| 摘要 |
PURPOSE:To realize the fault tolerance at a low cost by accumulating the use frequency of an auxiliary storage device, and outputting the information for requesting the backup of the auxiliary storage device, when the accumulation use frequency reaches a reference determined in advance. CONSTITUTION:By considering the cost per single backup and the cost for release at the time when an access disk is damaged, a cost function is formulated, and a backup time when the cost becomes lower is derived. That is, when a time interval for taking a backup, and the time until the access disk is damaged after taking the backup are denoted as T and X, respectively, the cost per unit time becomes as shown in the expressions I, II. In this regard, (a), C1 and C2 denote the data capacity which can be released per unit time, the cost required for release per one unit data capacity, and the cost per single backup, respectively. Accordingly, as for the expected cost per unit time, it will suffice that expected values of the expression I and II are calculated. It becomes a function of the time T, and the time when the cost becomes lower can be derived. In such a way, the fault tolerance can be realized at a low cost. |
