An exact analysis on expected seeks in shadowed disks

Abstract

Shadowed disk systems store identical data. Therefore, reads are satisfied by using the “nearer server rule” to choose the appropriate disk, whereas writes are satisfied by all disks. Such systems have been studied in order to derive approximate formulae for the expected seek distances traveled for reads and writes. In the present paper, earlier analytic models are studied again, under a different perspective which takes into account the disk scheduling policy applied and the total number of cylinders per disk. Our analysis is exact and results in new formulae for the expected seek distances traveled for reads and writes. The deviation of the earlier formulae with respect to the new exact formulae decrements with decreasing number of shadowed disks, increasing number of disk cylinders, as well as decreasing ration of reads vs. writes. Thus, it is verified that earlier results could be used as good approximations in the asymptotic case.