fsync

Impact on Larger Deployments

When the fsync command runs on mongos, it performs the fsync operation on the entire cluster. By setting the lock field to true, it sets a lock on the cluster, preventing additional writes.

To take a usable self-managed backup, before locking a sharded cluster:

• Ensure that no chunk migration, resharding, or DDL operations are active.

• Stop the balancer to prevent additional chunk migrations from starting.

Alternatives with Journaling

If your mongod has journaling enabled, use a file system or volume/block level snapshot tool to create a backup of the data set and the journal together as a single unit.

Lock Count

The fsync command returns a document includes a lockCount field. When run on mongod, the count indicates the number of fsync locks set on the server.

When run on a sharded cluster, mongos sends the fsync operation to each shard and returns the results, which includes the lockCount for each.

故障后的 fsync 锁

Fsync 锁在副本集或分片集群中的主节点上执行。

如果主节点因网络问题宕机或无法访问，集群会从可用的从节点中选举出新的主节点。如果带有 fsync 锁的主节点宕机，新的主节点不会保留 fsync 锁，并且可以处理写操作。备份操作过程中发生选举时，生成的备份可能不一致或无法使用。

要从主节点降级中恢复：

1. Run the fsyncUnlock command until the lock count reaches zero to release the lock on all nodes.

2. 发出fsync命令以在集群上重新建立 fsync 锁。

3. 重新启动备份。

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

Fsync Lock

The fsync command can lock an individual mongod instance or a sharded cluster through mongos. When run with the lock field set to true, the fsync operation flushes all data to the storage layer and blocks all additional write operations until you unlock the instance or cluster.

To lock the database, use the fsync command to set the lock field to true:

db.adminCommand( { fsync: 1, lock: true } )

The operation returns a document that includes the status of the operation and the lockCount:

{
   "info" : "now locked against writes, use db.fsyncUnlock() to unlock",
   "lockCount" : NumberLong(1),
   "seeAlso" : "http://dochub.mongodb.org/core/fsynccommand",
   "ok" : 1
}

When locked, write operations are blocked. Separate connections may continue read operations until the first attempt at a write operation, then they also wait until the sever or cluster is unlocked.

Fsync Unlock

To unlock a server of cluster, use the fsyncUnlock command:

db.adminCommand( { fsyncUnlock: 1 } )

Repeat this command as many times as needed to reduce the lock count to zero. Once the lock count reaches zero, the server or cluster can resume writes.

Check Lock Status

To check the state of the fsync lock, use db.currentOp(). Use the following JavaScript function in the shell to test if the server or cluster is currently locked:

serverIsLocked = function () {
                     var co = db.currentOp();
                     if (co && co.fsyncLock) {
                         return true;
                     }
                     return false;
                 }

After loading this function into your mongosh session, call it with the following syntax:

serverIsLocked()

This function will return true if the server or cluster is currently locked and false if the server or cluster is not locked.