Backups are generally performed on a regular schedule, such as daily. However, it takes a long time to backup large databases. In a 7 x 24 environment, you may not be able to stop your systems long enough to backup even small databases. Your availability and recovery plan should also contain measures to capture any data changes since the last backup. All major database management systems offer journaling functions that meet this need.
Power failures are the most common cause of abrupt system failures.
Disk mirroring can protect against these more serious failures. Often used in combination with RAID5, this approach requires that data be concurrently written to each unit in a set of identical disks, incurring minimal CPU overhead or increase in system complexity. However, to mirror all data you must buy twice as many disks. The fifth level of availability management offers a significantly greater protection since it includes software to automate all of the system administration tasks introduced by journaling and to alleviate any CPU overhead penalty. More importantly, even the best planned backup strategy and most complete DASD mirroring scheme cannot eliminate all types of user processing interruptions.