Integrated prefetching and caching in single and parallel disk systems

We study integrated prefetching and caching in single and parallel disk systems. In the first part of the paper we investigate approximation algorithms for the single disk problem. There exist two very popular approximation algorithms called Aggressive and Conservative for minimizing the total elapsed time. We give a refined analysis of the Aggressive algorithm, improving the original analysis by Cao et al. We prove that our new bound is tight. Additionally we present a new family of prefetching and caching strategies and give algorithms that perform better than Aggressive and Conservative. In the second part of the paper we investigate the problem of minimizing stall time in parallel disk systems. We present a polynomial time algorithm for computing a prefetching/caching schedule whose stall time is bounded by that of an optimal solution. The schedule uses at most 2(D-1) extra memory locations in cache. This is the first polynomial time algorithm that, using a small amount of extra resources, computes schedules whose stall times are bounded by that of optimal schedules not using extra resources. Our algorithm is based on the linear programming approach of [Journal of the ACM, 47 (2000) 969]. However, in order to achieve minimum stall times, we introduce the new concept of synchronized schedules in which fetches on the D disks are performed completely in parallel.