With the best possible mirrored protection configuration, the system continues to run after a single disk-related hardware failure. On some system units, the failed hardware can sometimes be repaired or replaced without having to turn off the system. If the failing component is one that cannot be repaired while the system is running, such as a bus or an I/O processor, the system typically continues to run after the failure. Maintenance can be deferred, the system can be shut down normally, and a long recovery time can be avoided.
Even if your system is not a large one, mirrored protection can provide valuable protection. A disk or disk-related hardware failure on an unprotected system leaves your system unusable for several hours. The actual time depends on the kind of failure, the amount of disk storage, your backup strategy, the speed of your tape unit, and the type and amount of processing the system performs. If you or your enterprise cannot tolerate this loss of availability, you should consider mirrored protection for your system, regardless of your system's size.