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.

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.

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.

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.

Standalone
Replica Set Member
Sharded Cluster Member

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

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. For example, connect a mongo shell to the instance and issue db.shutdownServer():
copy
```
db.getSiblingDB('admin').shutdownServer()
```
Once you have confirmed that the process is no longer running, restart the mongod instance:
- If you are using a configuration file:
  copy
  mongod -f <path/to/myconfig.conf>
- If you are using command-line options instead of a configuration file, update the --wiredTigerJournalCompressor option.
  copy
  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.

Perform a clean shutdown of the mongod instance. For example, connect a mongo shell to the instance and issue db.shutdownServer():
copy
```
db.getSiblingDB('admin').shutdownServer()
```
Update the configuration file to prepare to restart as a standalone:
- Set storage.journal.enabled to false.
- Comment out the replication settings for your deployment.
- Set parameter disableLogicalSessionCacheRefresh to true in the setParameter section.
For example:
copy
```
storage:
   journal:
      enabled: false
#replication:
#   replSetName: replA
setParameter:
   disableLogicalSessionCacheRefresh: true
```
If you use command-line options instead of a configuration file, you will have to update the command-line option during the restart.
Restart the mongod instance:
- If you are using a configuration file:
  copy
  mongod -f <path/to/myconfig.conf>
- If you are using command-line options instead of a configuration file,
  - Include the --nojournal option
  - Remove any replication command-line options (such as --replSet):
  - Set parameter disableLogicalSessionCacheRefresh to true in the --setParameter option.
    copy
    mongod --nojournal --setParameter disableLogicalSessionCacheRefresh=true ...
Perform a clean shutdown of the mongod instance:
copy
```
db.getSiblingDB('admin').shutdownServer()
```
Confirm that the process is no longer running.
Update the configuration file to prepare to restart as a replica set member with the new journal compressor:
- Remove the storage.journal.enabled setting.
- Uncomment the replication settings for your deployment.
- Remove the disableLogicalSessionCacheRefresh parameter.
- Remove storage.wiredTiger.engineConfig.journalCompressor setting to use the default journal compressor or specify a new value.
For example:
copy
```
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.
Restart the mongod instance as a replica set member:
- If you are using a configuration file:
  copy
  mongod -f <path/to/myconfig.conf>
- If you are using command-line options instead of a configuration file:
  - Remove the --nojournal option.
  - Remove the --wiredTigerJournalCompressor command-line option to use the default journal compressor or update to a new value.
  - Include your replication command-line options as well as any additional options for your replica set member.
  - Remove the disableLogicalSessionCacheRefresh parameter.
  copy
  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.

Perform a clean shutdown of the mongod instance. For example, connect a mongo shell to the instance and issue db.shutdownServer():
copy
```
db.getSiblingDB('admin').shutdownServer()
```
Update the configuration file to prepare to restart as a standalone:
- Set storage.journal.enabled to false.
- Set parameter skipShardingConfigurationChecks to true.
- Set parameter disableLogicalSessionCacheRefresh to true in the setParameter section.
- Comment out the replication settings for your deployment.
- Comment out the sharding.clusterRole setting.
- Set the net.port to the member’s current port, if it is not explicitly set.
For example:
copy
```
storage:
   journal:
      enabled: false
setParameter:
   skipShardingConfigurationChecks: true
   disableLogicalSessionCacheRefresh: 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.
Restart the mongod instance:
- If you are using a configuration file:
  copy
  mongod -f <path/to/myconfig.conf>
- If you are using command-line options instead of a configuration file:
  - Include the --nojournal option.
  - Set parameter skipShardingConfigurationChecks to true.
  - Set parameter disableLogicalSessionCacheRefresh to true in the --setParameter option.
  - Remove any replication command-line options (such as --replSet).
  - Remove --shardsvr/--configsvr option.
  - Explicitly include --port set to the instance’s current port.
  copy
  mongod --nojournal --setParameter skipShardingConfigurationChecks=true --setParameter disableLogicalSessionCacheRefresh=true --port <samePort> ...
Perform a clean shutdown of the mongod instance:
copy
```
db.getSiblingDB('admin').shutdownServer()
```
Confirm that the process is no longer running.
Update the configuration file to prepare to restart with the new journal compressor:
- Remove the storage.journal.enabled setting.
- Remove the skipShardingConfigurationChecks parameter setting.
- Remove the disableLogicalSessionCacheRefresh parameter setting.
- Uncomment the replication settings for your deployment.
- Uncomment the sharding.clusterRole setting.
- Remove storage.wiredTiger.engineConfig.journalCompressor setting to use the default journal compressor or specify a new value.
For example:
copy
```
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.
Restart the mongod instance as a replica set member:
- If you are using a configuration file:
  copy
  mongod -f <path/to/myconfig.conf>
- If you are using command-line options instead of a configuration file:
  - Remove the --nojournal option.
  - Remove the skipShardingConfigurationChecks parameter setting.
  - Remove the disableLogicalSessionCacheRefresh parameter.
  - Remove the --wiredTigerJournalCompressor command-line option to use the default journal compressor or update to a new value.
  - Include --shardsvr/--configsvr option.
  - Include your replication command-line options as well as any additional options for your replica set member.
  copy
  mongod --shardsvr --wiredTigerJournalCompressor <differentCompressor|none> --replSet ...

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