Andrew S. Tanenbaum (OS) Edition 4 Exercise 5 Question 32 (Page No. 432)

Question

A slight modification of the elevator algorithm for scheduling disk requests is to always scan in the same direction. In what respect is this modified algorithm better than the elevator algorithm?

Answer 1

This modified algorithm is know as “Circular SCAN (CSCAN)” algorithm. Like SCAN, C-SCAN moves the head from one end of the disk to the other, servicing requests along the way. When the head reaches the other end, however, it immediately returns to the beginning of the disk without servicing any requests on the return trip.(Like TypeWriter)

Benefit:

It provides a more uniform waiting time.

How?

Visualize – Keeping this thing in mind: ( Treats the cylinders as a circular list – that wraps around from the last cylinder to the first one )