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Journaling

Manage Journaling

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  • Procedures

MongoDB uses write ahead logging to an on-disk journal to guarantee write operation durability.

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.

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

Get Commit Acknowledgement

You can get commit acknowledgement with the Write Concern and the j option. For details, see Write Concern.

Monitor Journal Status

The serverStatus command/db.serverStatus() method returns wiredTiger.log, which contains statistics on the journal.

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 --repair.

Change WiredTiger 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.

Use the following procedure to change the journal compressor for a standalone mongod instance:

  1. Update storage.wiredTiger.engineConfig.journalCompressor to the new value.

    If you use command-line options instead of a configuration file, you must update the --wiredTigerJournalCompressor command-line option during the restart below.

  2. Perform a clean shutdown of the mongod instance. For example, connect mongosh to the instance and issue db.shutdownServer():

    db.getSiblingDB('admin').shutdownServer()

  3. Once you have confirmed that the process is no longer running, restart the mongod instance:

    • If you are using a configuration file:

      mongod -f <path/to/myconfig.conf>

    • If you are using command-line options instead of a configuration file, update --wiredTigerJournalCompressor to the new value.

      mongod --wiredTigerJournalCompressor <differentCompressor|none>  ...

Use the following procedure to change the journal compressor for a member of a replica set:

  1. Perform a clean shutdown of the mongod instance. For example, connect mongosh to the instance and issue db.shutdownServer():

    db.getSiblingDB('admin').shutdownServer()

  2. Update storage.wiredTiger.engineConfig.journalCompressor to the new value.

    If you use command-line options instead of a configuration file, you must update the command-line options during the restart below.

  3. Restart the mongod instance:

    • If you are using a configuration file:

      mongod -f <path/to/myconfig.conf>

    • If you are using command-line options instead of a configuration file, update --wiredTigerJournalCompressor to the new value.

      mongod --wiredTigerJournalCompressor <differentCompressor|none> ...

Use the following procedure to change the journal compressor for a member of a shard replica set or config server replica set:

  1. Perform a clean shutdown of the mongod instance. For example, connect mongosh to the instance and issue db.shutdownServer():

    db.getSiblingDB('admin').shutdownServer()

  2. Update storage.wiredTiger.engineConfig.journalCompressor to the new value.

    If you use command-line options instead of a configuration file, you must update the command-line options during the restart below.

  3. Restart the mongod instance:

    • If you are using a configuration file:

      mongod -f <path/to/myconfig.conf>

    • If you are using command-line options instead of a configuration file, update --wiredTigerJournalCompressor to the new value.

      mongod --shardsvr --wiredTigerJournalCompressor <differentCompressor|none> --replSet ...

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