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.
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.