Remove Shards from an Existing Sharded Cluster¶

On this page

Ensure the Balancer Process is Enabled
Determine the Name of the Shard to Remove
Remove Chunks from the Shard
Check the Status of the Migration
Move Unsharded Data
Finalize the Migration

To remove a shard you must ensure the shard’s data is migrated to the remaining shards in the cluster. This procedure describes how to safely migrate data and how to remove a shard.

This procedure describes how to safely remove a single shard. Do not use this procedure to migrate an entire cluster to new hardware. To migrate an entire shard to new hardware, migrate individual shards as if they were independent replica sets.

To remove a shard, first connect to one of the cluster’s mongos instances using mongo shell. Then use the sequence of tasks in this document to remove a shard from the cluster.

Ensure the Balancer Process is Enabled¶

To successfully migrate data from a shard, the balancer process must be enabled. Check the balancer state using the sh.getBalancerState() helper in the mongo shell. For more information, see the section on balancer operations.

Determine the Name of the Shard to Remove¶

To determine the name of the shard, connect to a mongos instance with the mongo shell and either:

Use the listShards command, as in the following:
```
db.adminCommand( { listShards: 1 } )
```
Run either the sh.status() or the db.printShardingStatus() method.

The shards._id field lists the name of each shard.

Remove Chunks from the Shard¶

From the admin database, run the removeShard command. This begins “draining” chunks from the shard you are removing to other shards in the cluster. For example, for a shard named mongodb0, run:

use admin
db.runCommand( { removeShard: "mongodb0" } )

This operation returns immediately, with the following response:

{
    "msg" : "draining started successfully",
    "state" : "started",
    "shard" : "mongodb0",
    "ok" : 1
}

Depending on your network capacity and the amount of data, this operation can take from a few minutes to several days to complete.

Check the Status of the Migration¶

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:

use admin
db.runCommand( { removeShard: "mongodb0" } )

The command returns output similar to the following:

{
     "msg" : "draining ongoing",
    "state" : "ongoing",
    "remaining" : {
        "chunks" : 42,
        "dbs" : 1
    },
    "ok" : 1
}

In the output, the remaining document displays the remaining number of chunks that MongoDB must migrate to other shards and the number of MongoDB databases that have “primary” status on this shard.

Continue checking the status of the removeShard command until the number of chunks remaining is 0. Always run the command on the admin database. If you are on a database other than admin, you can use sh._adminCommand to run the command on admin.

Move Unsharded Data¶

If the shard is the primary shard for one or more databases in the cluster, then the shard will have unsharded data. If the shard is not the primary shard for any databases, skip to the next task, Finalize the Migration.

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.

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" }
```
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:
```
db.runCommand( { movePrimary: "products", to: "mongodb1" })
```
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 }
```

Finalize the Migration¶

To clean up all metadata information and finalize the removal, run removeShard again. For example, for a shard named mongodb0, run:

use admin
db.runCommand( { removeShard: "mongodb0" } )

A success message appears at completion:

{
    "msg" : "removeshard completed successfully",
    "state" : "completed",
    "shard" : "mongodb0",
    "ok" : 1
}

Once the value of the state field is “completed”, you may safely stop the processes comprising the mongodb0 shard.

← Manage Sharded Cluster Balancer Sharded Cluster Data Management →

Was this page helpful?

Yes No

Thank you for your feedback!

We're sorry! You can Report a Problem to help us improve this page.