Priority queueing in RAID disk arrays with an NVS cache

We consider a cached RAID5 disk array which partially consists of non-volatile storage (NVS) providing a fast-write capability. Blocks written in this manner are destaged from the NVS cache onto disk asynchronously and at a lower priority than disk read requests. We consider three queueing disciplines to prioritize read misses with respect to destages: (i) Non-preemptive priorities. (ii) Preemptions only after completion of a seek (split-seek option). (iii) Preemption during the latency phase (split-latency option). It can be concluded from the simulation results for two workloads that unlike the FCFS policy, the increase in read response time for all three disciplines is quite moderate up to the point of full disk utilization. This improvement in read response time is at the cost of some wasted processing, but it is shown by applying the vacationing server model to the split-seek option that the reduction in the maximum throughput with respect to the FCFS policy is negligibly small