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一、如何移除分片

1、确认balancer已经开启

mongos> sh.getBalancerState()
true

2、移除分片

注：在admin db下执行命令。

mongos> use admin
switched to db admin
mongos> db.runCommand( { removeShard: "shard3" } )
{
	"msg" : "draining started successfully",
	"state" : "started",
	"shard" : "shard3",
	"ok" : 1
}

3、检查迁移的状态

同样执行

mongos> use admin
switched to db admin
mongos> db.runCommand( { removeShard: "shard3" } )
{
	"msg" : "draining ongoing",
	"state" : "ongoing",
	"remaining" : {
		"chunks" : NumberLong(3),
		"dbs" : NumberLong(0)
	},
	"ok" : 1
}

remaining中的chunks表示还有多少数据块未迁移。

4、移除未分片数据

In a cluster, a database with unsharded collections stores those collections only on a single shard.

That shard becomes the primary shard for that database. (Different databases in a cluster can have different primary shards.)

WARNING

Do not perform this procedure until you have finished draining the shard.

1）To determine if the shard you are removing is the primary shard for any of the cluster’s databases, issue one of the following methods:

sh.status()

db.printShardingStatus()

In the resulting document, the databases field lists each database and its primary shard.

For example, the following database field shows that the products database uses MongoDB0 as the primary shard:

{  "_id" : "products",  "partitioned" : true,  "primary" : "mongodb0" }

2）To move a database to another shard, use the movePrimary command. For example, to migrate all remaining unsharded data from mongodb0 to mongodb1, 

issue the following command:

use admin

db.runCommand( { movePrimary: "products", to: "mongodb1" })       --products为db name

This command does not return until MongoDB completes moving all data, which may take a long time. 

The response from this command will resemble the following:

{ "primary" : "mongodb1", "ok" : 1 }

If you use the movePrimary command to move un-sharded collections, you must either restart all mongos instances,

 or use the flushRouterConfig command on all mongos instances before writing any data to the cluster. 

 This action notifies the mongos of the new shard for the database.

If you do not update the mongos instances’ metadata cache after using movePrimary, the mongos may not write data to the correct shard. 

To recover, you must manually intervene.

根据上面所说，迁移非分片表 时 最好停机，在运行db.runCommand( { movePrimary: "products", to: "mongodb1" })  命令完成之后，

刷新所有mongos后(所有mongos上运行db.runCommand("flushRouterConfig"))，再对外提供服务。当然也可以重新启动所有mongos实例 。

5、完成迁移

mongos> use admin
switched to db admin
mongos> db.runCommand( { removeShard: "shard3" } )
{
	"msg" : "removeshard completed successfully",
	"state" : "completed",
	"shard" : "shard3",
	"ok" : 1
}

如果state为 completed，表示已完成迁移。

二、添加分片

1、首先确认balancer已经开启

mongos> sh.getBalancerState()
true

2、执行添加分片的命令

如果出现以下错误，删除目标shard3上的test1数据库，再次执行命令

mongos> sh.addShard("shard3/192.168.137.138:27019")
{
	"ok" : 0,
	"errmsg" : "can't add shard shard3/192.168.137.138:27019 because a local database 'test1' exists in another shard1:shard1/192.168.137.111:27017,192.168.137.75:27017"
}

mongos> sh.addShard("shard3/192.168.137.138:27019")
{ "shardAdded" : "shard3", "ok" : 1 }

最后运行sh.status()命令确认迁移是否成功，可能会花比较长的时间。

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