MongoDB uses write ahead logging to an on-disk journal to guarantee write operation durability and to provide crash resiliency. Before applying a change to the data files, MongoDB writes the change operation to the journal. If MongoDB should terminate or encounter an error before it can write the changes from the journal to the data files, MongoDB can re-apply the write operation and maintain a consistent state.
With journaling enabled, if mongod stops unexpectedly, the program can recover everything written to the journal, and the data remains in 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. See commitIntervalMs for more information on the default.
With journaling, if you want a data set to reside entirely in RAM, you need enough RAM to hold the data set plus the “write working set.” The “write working set” is the amount of unique data you expect to see written between re-mappings of the private view. For information on views, see Storage Views used in Journaling.
Changed in version 2.0: For 64-bit builds of mongod, journaling is enabled by default.
To enable journaling, start mongod with the --journal command line option.
If no journal files exist, when mongod starts, it must preallocate new journal files. During this operation, the mongod is not listening for connections until preallocation completes: for some systems this may take a several minutes. During this period your applications and the mongo shell are not available.
Do not disable journaling on production systems. 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¶
Avoid Preallocation Lag¶
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.
Create a temporary directory into which to create a set of journal files:
Create a set of journal files by staring a mongod instance that uses the temporary directory:
mongod --port 10000 --dbpath ~/tmpDbpath --journal
[initandlisten] waiting for connections on port 10000
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/
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:
The serverStatus command returns database status information that is useful for assessing performance.
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¶
Changed in version 2.0.
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.