Journaling is a feature that helps protect the file system against power outages or hardware component failures, reducing the need for directory repairs. This document explains some of the benefits of using this feature and how it works.
Journaling for the Mac OS Extended (HFS Plus) file system enhances computer availability and fault resilience, which is especially noteworthy for servers. Journaling protects the integrity of the file system on Xserve and other computers using Mac OS X Server in the event of an unplanned shutdown or power failure. It also helps to maximize the uptime of servers and connected storage devices by expediting repairs to the affected volumes when the system restarts.
When you enable journaling on a disk, a continuous record of changes to files on the disk is maintained in the journal. If your computer stops because of a power failure or some other issue, the journal is used to restore the disk to a known-good state when the server restarts.
Although you may experience loss of user data that was buffered at the time of the failure, the file system is returned to a consistent state. In addition, restarting the computer is much faster. Always remember to back up your data as frequently as necessary.