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compact (database command)

Definition

compact

Rewrites and defragments all data and indexes in a collection. On WiredTiger databases, this command will release unneeded disk space to the operating system.

Compatibility

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 and Flex clusters. For more information, see Unsupported Commands.

Syntax

The command has the following syntax:

db.runCommand(
 {
 compact: <collection name>
 }
)

Command Fields

The command takes the following fields:

Note

Starting in MongoDB 4.2

MongoDB removes the MMAPv1 storage engine and the MMAPv1 specific options paddingFactor, paddingBytes, preservePadding for compact.

Field
Type
Description

compact

string

The name of the collection.

force

flag

Optional. Starting in v4.4, if specified, forces compact to run on the primary in a replica set. Before v4.4, this boolean field enabled compact to run on the primary in a replica set if the value was true and returned an error when run on a primary if the value was false, because the command blocked all other operations.

Starting in v4.4, compact does not block MongoDB CRUD Operations on the database it is compacting.

comment

any

Optional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations:

A comment can be any valid BSON type (string, integer, object, array, etc).

compact Required Privileges

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

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

To add the dbAdmin or the custom role to an existing user, use db.grantRolesToUser() or db.updateUser(). The following operation grants the custom compact role to the myCompactUser 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 the db.createUser() method when creating the user.

use admin
db.createUser(
 {
 user: "myCompactUser",
 pwd: "myCompactUserPassword",
 roles: [
 { role: "dbAdmin", db: "<database>" } | "myCustomCompactRole"
 ]
 }
)

Behavior

Monitoring Progress

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

Operation Termination

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.

Disk Space

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 the collStats 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 dryRun flag. Calling compact on a collection will compact both the collection and its associated indexes.

compact may require additional disk space to run.

Replica Sets

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):

    • A secondary node can replicate while compact is running.

    • Reads are permitted.

To run compact on a cluster

1

Compact the secondary nodes.

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

2

Reassign the primary node.

To step down the current primary and trigger an election, use the rs.stepDown() method. To nominate a particular secondary node, adjust the member priority.

3

Compact the old primary.

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

Version Specific Considerations for Secondary Nodes

Blocking behavior on secondary nodes is version specific.

Version

Blocking Behavior

4.4

  • No replication is possible.

  • Reads are not permitted.

Post 4.4.17, 5.0.12, and 6.0.2

  • A secondary node can replicate while compact is running.

  • Reads permitted.

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

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

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

Sharded Clusters

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.

Index Building

mongod rebuilds all indexes in parallel following the compact operation.

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