compact (database command)

Definition

compact: Rewrites and defragments all data and indexes in a collection. On WiredTiger databases, this command releases 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.

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

Syntax

The command has the following syntax:

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

Command Fields

The command takes the following fields:

Field	Type	Description
`compact`	string	The name of the collection.
`force`	flag	Optional. If specified, forces `compact` to run on the primary in a replica set. `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: mongod log messages, in the `attr.command.cursor.comment` field. Database profiler output, in the `command.comment` field. `currentOp` output, in the `command.comment` field. 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 system collections, you must:

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

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, 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 restart the server before compact finishes, the operation terminates and may fail its attempt to release disk space back to the operating system.

Disk Space

The compact command attempts to reduce disk space for data and indexes in a collection by releasing obsolete blocks to the operating system. Effectiveness depends on how many blocks are available to release and their location in the data file.

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. Calling compact on a collection compacts both the collection and its associated indexes.

compact may require additional disk space to run.

Backups

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.

Multi-Shard Transactions

Note

If your deployment uses secondary reads and multi-shard transactions, consider running an initial sync instead of compact.

When compact runs, WiredTiger temporarily adjusts its block-manager strategy to accommodate the compact workload. This adjustment can slow checkpoint operations and increase the time that a member takes to commit a transaction. Delayed transaction commits can cause significant read latency, replication lag, and operational queuing.

Replica Sets

You can use compact on collections and indexes in a replica set; however, consider the following:

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

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.

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.

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

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

Index Size Differences Across Members

Each node applies writes to its indexes independently. Index storage size can differ across the members of a replica set, even when the members hold identical data. A primary node applies writes directly from client operations. A secondary node applies the same writes from the oplog, often in different batches and order. These write pattern differences change how the storage engine packs and compresses index pages. As a result, an index on one member can grow larger on disk than the same index on another member.

To bring an affected index back in line with the same index on other members, use one of the following remediation options:

Run compact on the affected node.
Perform an initial sync of the affected node.
Drop and recreate the affected index.

When you plan storage capacity for a replica set, account for this variation in index size across 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.

Example

The following operation runs the compact command on the movies collection:

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

Running compact returns output similar to the following:

{ bytesFreed: 27859, ok: 1 }

Back

collMod

compactStructuredEncryptionData