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Reduces the lock count on the server or cluster. To enable write operations, the lock count must be zero.

Starting in MongoDB 7.0.2 (also available starting in 6.0.11, and 5.0.22) the fsync and fsyncUnlock commands can run on mongos to lock and unlock a sharded cluster.

Use this command to unblock writes after you finish a backup operation.


Servers maintain an fsync lock count. The fsync command with the lock field set to true increments the lock count while the fsyncUnlock command decrements it. To enable writes on a locked server or cluster, call the fsyncUnlock command until the lock count reaches zero.

fsyncUnlock is an administrative operation. Typically you will use fsyncUnlock following a database backup operation.


In mongosh, this command can also be run through the db.fsyncUnlock() helper method.

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

The command has the following syntax:

fsyncUnlock: 1,
comment: <any>

The comment field is optional and may contain a comment of any data type.

The operation returns a document with the following fields:

Information on the status of the operation
The number of locks remaining on the instance after the operation.
The status code.

Consider a situation where db.fsyncLock() has been issued two times. The following fsyncUnlock operation reduces the locks taken by db.fsyncLock() by 1:

db.adminCommand( { fsyncUnlock: 1 } )

The operation returns the following document:

{ "info" : "fsyncUnlock completed", "lockCount" : NumberLong(1), "ok" : 1 }

As the lockCount is greater than 0, the mongod instance is locked against writes. To unlock the instance for writes, run the unlock operation again:

db.adminCommand( { fsyncUnlock: 1 } )

The operation returns the following document:

{ "info" : "fsyncUnlock completed", "lockCount" : NumberLong(0), "ok" : 1 }

The mongod instance is unlocked for writes.