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.>

This paper explores how choice of sparing methods impacts the performance of RAID level 5 (or parity striped) disk arrays. The three sparing methods examined are dedicated sparing, distributed sparing, and parity sparing. For database type workloads with random single block reads and writes, array performance is compared in four different modes - normal mode (no disks have failed), degraded mode (a disk has failed and its data has not been reconstructed), rebuild mode (a disk has failed and its data is being reconstructed), and copyback mode(which is needed for distributed sparing and parity sparing when failed disks are replaced with new disks). Attention is concentrated on small disk subsystems (fewer than 32 disks) where choice of sparing method has significant impact on array performance, rather than large disk subsystems (64 or more disks). It is concluded that, for disk subsystems with a small number of disks, distributed sparing offers major advantages over dedicated sparing in normal, degraded and rebuild modes of operation, even if one has to pay a copyback penalty. Furthermore, it is better than parity sparing in rebuild mode and similar to it in other operating modes, making it the sparing method of choice.