Journaling Volume is a feature that protects your files and the file system against the system crash, power outages, virus infection and other unexpected situations. Journaling was first introduced in Mac OS X Server 10.2.2.
Journaling in HFS + is done to enhance the availability of the computer and the fault resilience feature of the file system. It even protects the integrity of your file system on the servers (like Xserve or other servers that use Mac OS X) even when the system is shutdown or crashed. With journaled Volumes, you can even maximize the uptime of the servers and also the connected devices. In case of any disastrous situations, it prevents the disk form getting into the inconsistent state and also reduces the need for disk repair even if the server fails.
How is it done?
When you enable journal on a disk or volume, a special file called Journal file will be created. This Journal File is a special type of file used in HFS + that stores continuous updates of the files, directories and bitmaps constantly before it has been sent to the original file system.
In the event of system crash, power failure or other types of hardware component failures, using this journal file the server automatically tracks all the file system operations and restores the disk up to the last updated point i.e. to a good, consistent state even after a failure. Also the file system logs all the transactions continuously, hence in case if the server fails while accomplishing any operation, you no need to worry. As the file system can easily “replay” the information that is stored in its log and the operation will be completed when the server is restarted.
Why is journaling needed?
Failure or sudden system shut down will interrupt the read and write processes that causes incompatibility between the directory of a file system and the location and structure of the stored files.
So, if the volume is not journaled, after failure the file system will be in an unknown state i.e. there will be no traces of the previous updates. In such situations, the server will perform the consistency check, which has to go through the entire file system, block by block and reconnect all the files and file system before restarting the server and resuming the services. This process may take hours of your time depending on the amount of the data stored on the hard drive.
But, if you have journaled the volumes, the even in case of failure OS can trace the files with the help of journal file, that stores the complete information until the last update. Thus, there is no need to perform the consistency check during start up. And when the server is restarted, Mac OS X just replays the transactions that were stored in the journal and resumes all the operations that were interrupted. It also recovers all the unsaved data and saves them in their original location, where they were stored before crash.
However, with journaled volume, the file system will be returned to the consistent state and also the restarting the computer is much faster. The updates or transaction details will be saved until the last scheduled time; hence there might be loss of data
that were stored till the time of failure. In such cases you can just make use of Remo Recover Mac and reclaim your missing files easily. Always remember to back up your data as frequently as possible.
How to enable and disable journaling in Mac OS X?
In order to turn ON or OFF the journaling feature, you need to use Disk Utility and follow these:
- Open the Disk Utility; Go to Applications and then Utilities
- Select the Volume to be Journal enabled or disabled
- Then, if you want to enable then, click on the Enable Journaling button from the File menu
- In case you want to disable the journaling, select the Disable Journaling tab from the File menu
Note: In Mac OS X 10.4 and later, you need to press Option to make Disable Journaling visible in the File menu.