unshardCollection

Definition

unshardCollection

Unshards an existing sharded collection and moves the collection data onto a single shard. When you unshard a collection, the collection cannot be partitioned across multiple shards and the shard key is removed.

New in version 8.0.

Tip

In mongosh, this command can also be run through the sh.unshardCollection().

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

This command must run on the admin database.

Note

Unsharding a collection is a write-intensive operation that can result in an increased oplog growth rate. To help mitigate this, consider the following configuration changes:

• To prevent unbounded oplog growth, set a fixed oplog size.

• To reduce the chance of secondaries becoming stale, increase the oplog size.

For more details, see the Replica Set Oplog.

Compatibility

This command is available in deployments hosted in the following environments:

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB

• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

db.adminCommand( {
   unshardCollection: "<database>.<collection>",
   toShard: "<shard-id>"
} )

Command Fields

Considerations

• unshardCollection can only be run on sharded clusters.

• unshardCollection can only operate on sharded collections.

• unshardCollection can only operate on a single collection at a time.

• unshardCollection has a 5 minute minimum duration.

• You must rebuild Atlas Search indexes after unshardCollection runs.

• You cannot make topology changes, such as adding or removing shards or transitioning between embedded and dedicated config servers, until unshardCollection completes.

• You cannot run the following operations on the collection that is being unsharded while unshardCollection is in progress:

  • collMod

  • convertToCapped

  • createIndexes

  • db.collection.createIndexes()

  • drop

  • db.collection.drop()

  • dropIndexes

  • db.collection.dropIndex()

  • renameCollection

  • db.collection.renameCollection()

• You cannot run the following operations on the cluster while unshardCollection is in progress:

  • addShard

  • removeShard

  • db.createCollection()

  • dropDatabase

  • transitionToDedicatedConfigServer

  • transitionFromDedicatedConfigServer

• Index builds that occur while unshardCollection is in progress might silently fail.

  • Do not create indexes while unshardCollection is in progress.

  • Do not call unshardCollection if there are ongoing index builds.

Requirements

Before you unshard your collection, ensure that you meet the following requirements:

• Your application can tolerate a period of two seconds where the affected collection blocks writes. During the time period where writes are blocked, your application experiences an increase in latency.

• Your database meets these resource requirements:

  • Ensure the shard you are moving the collection to has enough storage space for the collection and its indexes. The destination shard requires at least ( Collection storage size + Index Size ) * 2 bytes available.

  • Ensure that your I/O capacity is below 50%.

  • Ensure that your CPU load is below 80%.

Examples

db.adminCommand( {
    unshardCollection: "sales.eu_accounts"
} )

db.adminCommand( {
    unshardCollection: "sales.eu_accounts",
    toShard: "shard1"
} )