取和写入。MongoDB resumes application reads and writes after the update. MongoDB在更新后恢复应用程序的读写操作。The range move requires all writes to be acknowledged by majority of the members of the replica set both before and after committing the range move to config servers.范围移动要求在将范围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 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，然后才能进行进一步的清理（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，请参阅辅助节流阀以获取示例。

Maximum Number of Documents Per Range to Migrate每个范围要迁移的最大文档数

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. 默认情况下，如果范围中的文档数量大于配置的范围大小除以平均文档大小的结果的2倍，MongoDB将无法移动该范围。If MongoDB can move a sub-range of a chunk and reduce the size to less than that, the balancer does so by migrating a range. 如果MongoDB可以移动一个区块的子范围并将大小缩小到小于这个范围，那么均衡器就可以通过迁移一个范围来实现这一点。db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.db.collection.stats()包含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:对于太大而无法迁移的块：

• The balancer setting attemptToBalanceJumboChunks allows the balancer to migrate chunks too large to move as long as the chunks are not labeled jumbo. 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 Ranges that Exceed Size Limit for details.有关详细信息，请参阅超出大小限制的平衡范围。

  When issuing moveRange and moveChunk commands, it's possible to specify the forceJumbo option to allow for the migration of ranges that are too large to move. 在发出moveRange和moveChunk命令时，可以指定forceJumbo选项以允许迁移太大而无法移动的范围。The ranges may or may not be labeled jumbo.范围可以标记为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, rangeDeleterBatchDelayMS, and rangeDeleterHighPriority parameters. 您可以使用rangeDeleterBatchSize、rangeDeleterBatchDelayMS和rangeDeleterHighPriority参数来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设置为一个小值，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。
• To prioritize range deletions, you can set rangeDeleterHighPriority to true. Range deletions are potentially long-running background tasks that might negatively impact the throughput of user operations when the system is under heavy load.要确定范围删除的优先级，可以将rangeDeleterHighPriority设置为true。范围删除是潜在的长期运行的后台任务，当系统处于重负载下时，可能会对用户操作的吞吐量产生负面影响。

Change Streams and Orphan Documents更改流和孤立文档

Starting in MongoDB 5.3, during range migration, change stream events are not generated for updates to orphaned documents.从MongoDB 5.3开始，在范围迁移过程中，不会为孤立文档的更新生成更改流事件。