mongodb分片介绍—— 基于范围（数值型）的分片 或者 基于哈希的分片

砂拉豆

均衡
　　The balancer is a background process that manages chunk migrations. The balancer can run from any of themongos instances in a cluster.
　　当集群中数据的不均衡发生时,均衡器会将数据块从数据块数目最多的分片迁移到数据块最少的分片上,举例来讲:如果集合 users 在 分片1 上有100个数据块,在 分片2 上有50个数据块,均衡器会将数据块从 分片1一直向 分片2 迁移,一直到数据均衡为止.
　　The shards manage chunk migrations as a background operation between an origin shard and a destination shard. During a chunk migration, the destination shard is sent all the current documents in the chunk from theorigin shard. Next, the destination shard captures and applies all changes made to the data during the migration process. Finally, the metadata regarding the location of the chunk on config server is updated.
　　If there’s an error during the migration, the balancer aborts the process leaving the chunk unchanged on the origin shard. MongoDB removes the chunk’s data from the origin shard after the migration completes successfully.