The risk in SSTF

A nightly backup on a mechanical disk has started overrunning its window. The disk has cylinders 0 to 199. At one sampled moment the head is at cylinder 100 and these requests are queued, in arrival order:

42, 187, 90, 175, 20, 140, 70, 15

The current driver serves them strictly in arrival order. The team is choosing a replacement policy and has computed the total head movement for each:

| Policy | Head movement | |---|---| | FCFS | 785 | | SSTF | 257 | | SCAN (to cylinder 199, then back) | 283 | | LOOK | 259 | | C-SCAN | 388 |

Seek time dominates on this workload, so head movement is a fair proxy for elapsed time.