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fsync¶
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Definition¶
-
fsync
¶ Forces the
mongod
process to flush all pending writes from the storage layer to disk. Optionally, you can usefsync
to lock themongod
instance and block write operations for the purpose of capturing backups.As applications write data, MongoDB records the data in the storage layer and then writes the data to disk within the
syncdelay
interval, which is 60 seconds by default. Runfsync
when you want to flush writes to disk ahead of that interval.The
fsync
command has the following syntax:The
fsync
command has the following fields:Field Type Description fsync
integer Enter “1” to apply fsync
.async
Boolean Optional. Runs fsync
asynchronously. By default, thefsync
operation is synchronous.lock
Boolean Optional. Locks mongod
instance and blocks all write operations.
Behavior¶
An fsync
lock is only possible on individual
mongod
instances of a
sharded cluster, not on the entire cluster. To backup an entire sharded
cluster, please see Backup and Restore Sharded Clusters for
more information.
If your mongod
has journaling enabled,
consider using another method to create a
back up of the data set.
After fsync
, with lock, runs on a mongod
,
all write operations will block until a subsequent unlock. Read operations may also
block. As a result, fsync
,
with lock, is not a reliable mechanism for making
a mongod
instance operate in a read-only mode.
Examples¶
Run Asynchronously¶
The fsync
operation is synchronous by default To run
fsync
asynchronously, use the async
field set to
true
:
The operation returns immediately. To view the status of the
fsync
operation, check the output of
db.currentOp()
.
Lock mongod
Instance¶
The primary use of fsync
is to lock the mongod
instance in order to back up the files within mongod
’s dbpath
.
The operation flushes all data to the storage layer and
blocks all write operations until you unlock the mongod
instance.
To lock the database, use the lock
field set to true
:
You may continue to perform read operations on a mongod
instance that has a
fsync
lock. However, after the first write operation all
subsequent read operations wait until you unlock the mongod
instance.
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 mongod
instance is
currently locked:
After loading this function into your mongo
shell session
call it, with the following syntax:
This function will return true
if the mongod
instance is
currently locked and false
if the mongod
is not locked. To
unlock the mongod
, make a request for an unlock using the
following operation:
Unlock mongod
Instance¶
To unlock the mongod
instance, use db.fsyncUnlock()
: