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

Enable Journaling

For 64-bit builds of mongod, journaling is enabled by default.

To enable journaling, start mongod with the --journal command line option.

Disable Journaling

Warning

Do not disable journaling on production systems. 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 Acknowledgment

You can get commit acknowledgment 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.

  1. Create a temporary directory into which to create a set of journal files:

    mkdir ~/tmpDbpath
    
  2. Create a set of journal files by starting a mongod instance that uses the temporary directory:

    mongod --port 10000 --dbpath ~/tmpDbpath --journal
    
  3. When you see the following log output, indicating mongod has the files, press CONTROL+C to stop the mongod instance:

    [initandlisten] waiting for connections on port 10000
    
  4. 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:

    mv ~/tmpDbpath/journal /data/db/
    
  5. Start the new mongod instance:

    mongod --port 27017 --dbpath /data/db --journal
    

Monitor Journal Status

Use the following commands and methods to monitor journal status:

  • serverStatus

    The serverStatus command returns database status information that is useful for assessing performance.

  • journalLatencyTest

    Use journalLatencyTest to measure how long it takes on your volume to write to the disk in an append-only fashion. You can run this command on an idle system to get a baseline sync time for journaling. You can also run this command on a busy system to see the sync time on a busy system, which may be higher if the journal directory is on the same volume as the data files.

    The journalLatencyTest command also provides a way to check if your disk drive is buffering writes in its local cache. If the number is very low (i.e., less than 2 milliseconds) and the drive is non-SSD, the drive is probably buffering writes. In that case, enable cache write-through for the device in your operating system, unless you have a disk controller card with battery backed RAM.

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.

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