Data Partitioning with Chunks使用区块进行数据分区

Initial Chunks初始区块

Range Size范围大小

The default range size in MongoDB is 128 megabytes. MongoDB中默认的范围大小是128兆字节。You can increase or reduce the chunk size. Consider the implications of changing the default chunk size:您可以增加或减少区块大小。考虑更改默认区块大小的含义：

1. Small ranges lead to a more even distribution of data at the expense of more frequent migrations. 小范围导致数据分布更加均匀，而代价是更频繁的迁移。This creates expense at the query routing (mongos) layer.这在查询路由（mongos）层产生了开销。
2. Large ranges lead to fewer migrations. 大范围导致迁移减少。This is more efficient both from the networking perspective and in terms of internal overhead at the query routing layer. 从网络的角度和查询路由层的内部开销来看，这都更有效。But, these efficiencies come at the expense of a potentially uneven distribution of data.但是，这些效率是以潜在的不均衡数据分布为代价的。
3. Range size affects the Maximum Number of Documents Per Range to Migrate.范围大小会影响要迁移的每个范围的最大文档数。
4. Range size affects the maximum collection size when sharding an existing collection. 分割现有集合时，范围大小会影响最大集合大小。Post-sharding, data range size does not constrain collection size.分片后，数据范围大小不限制集合大小。

For many deployments, it makes sense to avoid frequent and potentially spurious migrations at the expense of a slightly less evenly distributed data set.对于许多部署来说，避免频繁和潜在的虚假迁移是有意义的，而牺牲的是分布稍微不均匀的数据集。

Range Migration范围迁移

MongoDB migrates data ranges in a sharded cluster to distribute the data of a sharded collection evenly among shards. Migrations may be either:MongoDB迁移一个分片集群中的数据范围，使分片集合的数据在各分片之间均匀分布。迁移可能是：

• Manual. Only use manual migration in limited cases, such as to distribute data during bulk inserts. 手动。仅在有限的情况下使用手动迁移，例如在大容量插入期间分发数据。See Migrating Chunks Manually for more details.有关更多详细信息，请参阅手动迁移区块。

• Automatic. 自动。The balancer process automatically migrates data when there is an uneven distribution of a sharded collection's data across the shards. 当分片集合的数据在各分片之间分布不均匀时，平衡器进程会自动迁移数据。See Migration Thresholds for more details.有关详细信息，请参阅迁移阈值。

For more information on the sharded cluster balancer, see Sharded Cluster Balancer.有关分片集群平衡器的更多信息，请参阅分片集群平衡器。

Balancing平衡

The balancer is a background process that manages data migrations. 平衡器是管理数据迁移的后台进程。If the difference in amount of data between the largest and smallest shard exceed the migration thresholds, the balancer begins migrating data across the cluster to ensure an even distribution.如果最大和最小分片之间的数据量差异超过迁移阈值，则平衡器开始在集群中迁移数据，以确保均匀分布。

You can manage certain aspects of the balancer. The balancer also respects any zones created as a part of configuring zones in a sharded cluster.您可以管理平衡器的某些方面。平衡器还尊重作为在分片集群中配置区域的一部分而创建的任何区域。

See Sharded Cluster Balancer for more information on the balancer.有关平衡器的详细信息，请参阅分片集群平衡器。

Indivisible/Jumbo Chunks不可分割/大块

In some cases, chunks can grow beyond the specified chunk size but cannot undergo a split. 在某些情况下，块可以增长到指定的块大小之外，但不能进行拆分。The most common scenario is when a chunk represents a single shard key value. 最常见的场景是当一个chunk表示单个分片键值时。Since the chunk cannot split, it continues to grow beyond the chunk size, becoming a jumbo chunk. 由于区块无法分割，它会继续增长，超过区块大小，成为一个巨型的区块。These jumbo chunks can become a performance bottleneck as they continue to grow, especially if the shard key value occurs with high frequency.随着这些巨型块的不断增长，它们可能会成为性能瓶颈，尤其是在分片键值高频率出现的情况下。

Starting in MongoDB 5.0, you can reshard a collection by changing a document's shard key.从MongoDB 5.0开始，您可以通过更改文档的分片键来重新分片集合。

Starting in MongoDB 4.4, MongoDB provides the refineCollectionShardKey command. 从MongoDB 4.4开始，MongoDB提供了refineCollectionShardKey命令。Refining a collection's shard key allows for a more fine-grained data distribution and can address situations where the existing key insufficient cardinality leads to jumbo chunks.细化集合的分片键可以实现更细粒度的数据分布，并可以解决现有键基数不足导致巨型块的情况。

To learn whether you should reshard your collection or refine your shard key, see Change a Shard Key.要了解是应该重新发布集合还是细化分片键，请参阅更改分片键。

For more information, see:有关详细信息，请参阅：

• Clear jumbo Flag清除jumbo标志
• Maximum Number of Documents Per Range to Migrate每个范围要迁移的最大文档数