AN ECONOMICAL BACKUP WARNING STRATEGY FOR A HARD-DISK

A hard disk has, in recent years, become an essential component for a personal computer as well as a work station. A hard disk stores a vari ety of files, which are daily updated. The files in a hard disk are, h owever, lost because of human errors and a failure of the hard disk. T his is called a hard disk failure. In order to protect us from such a serious loss, it is important to make a backup copy of files on a magn etic tape or on several floppy disks. Frequent backup operations requi re much cost in backup operations themselves, while rare backup operat ions would make the loss at a hard disk failure very large. This indic ates that the backup timing should adequately be determined considerin g the above two factors. This study discusses an economical backup war ning strategy, which prescribes the time to give us a warning for a ba ckup operation at age T, where age refers to the elapsed time since th e previous backup operation or a recovery operation whichever occurred most recently. When a job accessing the hard disk is being processed at the warning time, the backup operation is conducted after the proce ss of the job is finished. The expected cost per unit time under this strategy is formulated as an objective function. The existence of an e conomical warning time that minimizes the expected cost is shown. Nume rical examples are also presented to illustrate the theoretical underp innings of the economical backup warning strategy formulation.