Disk Scheduling
Disk access time = seek time + rotational latency + transfer time. Disk-scheduling algorithms try to minimise seek time (head movement).
Request queue: 98, 183, 37, 122, 14, 124, 65, 67; head at 53; cylinders 0–199.
| Algorithm | Order served | Total head movement |
|---|---|---|
| FCFS | 98, 183, 37, 122, 14, 124, 65, 67 | 640 |
| SSTF | 65, 67, 37, 14, 98, 122, 124, 183 | 236 |
| SCAN (toward 0) | 37, 14, [0], 65, 67, 98, 122, 124, 183 | 236 |
| C-SCAN (toward 199) | 65, 67, 98, 122, 124, 183, [199, 0], 14, 37 | 382* |
| LOOK (toward 0) | 37, 14, 65, 67, 98, 122, 124, 183 | 208 |
*Includes the 199 → 0 return sweep. Some textbooks exclude it, giving 183.
- SSTF can starve requests far from the head.
- SCAN (elevator) gives more uniform waiting times.
- C-SCAN treats the cylinders as a circular list.
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