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Manage Journaling

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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.

Note

Starting in MongoDB 4.0, you cannot specify --nojournal option or storage.journal.enabled: false for replica set members that use the WiredTiger storage engine.

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.

To disable journaling for a standalone deployment, 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.

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 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.

  2. Perform a clean shutdown of the mongod instance:

    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 the --wiredTigerJournalCompressor option.

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

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

Note

The following procedure involves restarting the replica member as a standalone without the journal.

  1. Perform a clean shutdown of the mongod instance:

    db.getSiblingDB('admin').shutdownServer()
    
  2. Update the configuration file to prepare to restart as a standalone:

    For example:

    storage:
       journal:
          enabled: false
    #replication:
    #   replSetName: replA
    

    If you use command-line options instead of a configuration file, you will have to update the command-line option during the restart.

  3. Restart the mongod instance:

  4. Perform a clean shutdown of the mongod instance:

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

    Confirm that the process is no longer running.

  5. Update the configuration file to prepare to restart as a replica set member with the new journal compressor:

    For example:

    storage:
       wiredTiger:
          engineConfig:
             journalCompressor: <newValue>
    replication:
       replSetName: replA
    

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

  6. Restart the mongod instance as a replica set member:

    • 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:

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

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

Note

The following procedure involves restarting the replica member as a standalone without the journal.

  1. Perform a clean shutdown of the mongod instance:

    db.getSiblingDB('admin').shutdownServer()
    
  2. Update the configuration file to prepare to restart as a standalone:

    For example:

    storage:
       journal:
          enabled: false
    setParameter:
       skipShardingConfigurationChecks: true
    #replication:
    #   replSetName: shardA
    #sharding:
    #   clusterRole: shardsvr
    net:
      port: 27218
    

    If you use command-line options instead of a configuration file, you will have to update the command-line option during the restart.

  3. Restart the mongod instance:

  4. Perform a clean shutdown of the mongod instance:

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

    Confirm that the process is no longer running.

  5. Update the configuration file to prepare to restart with the new journal compressor:

    For example:

    storage:
       wiredTiger:
          engineConfig:
             journalCompressor: <newValue>
    replication:
       replSetName: shardA
    sharding:
       clusterRole: shardsvr
    net:
      port: 27218
    

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

  6. Restart the mongod instance as a replica set member:

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