Disk scheduling policies with lookahead

Abstract

Advances in magnetic recording technology have resulted in a rapid increase in disk capacities, but improvements in the mechanical characteristics of disks have been quite modest. For example the access time to random disk blocks has decreased by a mere factor of two, while disk capacities have increased by several orders of magnitude. High performance OLTP applications subject disks to a very demanding workload, since they require high access rates to randomly distributed disk blocks and gain limited benef£t from caching and prefetching. We address this problem by re-evaluating the performance of some well known disk scheduling methods, before proposing and evaluating extensions to them. A variation to CSCAN takes into account rotational latency, so that the service time of further requests is reduced. A variation to SATF considers the sum of service times of several successive requests in scheduling the next request, so that the arm is moved to a (temporal) neighborhood with many requests. The service time of further requests is discounted, since their immediate processing is not guaranteed. A variation to the SATF policy prioritizes reads with respect to writes and processes winner write requests conditionally, i.e., when the ratio of their service time to that of the winner read request is smaller than a certain threshold. We review previous work to put our work into the proper perspective and discuss plans for future work.