Improving parallel-disk buffer management using randomized writeback

We address the problems of I/O scheduling and buffer management for general reference strings in a parallel I/O system. Using the standard parallel disk model with D disks and a shared I/O buffer of size M, we study the performance of online algorithms that use bounded global M-block lookahead. We introduce the concept of write-back whereby blocks are dynamically relocated between disks during the course of the computation. Write-back allows the layout to be altered to suit different access patterns in different parts of the reference string. We show that any bounded-lookahead online algorithm that uses purely deterministic policies must have a competitive ratio of Ω (D). We show how to improve the performance by using randomization, and present a novel algorithm, RAND-WB, using a randomized write-back scheme. RAND-WB has a competitive ratio of θ(√D), which is the best achievable by any online algorithm with only global M-block lookahead. If the initial layout of data on the disks is uniformly random, RAND-WB has a competitive ratio of θ(log D)