Disk access is slow (milliseconds) compared to CPU/RAM (nanoseconds). The mechanical seek time (moving the disk arm) dominates. With multiple I/O requests, the order of servicing them greatly affects performance.
Disk scheduling algorithms decide the order to service pending I/O requests to minimize seek time and improve throughput.
Common algorithms:
- FCFS: Process requests in arrival order (simple, but can be slow)
- SSTF: Pick closest request first (minimizes seek, but may starve)
- SCAN (Elevator): Move in one direction, service requests, reverse at end
- C-SCAN: Like SCAN but only services in one direction, jumps back
- LOOK/C-LOOK: Like SCAN/C-SCAN but stops at last request (not end of disk)
- SSTF reduces average seek but can starve distant requests
- SCAN gives uniform wait times (like elevator algorithm)
- C-SCAN provides more uniform wait than SCAN
- LOOK is more efficient (doesn’t go to disk end unnecessarily)
- Built from: Disk Structure, O System
- Builds into: Disk Management
- Related: FCFS, SSTF, SCAN, C-SCAN
- Contrasts with: Swap Space (different disk usage pattern)
- SSTF can cause starvation for requests at disk edges
- Request merging (adjacent sectors) can improve throughput
- Modern disks do their own scheduling (NCQ) — OS scheduling may be ignored