Comparison of Disk Scheduling Algorithms

September 12, 2025

Disk Scheduling Algorithms Comparison

🔷 Given Example (Common for All)

Request queue:
98, 183, 37, 122, 14, 124, 65, 67
Initial head position: 53

1. Summary Table

Algorithm	Order Strategy	Total Head Movement	Advantages	Disadvantages
FCFS	Arrival order	640	Simple, fair	Very high seek time
SSTF	Closest request first	236	Good performance	Starvation possible
SCAN	Move to end, reverse	331	Fair, better than FCFS	Goes to disk ends unnecessarily
C-SCAN	One direction, circular	382	Uniform wait time	Extra jump cost
LOOK	SCAN but stops at last request	299	Efficient, less movement	Slight waiting issues
C-LOOK	Circular LOOK	322	Uniform + avoids ends	Jump overhead

2. Key Differences

🔴 FCFS (First-Come First-Served)

Processes requests in arrival order
❌ Large head movement
✔️ Simple and fair

🟡 SSTF (Shortest Seek Time First)

Chooses nearest request
✔️ Reduces seek time
❌ May cause starvation

🔵 SCAN (Elevator Algorithm)

Moves in one direction → reverses
✔️ Balanced performance
❌ Goes to disk ends even if unnecessary

🟣 C-SCAN (Circular SCAN)

Moves in one direction only
✔️ Uniform waiting time
❌ Extra movement (jump to start)

🟢 LOOK

Like SCAN but stops at last request
✔️ Avoids unnecessary movement
✔️ More efficient than SCAN

🟠 C-LOOK

Like C-SCAN but stops at last request
✔️ Uniform waiting + efficiency
❌ Jump overhead still exists

3. Performance Insight

🔷 Best for Performance (Low Seek Time)

👉 LOOK / SSTF

🔷 Best for Fairness

👉 SCAN / C-SCAN / C-LOOK

🔷 Best for Simplicity

👉 FCFS

4. Key Trade-offs

Goal	Best Algorithm
Minimum seek time	SSTF
Avoid starvation	SCAN / LOOK
Uniform wait time	C-SCAN / C-LOOK
Simplicity	FCFS

5. Conceptual Understanding

👉 Disk scheduling is about trade-offs:

Performance vs Fairness
Efficiency vs Simplicity

6. Final Takeaways

FCFS → simple but inefficient
SSTF → fast but unfair
SCAN → balanced
LOOK → optimized SCAN
C-SCAN → uniform waiting
C-LOOK → best balance of uniformity + efficiency

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