compact (original) (raw)

compact

Attempts to release unneeded disk space to the operating system.

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

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

Important

This command is not supported in M0, M2, M5, and Flex clusters. For more information, see Unsupported Commands.

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

The command has the following syntax:

db.runCommand(

   {

     compact: <string>,

     dryRun: <boolean>,

     force: <boolean>, // Optional

     freeSpaceTargetMB: <int>, // Optional

     comment: <any>, // Optional

   }

)

The command takes the following fields:

For clusters enforcing authentication, you must authenticate as a user with the compact privilege action on the target collection. The dbAdmin andhostManager roles provide the required privileges for runningcompact against non-system collections.

For system collections, you must:

1. Create a custom role that grants the compact action on the system collection.
2. Grant that role to a new or existing user.
3. Authenticate as that user to perform the compactcommand.

For example, the following operations create a custom role that grants the compact action against the specified database and collection:

use admin

db.createRole(

  {

    role: "myCustomCompactRole",

    privileges: [

      {

        resource: { "db" : "<database>" , "collection" : "<collection>" },

        actions: [ "compact" ]

      }

    ],

    roles: []

  }

)

For more information on configuring the resource document, seeResource Document on Self-Managed Deployments.

To add the dbAdmin, hostManager, or the custom role to an existing user, use db.grantRolesToUser() or db.updateUser(). The following operation grants the custom compact role to themyCompactUser on the admin database:

use admin

db.grantRolesToUser("myCompactUser", [ "dbAdmin" | "myCustomCompactRole" ] )

To add the dbAdmin or the custom role to a new user, specify the role to the roles array of thedb.createUser() method when creating the user.

use admin

db.createUser(

  {

     user: "myCompactUser",

     pwd: "myCompactUserPassword",

     roles: [

       { role: "dbAdmin", db: "<database>" } | "myCustomCompactRole"

     ]

   }

)

To check the compact operation's progress, monitor themongod log file or run db.currentOp()from another shell instance.

If you terminate compact with the db.killOp() method or restart the server before the operation finishes, compact ends and may fail its attempt to release disk space back to the operating system.

The compact command attempts to reduce the disk space consumed for data and indexes in a collection by releasing obsolete blocks back to the operating system. The effectiveness of compact is relative to how many blocks are available to be released and where in the data file the blocks are.

To see how the storage space changes for the collection, run thecollStats command before and after compaction. You can use the output metric collStats.freeStorageSize to view the amount of storage available for reuse.

The operation is iterative and operates on segments of the data file in each pass. To view an estimate of how much space compact will release, use the dryRunflag. Calling compact on a collection will compact both the collection and its associated indexes.

compact may require additional disk space to run.

Compaction regularly checkpoints the database, which can lead to synchronization overhead. On high-traffic databases, this can potentially delay or prevent operational tasks such as taking backups. To avoid unexpected disruptions, disable compaction before taking a backup.

You can use compact on collections and indexes that are stored in a replica set, however there are some important considerations:

• The primary node does not replicate the compact command to the secondaries.
• You should run compact on secondary nodes whenever possible. If you cannot run compact on secondaries, see the force option.
• Starting in MongoDB 6.1.0 (and 6.0.2 and 5.0.12):
  • A secondary node can replicate while compact is running.
  • Reads are permitted.

To run compact on a cluster

Run compact on one of the secondary nodes. When compactfinishes, repeat the operation on each of the remaining secondaries in turn.

After stepping down, the old primary node becomes a secondary node. Run compact on the old primary node.

• A secondary node can replicate while compact is running.
• Reads are permitted.

While the compact command is running, the replica set remains in aSECONDARY status.

For more information about replica set member states, see See Replica Set Member States.

For replica set maintenance and availability, seePerform Maintenance on Self-Managed Replica Set Members.

compact only applies to mongod instances. In a sharded environment, run compact on each shard separately as a maintenance operation.

You cannot issue compact against a mongos instance.

mongod rebuilds all indexes in parallel following thecompact operation.

If you try to run multiple concurrent compact commands on the same collection, MongoDB returns an error.

The following operation runs the compact command on themovies collection:

db.runCommand( { compact: "movies" } )

{ bytesFreed: 27859, ok: 1 }

The following operation performs a dry run of the compact command on themovies collection:

db.runCommand( {

   compact: "movies",

   dryRun: true

} )

{ estimatedBytesFreed: 27859, ok: 1 }

• autoCompact