C-LOOK Disk Scheduling (Circular LOOK)

🔷 What is C-LOOK?

C-LOOK is an improved version of C-SCAN.

👉 Key idea:

• Head moves in one direction only

• Services requests only up to the last request

• Then jumps to the first request (not to disk end)

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1. How C-LOOK Works

🔷 Steps

1. Start at current head position

2. Move in one direction (e.g., increasing cylinders)

3. Service all requests in that direction

4. Stop at the last request (not end of disk)

5. Jump to the lowest request

6. Continue in the same direction

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2. Example 

🔹 Given:

• Request queue:

  98, 183, 37, 122, 14, 124, 65, 67
  

• Initial head position:

  53
  

• Direction:
  👉 Moving toward higher cylinders

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🔷 Step-by-Step Execution

Step 1: Move Forward (≥ 53)

Requests:

65, 67, 98, 122, 124, 183

Service in order:

53 → 65 → 67 → 98 → 122 → 124 → 183

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Step 2: Jump to Lowest Request

Remaining:

14, 37

Jump:

183 → 14   (no servicing in between)

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Step 3: Continue Forward

14 → 37

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🔷 Final Order of Service

53 → 65 → 67 → 98 → 122 → 124 → 183 → 14 → 37

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3. Total Head Movement

53→65 = 12  
65→67 = 2  
67→98 = 31  
98→122 = 24  
122→124 = 2  
124→183 = 59  
183→14 = 169  
14→37 = 23  

👉 Total = 322 cylinders

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 4. Advantages

    1. Uniform Waiting Time

• Similar to C-SCAN

• Fair distribution of service

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    2. Less Unnecessary Movement

• Does not go to disk ends

• Better than C-SCAN

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     3. Predictable Behavior

• Always moves in one direction

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5. Disadvantages

    1. Jump Cost

• Large jump from last → first request

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    2. Slightly Higher Movement than LOOK

• Due to circular jump

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6. Key Insight

👉 Differences:

• SCAN → goes to disk end

• LOOK → stops at last request

• C-SCAN → circular, goes to disk end

• C-LOOK → circular, stops at last request

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7. Comparison Snapshot

Algorithm	Movement Style	Efficiency
SCAN	Back & forth to ends	Good
LOOK	Back & forth to last request	Better
C-SCAN	One direction to ends	Uniform
C-LOOK	One direction to last request	Efficient + uniform

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8. Simple Analogy

• C-SCAN → Elevator goes to top floor, then returns to ground floor without stopping

• C-LOOK → Elevator goes only to highest requested floor, then jumps to lowest request

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9. Key Takeaways

• C-LOOK = optimized C-SCAN

• Avoids unnecessary travel to disk ends

• Provides uniform waiting time

• Widely used in modern systems