. In this paper we study three popular on-line disk scheduling algorithms, FCFS, SSTF, and LOOK, using competitive analysis. Our results show that, in a competitive sense, the performance of LOOK is better than those of SSTF and FCFS. As a by-product, our analysis also reveals quantitatively the role played by the size of the window, which in our model is a waiting buffer that holds a fixed number of requests waiting to be serviced next. The window, in some sense, offers the lookahead ability which is mentioned in several on-line problems. 1. Introduction Disk scheduling is a problem that is of practical importance and theoretical interest in the study of computer systems, in particular, in the areas of databases and operating systems. The goal of disk scheduling is to devise a policy for servicing disk requests in an on-line fashion so as to minimize the total disk access time. Traditionally, measuring the performance of a disk scheduling algorithm often resorts to either probabili..