Optimal prefetching and caching for parallel I/O sytems

Abstract

We address the problem of prefetching and caching in a parallel I/O system and present a new algorithm for optimal parallel-disk scheduling. Traditional buffer management algorithms that minimize the number of I/O disk accesses, are substantially suboptimal in a parallel I/O system where multiple I/Os can proceed simultaneously.We present a new algorithm SUPERVISOR for parallel-disk I/O scheduling. We show that in the off-line case, where apriori knowledge of all the requests is available, SUPERVISOR performs the minimum number of I/Os to service the given I/O requests. This is the first parallel I/O scheduling algorithm that is provably offline optimal. In the on-line case, we study SUPERVISOR in the context of global L-block lookahead, which gives the buffer management algorithm a lookahead consisting of L distinct requests. We show that the competitive ratio of SUPERVISOR, with global L-block lookahead, is T(M - L + D), when L ≤ M, and T(MD/L), when L > M, where the number of disks is D and buffer size is M.