Survey and analysis of disk scheduling methods

Abstract

Performance of many important computer applications depends on the performance of Hard Disk Drives (HDDs) . Disk capacities and transfer rates have been increasing rapidly, but the improvement in disk access time is disappointingly slow. Caching and prefetching are two method to alleviate this delay, which is 6-7 orders of magnitude longer than the processor cycle time. Disk scheduling is desirable when the data is not cached and a disk access is required. This paper is concerned with the analysis of two disk arm scheduling methods: SATF (shortest access time first) which outperforms SCAN, while both methods outperform FCFS scheduling. We propose improvements to a recent analysis of the SCAN policy and carry out an empirical investigation of SATF performance to derive a relationship between the queue-length and mean service time. A review of variations of SCAN and SATF is provided, since they have been utilized in conjunction with multilevel disk scheduling methods. We also discuss recent developments to improve the performance of high capacity HDDs, which allow multiple tracks to be accessed without incurring seeks.