Sharded Cluster Balancer分片式集群平衡器

Adding and Removing Shards from the Cluster在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. 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，因为新的分片没有数据。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，请参阅向集群添加分片教程。

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造成类似的不平衡，因为驻留在该分片上的数据必须在整个集群中重新分布。While MongoDB begins draining a removed shard immediately, it can take some time before the cluster balances. 虽然MongoDB会立即开始耗尽已删除的分片，但集群平衡可能需要一些时间。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.当你在一个块分布不均匀的集群中删除一个分片时，平衡器首先从耗尽的分片中删除块，然后平衡剩余的不均匀块分布。

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.为了尽量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一定阈值后才开始平衡。

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. 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.如果（该集合的）分片之间的数据差异小于集合配置范围大小的三倍，则认为集合是平衡的。对于128MB的默认范围大小，对于给定的集合，两个分片必须具有至少384MB的数据大小差才能进行迁移。

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. However, the balancer does not wait for the current migration's delete phase to complete before starting the next range migration. 为了从分片迁移数据，平衡器一次迁移一个范围的数据。但是，平衡器在开始下一次范围迁移之前不会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.有关范围迁移过程和删除阶段，请参阅范围迁移。

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集群严重不平衡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.此行为也会影响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. Range 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.在某些情况下，删除阶段可能会持续更长时间。范围迁移得到了增强，以便在删除阶段发生故障转移时更具弹性。即使副本集的主文档在此阶段崩溃或重新启动，孤立文档也会被清理。

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，_seconditionThrottle值决定迁移何时继续进行范围内的下一个文档。

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 moved during range migration must receive the requested acknowledgment before proceeding with the next document.如果平衡器的_seconditionThrottle设置设置为写入关注，则在范围迁移期间移动的每个文档在继续处理下一个文档之前都必须收到请求的确认。
• 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_seconditionThrottle设置，迁移过程不会等待复制到辅助文档，而是继续处理下一个文档。

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 range migration have the following replication policy:独立于任何_seconditionThrottle设置，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. MongoDB resumes application reads and writes after the update. MongoDB在用范围位置更新配置服务器之前，会短暂暂停对源分片上迁移到的集合的所有应用程序读写。MongoDB在更新后恢复应用程序读写。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，所有写入都要得到副本集大多数成员的确认。
• 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，在进一步清理（来自其他传出迁移）或新的传入迁移之前，必须将所有写入复制到大多数服务器。

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_seconditionThrottle设置，请参阅Secondary Throttle以获取示例。

Maximum Number of Documents Per Range to Migrate每个区域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，如果范围内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，则MongoDB无法移动范围。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可以移动块的子范围并将大小减小到小于该范围，则平衡器会通过迁移范围来实现。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，请参阅超过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.这些范围可能被标记为巨型，也可能没有。

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.您可以使用rangeDeleterBatchSize和rangeDeleterBatchDelayMS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量，可以将rangeDeleterBatchSize设置为较小的值，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设置为高于当前默认值20毫秒。

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，在范围迁移期间，不会为孤立文档的更新生成更改流事件。