Scheduling Methods for Disk Requests

Abstract

Disk scheduling changes the sequence order to serve the requests that are waiting in the queue. While data placement reduces the access time of a disk request, scheduling reduces the waiting time of a request. Thus, the response time is found as: Response time = Waiting time + Access time The longer the waiting queue, the more useful is the scheduling method. When there is no waiting queue, any scheduling methods perform the same. Expected waiting time and queue length can be found using queueing theory. The queueing theory is out of the scope of this book. In general, a disk scheduling policy changes the service order of waiting requests. A disk scheduling policy accepts the waiting requests and serves them in the new service sequence. Notice that the service sequence may or may not be the same as the incoming order of the waiting requests. In this chapter, we shall describe two common disk scheduling methods. First, we shall describe the simple first-in-first-out method. After that, we shall describe the efficient SCAN algorithm in the following sections.