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- Manage Journaling
Manage Journaling¶
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MongoDB uses write ahead logging to an on-disk journal to guarantee write operation durability. The MMAPv1 storage engine also requires the journal in order to provide crash resiliency.
The WiredTiger storage engine does not require journaling to guarantee a consistent state after a crash. The database will be restored to the last consistent checkpoint during recovery. However, if MongoDB exits unexpectedly in between checkpoints, journaling is required to recover writes that occurred after the last checkpoint.
With journaling enabled, if mongod
stops unexpectedly, the
program can recover everything written to the journal. MongoDB will
re-apply the write operations on restart and maintain a consistent
state. By default, the greatest extent of lost writes, i.e., those not
made to the journal, are those made in the last 100 milliseconds, plus
the time it takes to perform the actual journal writes. See
commitIntervalMs
for more information on
the default.
Procedures¶
Disable Journaling¶
Warning
Do not disable journaling on production systems.
In version 3.6, the
--nojournal
option is deprecated for replica set members using the WiredTiger storage engine.Replica set members which use the WiredTiger storage engine should not use the
--nojournal
option. For more information about journaling, see Manage Journaling.When using the MMAPv1 storage engine without a journal, if your
mongod
instance stops without shutting down cleanly unexpectedly for any reason, (e.g. power failure) and you are not running with journaling, then you must recover from an unaffected replica set member or backup, as described in repair.
To disable journaling, start mongod
with the
--nojournal
command line option.
Get Commit Acknowledgement¶
You can get commit acknowledgement with the Write Concern and
the j
option. For details, see
Write Concern.
Avoid Preallocation Lag for MMAPv1¶
With the MMAPv1 storage engine, MongoDB may
preallocate journal files if the mongod
process determines
that it is more efficient to preallocate journal files than create new
journal files as needed.
Depending on your filesystem, you might experience a preallocation lag
the first time you start a mongod
instance with journaling
enabled. The amount of time required to pre-allocate files might last
several minutes; during this time, you will not be able to connect to
the database. This is a one-time preallocation and does not occur with
future invocations.
To avoid preallocation lag, you can
preallocate files in the journal directory by copying them from
another instance of mongod
.
Preallocated files do not contain data. It is safe to later remove
them. But if you restart mongod
with journaling,
mongod
will create them again.
Example
The following sequence preallocates journal files for an
instance of mongod
running on port 27017
with a
database path of /data/db
.
For demonstration purposes, the sequence starts by creating a set of journal files in the usual way.
Create a temporary directory into which to create a set of journal files:
Create a set of journal files by starting a
mongod
instance that uses the temporary directory. For example:Tip
To avoid downtime, give each config server a logical DNS name (unrelated to the server’s physical or virtual hostname). Without logical DNS names, moving or renaming a config server requires shutting down every
mongod
andmongos
instance in the sharded cluster.Warning
Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.
When you see the following log output, indicating
mongod
has the files, press CONTROL+C to stop themongod
instance:Preallocate journal files for the new instance of
mongod
by moving the journal files from the data directory of the existing instance to the data directory of the new instance:Start the new
mongod
instance. For example:Tip
To avoid downtime, give each config server a logical DNS name (unrelated to the server’s physical or virtual hostname). Without logical DNS names, moving or renaming a config server requires shutting down every
mongod
andmongos
instance in the sharded cluster.Warning
Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.
Monitor Journal Status¶
The serverStatus
command/db.serverStatus()
method returns wiredTiger.log
, which contains
statistics on the WiredTiger journal.
Change the Group Commit Interval for MMAPv1¶
For the MMAPv1 storage engine, you can set the
group commit interval using the --journalCommitInterval
command line option. The allowed
range is 2
to 300
milliseconds.
Lower values increase the durability of the journal at the expense of disk performance.
Recover Data After Unexpected Shutdown¶
On a restart after a crash, MongoDB replays all journal files in the
journal directory before the server becomes available. If MongoDB must
replay journal files, mongod
notes these events in the log
output.
There is no reason to run repairDatabase
in these
situations.
Change WT Journal Compressor¶
With the WiredTiger storage engine, MongoDB, by default, uses the
snappy
compressor for the journal. To specify a different
compressions algorithm or no compression for a mongod
instance:
Tip
If you encounter an unclean shutdown for a mongod
during this procedure, you must use the old compressor settings to
recover using the journal files. Once recovered, you can retry the
procedure.
Update the
storage.wiredTiger.engineConfig.journalCompressor
setting to the new value.If you use command-line options instead of a configuration file, you will have to update the
--wiredTigerJournalCompressor
command-line option during the restart below.Perform a clean shutdown of the
mongod
instance:Once you have confirmed that the process is no longer running, restart the
mongod
instance:If you are using a configuration file:
If you are using command-line options instead of a configuration file, update the
--wiredTigerJournalCompressor
option.