fsync
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Definition
fsync
Flushes all pending writes from the storage layer to disk. When the
lock
field is set totrue
, it sets a lock on the server or cluster to prevent additional writes until the lock is released.Starting in MongoDB 4.4.25, the
fsync
andfsyncUnlock
commands can run onmongos
to lock and unlock a sharded cluster.As applications write data, MongoDB records the data in the storage layer and then writes the data to disk.
Run
fsync
when you want to flush writes to disk.To provide durable data, WiredTiger uses checkpoints. For more details, see Journaling and the WiredTiger Storage Engine.
Important
Servers maintain an fsync lock count. The
fsync
command with thelock
field set totrue
increments the lock count while thefsyncUnlock
command decrements it. To enable writes on a locked server or cluster, call thefsyncUnlock
command until the lock count reaches zero.Use this command to block writes when you want to perform backup operations.
Tip
In the
mongo
Shell, this command can also be run through thedb.fsyncLock()
helper method.Helper methods are convenient for
mongo
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
fsync
command has the following syntax:db.adminCommand( { fsync: 1, lock: <Boolean>, fsyncLockAcquisitionTimeout: <integer>, comment: <any> } ) The
fsync
command has the following fields:FieldTypeDescriptionfsync
integerEnter "1" to applyfsync
.fsyncLockAcquisitionTimeoutMillis
integerOptional. Specifies the amount of time in milliseconds to wait to acquire locks. If the lock acquisition operation times out, the command returns a failed response.
Default:
90000
New in version 4.4.25.
lock
booleanOptional. Takes a lock on the server or cluster and blocks all write operations. Eachfsync
withlock
operation takes a lock.comment
anyOptional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations:
mongod log messages, in the
attr.command.cursor.comment
field.Database profiler output, in the
command.comment
field.currentOp
output, in thecommand.comment
field.
A comment can be any valid BSON type (string, integer, object, array, etc).
New in version 4.4.
Considerations
fsync
command with the lock
option 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.
Impact on Larger Deployments
New in version 4.4.25.
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.
Note
If the lockCount
field is greater than zero, all writes
are blocked on the server and cluster. To reduce the lock
count, use the fsyncUnlock
command.
Fsync Locks after Failures
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:
Run the
fsyncUnlock
command until the lock count reaches zero to release the lock on all nodes.Issue the
fsync
command to reestablish the fsync lock on the cluster.Restart the backup.
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.
Examples
Fsync Lock
Note
fsync
command with the lock
option 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.
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.
Important
The fsync lock operation maintains a lock count.
To unlock a server or cluster for writes, the lock count must be zero. That is, for the given number of times you perform an fsync lock, you must issue a corresponding number of unlock operations to unlock the server or cluster for writes.
Fsync Unlock
To unlock a server of cluster, use the fsyncUnlock
command:
db.adminCommnad( { 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 mongo
shell 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.