fsync (database command数据库命令)

Impact on Larger Deployments对大规模部署的影响

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.当fsync命令在mongos上运行时，它会在整个集群上执行fsync操作。通过将lock字段设置为true，它在集群上设置了一个锁，防止了额外的写入。

To take a usable self-managed backup, before locking a sharded cluster:要在锁定分片群集之前进行可用的自我管理备份，请执行以下操作：

• Ensure that no chunk migration, resharding, or DDL operations are active.确保没有块迁移、重新标记或DDL操作处于活动状态。
• Stop the balancer to prevent additional chunk migrations from starting.停止平衡器以防止启动额外的块迁移。

Alternatives with Journaling日志记录的替代方案

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.如果mongod启用了日志记录，请使用文件系统或卷/块级快照工具将数据集和日志作为一个单元一起创建备份。

Lock Count锁计数

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.fsync命令返回一个包含lockCount字段的文档。在mongod上运行时，计数表示服务器上设置的fsync锁的数量。

When run on a sharded cluster, mongos sends the fsync operation to each shard and returns the results, which includes the lockCount for each.在分片集群上运行时，mongos向每个分片发送fsync操作并返回结果，其中包括每个分片的lockCount。

Fsync Locks after Failures故障后Fsync锁定

Fsync locks execute on the primary in a replica set or sharded cluster.Fsync锁在副本集或分片集群的主服务器上执行。

If the primary goes down or becomes unreachable due to network issues, the cluster elects a new primary from the available secondaries. 如果主服务器因网络问题而停机或无法访问，集群将从可用的次服务器中选择一个新的主服务器。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.如果具有fsync锁的主服务器发生故障，则新主服务器不会保留fsync锁，可以处理写操作。在备份操作期间发生选举时，产生的备份可能不一致或不可用。

To recover from the primary going down:要从主故障中恢复：

1. Run the fsyncUnlock command until the lock count reaches zero to release the lock on all nodes.运行fsyncUnlock命令，直到锁计数达到零，以释放所有节点上的锁。
2. Issue the fsync command to reestablish the fsync lock on the cluster.发出fsync命令以在集群上重新建立fsync锁。
3. Restart the backup.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.此外，fsync锁是持久的。当旧主节点再次联机时，您需要使用fsyncUnlock命令释放节点上的锁。

Fsync LockFsync锁

The fsync command can lock an individual mongod instance or a sharded cluster through mongos. fsync命令可以通过mongos锁定单个mongod实例或分片集群。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.当将lock字段设置为true运行时，fsync操作会将所有数据刷新到存储层，并阻止所有其他写入操作，直到您解锁实例或集群。

To lock the database, use the fsync command to set the lock field to true:要锁定数据库，请使用fsync命令将lock字段设置为true：

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

The operation returns a document that includes the status of the operation and the lockCount:该操作返回一个文档，其中包括操作状态和lockCount：

{
   "info" : "now locked against writes, use db.fsyncUnlock() to unlock",
   "lockCount" : Long(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.锁定后，写入操作将被阻止。单独的连接可能会继续读取操作，直到第一次尝试写入操作，然后它们也会等待服务器或集群解锁。

Fsync UnlockFsync解锁

To unlock a server of cluster, use the fsyncUnlock command:要解锁群集的服务器，请使用fsyncUnlock命令：

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.根据需要重复此命令多次，以将锁计数减少到零。一旦锁计数达到零，服务器或集群就可以恢复写入。

Check Lock Status检查锁定状态

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:要检查fsync锁的状态，请使用db.currentOp()。在shell中使用以下JavaScript函数来测试服务器或集群当前是否已锁定：

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:将此函数加载到mongosh会话中后，使用以下语法调用它：

serverIsLocked()

This function will return true if the server or cluster is currently locked and false if the server or cluster is not locked.如果服务器或群集当前已锁定，则此函数将返回true，如果服务器或群集中未锁定，则返回false。