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compact¶
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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.
compact
has the following form:compact
takes the following fields:Starting in MongoDB 4.2
MongoDB removes the MMAPv1 storage engine and the MMAPv1 specific options
paddingFactor
,paddingBytes
,preservePadding
forcompact
.Field Type Description compact
string The name of the collection. force
boolean Optional. If
true
,compact
can run on the primary in a replica set. Iffalse
,compact
returns an error when run on a primary, because the command blocks all other operations.compact
blocks operations only for the database it is compacting.
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:
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:
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.
Behavior¶
Blocking¶
compact
only blocks operations for the database it is currently
operating on. Only use compact
during scheduled maintenance
periods.
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
orcompact
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 optionforce:true
above for information regarding compacting the primary. - Starting in MongoDB 4.2.18: a secondary is
not available when
compact
is running. The secondary does not enter theRECOVERING
state. - For previous MongoDB versions: on secondaries,
compact
forces the secondary to enter theRECOVERING
state. Read operations issued to an instance in theRECOVERING
state will fail. This prevents clients from reading during the operation. When the operation completes, the secondary returns toSECONDARY
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.
Capped Collections¶
On WiredTiger, the compact
command will attempt to compact the collection.