Analytical models for evaluating the performance of arrayed disk subsystems in normal, degraded, and rebuild mode are developed. Models for estimating rebuild time under the assumption that user requests get priority and can preempt rebuild activity are also developed. It is found that uncached disk arrays perform much worse than uncached nonarrays. It is determined that caches are necessary in arrayed subsystems. Furthermore, in degraded and rebuild modes, large building blocks minimize the worst response time seen by any user, whereas small building blocks minimize the average response time.