Disk Scheduling: FCFS, SSTF, SCAN & C-SCAN

Disk Scheduling Algorithm

Sweep direction

Head speed 60 cyl/s

Request Queue

Pending: —

Live Readouts

Head position (cylinder)

Serviced

0 / 0

Total head movement

0 cyl

Avg seek length

—

How it works

A hard-disk drive has one read/write head per surface, mounted on an arm that must physically move to the cylinder (track) holding the requested data before a transfer can start. That mechanical seek dominates disk latency, so the order in which pending I/O requests are serviced changes total wait time dramatically. The cost model is simple:

Total seek distance = Σ |C(i) − C(i−1)|
Average seek length = Total seek distance / number of requests

FCFS — services requests strictly in arrival order. Simple and fair, but the arm can zig-zag across the whole disk, maximizing total seek distance.
SSTF — always jumps to the closest pending cylinder. Minimizes seek distance greedily, but can starve far-away requests indefinitely.
SCAN (elevator) — sweeps in one direction servicing every request it passes, continues to the last cylinder, then reverses and sweeps back. Like a lift that visits every floor before changing direction.
C-SCAN — sweeps in one direction only; on reaching the end it jumps back to cylinder 0 without servicing on the return trip, then sweeps forward again. Produces a more uniform wait time across all cylinders than SCAN.
LOOK — like SCAN, but reverses as soon as the last request in the current direction is serviced, instead of travelling all the way to the disk's physical edge.

The graph above the track plots head position against service step — the classic "seek pattern" chart used to compare scheduling policies; its total up-and-down distance is exactly the total seek distance above.

Disk head Pending request Serviced request

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.