Docs Menu
Docs Home
MongoDB Manual
/ / /


On this page

  • Definition
  • Behavior
  • Example

Flushes all pending writes from the storage layer to disk and locks the server to prevent any additional writes until the lock is released.

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


mongosh Method

This page documents a mongosh method. This is not the documentation for database commands or language-specific drivers, such as Node.js.

For the database command, see the fsync command.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

For the legacy mongo shell documentation, refer to the documentation for the corresponding MongoDB Server release:

mongo shell v4.4

Servers maintain an fsync lock count. The fsyncLock() method increments the lock count while the fsyncUnlock() method decrements it. To unlock writes on a server or cluster, call the fsyncUnlock() method until the lock count reaches zero.

db.fsyncLock() has the syntax:


The operation returns a document with the following fields:

Information on the status of the operation.
Number of locks currently on the instance.
Link to the fsync command documentation.
The status code.

db.fsyncLock() is an administrative command. Use this method to lock a server or cluster before backup operations.

db.fsyncLock() ensures that the data files are safe to copy using low-level backup utilities such as cp, scp, or tar. A mongod started using the copied files contains user-written data that is indistinguishable from the user-written data on the locked mongod.

The data files of a locked mongod may change due to operations such as journaling syncs or WiredTiger snapshots. While this has no affect on the logical data (e.g. data accessed by clients), some backup utilities may detect these changes and emit warnings or fail with errors. For more information on MongoDB- recommended backup utilities and procedures, see MongoDB Backup Methods.

Fsync locks execute on the primary in a replica set or sharded cluster.

If the primary goes down or becomes unreachable due to network issues, the cluster elects a new primary from the available secondaries. If a primary with an fsync lock goes down, the new primary does not retain the fsync lock and can handle write operations. When elections occur during backup operations, the resulting backup may be inconsistent or unusable.

To recover from the primary going down:

  1. Run the db.fsyncUnlock() method until the lock count reaches zero to release the lock on all nodes.

  2. Issue the db.fsyncLock() command to reestablish the fsync lock on the cluster.

  3. Restart the backup.

Additionally, fsync locks are persistent. When the old primary comes online again, you need to run the db.fsyncUnlock() command to release the lock on the node.

The following operation runs db.fsyncLock():


The operation returns the following status document that includes the lockCount:

"info" : "now locked against writes, use db.fsyncUnlock() to unlock",
"lockCount" : NumberLong(1),
"seeAlso" : "",
"ok" : 1

If you run db.fsyncLock() again, the operation increments the lockCount:

"info" : "now locked against writes, use db.fsyncUnlock() to unlock",
"lockCount" : NumberLong(2),
"seeAlso" : "",
"ok" : 1

To unlock the instance for writes, you must run db.fsyncUnlock() twice to reduce the lockCount to 0.