Remove Shards from an Existing Sharded Cluster从现有分片群集中删除分片

Considerations注意事项

• A shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.分片移除可能会导致打开的变更流游标关闭，并且关闭的变更流游标可能无法完全恢复。
• You can safely restart a cluster during a shard removal process. 您可以在分片移除过程中安全地重新启动集群。If you restart a cluster during an ongoing draining process, draining continues automatically after the cluster components restart. 如果在正在进行的排放过程中重新启动群集，则在群集组件重新启动后，排放将自动继续。MongoDB records the shard draining status in the config.shards collection.MongoDB在config.shards集合中记录分片排出的状态。

Ensure the Balancer Process is Enabled确保平衡器进程已启用

To successfully migrate data from a shard, the balancer process must be enabled. 要成功地从分片迁移数据，必须启用平衡器进程。Check the balancer state using the sh.getBalancerState() helper in mongosh. 使用mongosh中的sh.getBalancerState()助手检查平衡器状态。For more information, see the section on balancer operations.有关更多信息，请参阅平衡器操作部分。

Determine the Name of the Shard to Remove确定要删除的分片的名称

To determine the name of the shard, connect to a mongos instance with mongosh and either:要确定分片的名称，请使用mongosh连接到mongos实例，然后：

• Use the listShards command, as in the following:使用listShards命令，如下所示：

  db.adminCommand( { listShards: 1 } )

• Run either the sh.status() or the db.printShardingStatus() method.运行sh.status()或db.printShardingStatus()方法。

The shards._id field lists the name of each shard.shards._id字段列出了每个分片的名称。

Remove Chunks from the Shard从分片中移除大块

From the admin database, run the removeShard command. 从管理数据库中，运行removeShard命令。This begins "draining" chunks from the shard you are removing to other shards in the cluster. 这开始将要移除的分片中的块“排出”到集群中的其他分片中。For example, for a shard named mongodb0, run:例如，对于名为mongodb0的分片，运行：

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".mongos将removeShard命令的写入关注点转换为"majority"。

This operation returns with the following response:此操作返回并返回以下响应：

{
   "msg" : "draining started successfully",
   "state" : "started",
   "shard" : "mongodb0",
   "note" : "you need to drop or movePrimary these databases",
   "dbsToMove" : [
      "fiz",
      "buzz"
   ],
   "ok" : 1,
   "operationTime" : Timestamp(1575398919, 2),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1575398919, 2),
      "signature" : {
         "hash" : BinData(0,"Oi68poWCFCA7b9kyhIcg+TzaGiA="),
         "keyId" : NumberLong("6766255701040824328")
      }
   }
}

The balancer begins migrating chunks from the shard named mongodb0 to other shards in the cluster. These migrations happens slowly to avoid placing undue load on the overall cluster. 平衡器开始将块从名为mongodb0的分片迁移到集群中的其他分片。这些迁移进行得很慢，以避免给整个集群带来不必要的负载。Depending on your network capacity and the amount of data, this operation can take from a few minutes to several days to complete.根据您的网络容量和数据量，此操作可能需要几分钟到几天的时间才能完成。

Check the Status of the Migration检查迁移状态

To check the progress of the migration at any stage in the process, run removeShard from the admin database again. For example, for a shard named mongodb0, run:要检查迁移过程中任何阶段的进度，请再次从admin数据库运行removeShard。例如，对于名为mongodb0的分片，运行：

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".mongos将removeShard命令的写入关注点转换为"majority"。

The command returns output similar to the following:该命令返回类似于以下内容的输出：

{
   "msg" : "draining ongoing",
   "state" : "ongoing",
   "remaining" : {
      "chunks" : NumberLong(2),
      "dbs" : NumberLong(2),
      "jumboChunks" : NumberLong(0)         // Available starting in 4.2.2 (and 4.0.14)
   },
   "note" : "you need to drop or movePrimary these databases",
   "dbsToMove" : [
      "fizz",
      "buzz"
   ],
   "ok" : 1,
   "operationTime" : Timestamp(1575399086, 1655),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1575399086, 1655),
      "signature" : {
         "hash" : BinData(0,"XBrTmjMMe82fUtVLRm13GBVtRE8="),
         "keyId" : NumberLong("6766255701040824328")
      }
   }
}

In the output, the remaining field includes the following fields:在输出中，remaining字段包括以下字段：

Continue checking the status of the removeShard command until the number of chunks remaining is 0.继续检查removeShard命令的状态，直到剩余的块数为0。

db.adminCommand( { removeShard: "mongodb0" } )

Move Databases to Another Primary Shard将数据库移动到另一个主分片

If the shard is the primary shard for one or more databases in the cluster, then you must make that database use a different shard as its primary shard. 如果分片是集群中一个或多个数据库的主分片，则必须使该数据库使用不同的分片作为其主分片。removeShard lists any databases that you need to move in the dbsToMove field in the command output. 在命令输出的dbsToMove字段中列出了需要移动的所有数据库。If the shard is not the primary shard for any databases, skip to the next task, Finalize the Migration.如果分片不是任何数据库的主分片，请跳到下一个任务完成迁移。

To move a database to another shard, use the movePrimary command.要将数据库移动到另一个分片，请使用movePrimary命令。

To migrate the fizz database from mongodb0 to mongodb1, issue the following command:

db.adminCommand( { movePrimary: "fizz", to: "mongodb1" })

mongos uses "majority" write concern for movePrimary.

This command does not return until MongoDB completes moving all data. 在MongoDB完成所有数据的移动之前，此命令不会返回。The response from this command will resemble the following:此命令的响应如下所示：

{
   "ok" : 1,
   "operationTime" : Timestamp(1575400369, 9),
   "$clusterTime" : {
   "clusterTime" : Timestamp(1575400369, 9),
   "signature" : {
      "hash" : BinData(0,"2Nz8QCcVXB0LJLm1hsXfpTCaM0M="),
      "keyId" : NumberLong("6766255701040824328")
   }
}
}

Finalize the Migration完成迁移

To clean up all metadata information and finalize the removal, run removeShard again. For example, for a shard named mongodb0, run:要清除所有元数据信息并完成删除，请再次运行removeShard。例如，对于名为mongodb0的分片，运行：

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".mongos将removeShard命令的写入关注点转换为"majority"。

A success message appears at completion:完成时会显示一条成功消息：

{
    "msg" : "removeshard completed successfully",
    "state" : "completed",
    "shard" : "mongodb0",
    "ok" : 1,
    "operationTime" : Timestamp(1575400370, 2),
    "$clusterTime" : {
       "clusterTime" : Timestamp(1575400370, 2),
       "signature" : {
          "hash" : BinData(0,"JjSRciHECXDBXo0e5nJv9mdRG8M="),
          "keyId" : NumberLong("6766255701040824328")
       }
    }
}

Once the value of the state field is "completed", you may safely stop the instances comprising the mongodb0 shard.一旦state字段的值“完成”，就可以安全地停止包含mongodb0分片的实例。