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compact

On this page

  • Definition
  • Syntax
  • Command Fields
  • compact Required Privileges
  • Behavior

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.

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

Changed in version 4.4.

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).

New in version 4.4.

Warning

Always have an up-to-date backup before performing server maintenance such as the compact operation.

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, create a custom role that grants the compact action on the system collection. You can then grant that role to a new or existing user and authenticate as that user to perform the compact command. For example, the following operations create a custom role that grants the compact action against 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.

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

Blocking

Changed in version 4.4.

Starting in v4.4, on WiredTiger, compact only blocks the following metadata operations:

compact does not block MongoDB CRUD Operations for the database it is currently operating on.

Before v4.4, compact blocked all operations for the database it was compacting, including MongoDB CRUD Operations, and was therefore recommended for use only during scheduled maintenance periods. Starting in v4.4, the compact command is appropriate for use at any time.

You may view the intermediate progress either by viewing the mongod log file or by running the db.currentOp() in another shell instance.

Operation Termination

If you terminate the operation with the db.killOp() method or restart the server before the compact operation has finished, be aware of the following:

  • If you have journaling enabled, the data remains valid and usable, regardless of the state of the compact operation. You may have to manually rebuild the indexes.

  • If you do not have journaling enabled and the mongod or compact terminates during the operation, it is impossible to guarantee that the data is in a valid state.

  • In either case, much of the existing free space in the collection may become un-reusable. In this scenario, you should rerun the compaction to completion to restore the use of this free space.

Disk Space

To see how the storage space changes for the collection, run the collStats command before and after compaction.

On WiredTiger, compact attempts to reduce the required storage space for data and indexes in a collection, releasing unneeded disk space to the operating system. The effectiveness of this operation is workload dependent and no disk space may be recovered. This command is useful if you have removed a large amount of data from the collection, and do not plan to replace it.

compact may require additional disk space to run on WiredTiger databases.

Replica Sets

compact commands do not replicate to secondaries in a replica set.

  • Compact each member separately.

  • Ideally run compact on a secondary. See option force above for information regarding compacting the primary.

  • Starting in MongoDB 5.0.3 (and 4.4.9 and 4.2.18): a secondary is not available when compact is running. The secondary does not enter the RECOVERING state.

  • For previous MongoDB versions: on secondaries, compact forces the secondary to enter the RECOVERING state. Read operations issued to an instance in the RECOVERING state will fail. This prevents clients from reading during the operation. When the operation completes, the secondary returns to SECONDARY state.

See Replica Set Member States for more information about replica set member states.

See Perform Maintenance on Replica Set Members for an example replica set maintenance procedure to maximize availability during maintenance operations.

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.

Capped Collections

On WiredTiger, the compact command will attempt to compact the collection.

Index Building

mongod rebuilds all indexes in parallel following the compact operation.

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