MongoDB 是一個基于分布式文件存儲的數據庫。由 C++ 語言編寫。旨在為 WEB 應用提供可擴展的高性能數據存儲解決方案。

MongoDB 是一個介于關系數據庫和非關系數據庫之間的產品，是非關系數據庫當中功能最豐富，最像關系數據庫的。

一、如何移除分片

1、確認balancer已經開啟

mongos> sh.getBalancerState()
true

2、移除分片

注：在admin db下執行命令。

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

3、檢查遷移的狀態

同樣執行

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

remaining中的chunks表示還有多少數據塊未遷移。

4、移除未分片數據

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為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.

根據上面所說，遷移非分片表 時 最好停機，在運行db.runCommand( { movePrimary: "products", to: "mongodb1" }) 命令完成之后，刷新所有mongos后(所有mongos上運行db.runCommand("flushRouterConfig"))，再對外提供服務。當然也可以重新啟動所有mongos實例 。

5、完成遷移

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

如果state為 completed，表示已完成遷移。

二、添加分片

1、首先確認balancer已經開啟

mongos> sh.getBalancerState()
true

2、執行添加分片的命令

如果出現以下錯誤，刪除目標shard3上的test1數據庫，再次執行命令

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 }

最后運行sh.status()命令確認遷移是否成功，可能會花比較長的時間。

以上內容是給大家介紹了Mongodb 刪除添加分片與非分片表維護的全部敘述，希望對大家有所幫助。