LOOK Disk Scheduling

🔷 What is LOOK?

LOOK is an improved version of SCAN.

👉 Key idea:

• The disk head does NOT go to the end of the disk

• It only goes as far as the last request in that direction, then reverses

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1. How LOOK Works

🔷 Steps

1. Start at current head position

2. Move in a chosen direction

3. Service all requests in that direction

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

5. Reverse direction

6. Continue servicing remaining requests

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2. Example 

🔹 Given:

• Request queue:

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

• Initial head position:

  53
  

• Direction:
  👉 Moving toward 0 (same as SCAN example)

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🔷 Step-by-Step Execution

Step 1: Move Left (toward 0)

Requests less than 53:

37, 14

Service in descending order:

53 → 37 → 14

👉 Stop at 14 (last request in this direction)
❌ Do NOT go to 0

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Step 2: Reverse Direction

Now move right and service remaining requests:

14 → 65 → 67 → 98 → 122 → 124 → 183

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🔷 Final Order of Service

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

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 3. Total Head Movement

53→37 = 16  
37→14 = 23  
14→65 = 51  
65→67 = 2  
67→98 = 31  
98→122 = 24  
122→124 = 2  
124→183 = 59  

👉 Total = 208 cylinders

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4. Advantages

✅ 1. Less Movement than SCAN

• Avoids unnecessary travel to disk ends

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✅ 2. Better Performance

• Reduces seek time

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✅ 3. Fairness

• No starvation (like SCAN)

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 5. Disadvantages

🔴 1. Slight Waiting Issues

• Requests just behind head may wait longer

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6. Key Insight

👉 Difference from SCAN:

• SCAN → goes to disk end (0 or max)

• LOOK → stops at last request

👉 Result:

• Saves time

• Improves efficiency

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7. Comparison Snapshot

Algorithm	Total Movement
FCFS	640
SSTF	236
SCAN	(higher than LOOK)
LOOK	208

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8. Simple Analogy

• SCAN → Elevator goes to top floor even if no one is there

• LOOK → Elevator stops at highest requested floor and turns back

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9. Key Takeaways

• LOOK = optimized SCAN

• Avoids unnecessary movement

• Improves efficiency

• Widely used in practice