Reliability analysis of some duplex disk systems

Abstract

Disk duplexing has been a standard and most suitable method for increasing the reliability and availability of secondary storage of a computer system. This paper analyses three different configurations of duplex disk subsystems: duplex disks under the control of a common control unit; duplex control units with each disk of a duplex pair accessible from a different control unit; duplex control units with each disk accessible from either control unit. Failure time distributions of all units are assumed to be exponential whereas repair time distributions are general. In order to have a model which is as general as possible but computationally tractable, the repair time distributions are taken to be coxian of order 2, which allows close approximation of a broad class of repair time distributions. Closed form analytical expressions have been obtained for steady-state availability (SSA) and mean time to system failure (MTSF) and numerical illustrations are given to compare the three subsystems. These results should prove useful to computer system engineers and designers.