In this work we address the problems of prefetching and I/O scheduling for read-once reference strings in a parallel I/O system. We use the standard parallel disk model with D disks a shared I/O bu er of sizeM. We design an on-line algorithm ASP (Adaptive Segmented Prefetching) with ML-block lookahead, L 1, and compare its performance to the best on-line algorithm with the same lookahead. We show that for any reference string the number of I/Os done by ASP is with a factor (C), C = minfpL;D1=3g, of the number of I/Os done by the optimal algorithm with the same amount of lookahead.