Integrated prefetching and caching in single and parallel disk systems

We study integrated prefetching and caching in single and parallel disk systems. There exist two very popular approximation algorithms called Aggressive and Conservative for minimizing the total elapsed time in the single disk problem. For D parallel disks, approximation algorithms are known for both the elapsed time and stall time performance measures. In particular, there exists a D-approximation algorithm for the stall time measure that uses D-1 additional memory locations in cache. In the first part of the paper we investigate approximation algorithms for the single disk problem. We give a refined analysis of the Aggressive algorithm, showing that the original analysis was too pessimistic. 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 3(D - 1) extra memory locations in cache. This is the first polynomial time algorithm for computing schedules with a minimum stall time. Our algorithm is based on the linear programming approach of [1]. 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.