SSTF Disk Scheduling (Shortest Seek Time First)

🔷 What is SSTF?

SSTF selects the disk request that is closest to the current head position.

👉 Key idea:

• Always service the request with minimum seek distance

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1. How SSTF Works

🔷 Step-by-Step

1. Start at current head position

2. Find the request nearest to current position

3. Move head to that request

4. Repeat until all requests are served

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2. Example 

🔹 Given:

• Request queue:

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

• Initial head position:

  53
  

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🔷 Step-by-Step Execution

Step 1: From 53

Closest request:

• 65 (distance = 12)

• 67 (14)

• 37 (16)

👉 Choose 65

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Step 2: From 65

Remaining:

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

Closest:

• 67 (2)

👉 Choose 67

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Step 3: From 67

Closest:

• 37 (30)

• 98 (31)

👉 Choose 37

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Step 4: From 37

Closest:

• 14 (23)

👉 Choose 14

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Step 5: From 14

Closest:

• 98 (84)

• 122 (108)

👉 Choose 98

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Step 6: From 98

Closest:

• 122 (24)

• 124 (26)

👉 Choose 122

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Step 7: From 122

Closest:

• 124 (2)

👉 Choose 124

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Step 8: From 124

Remaining:

• 183

👉 Choose 183

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🔷 Final Order of Service

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

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3. Total Head Movement

53→65 = 12  
65→67 = 2  
67→37 = 30  
37→14 = 23  
14→98 = 84  
98→122 = 24  
122→124 = 2  
124→183 = 59  

👉 Total = 236 cylinders

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4. Advantages

✅ 1. Reduced Seek Time

• Much less movement than FCFS (640 → 236)

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✅ 2. Better Performance

• Faster response time

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5. Disadvantages

🔴 1. Starvation

• Requests far from head may wait indefinitely

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🔴 2. Not Fair

• Favors nearby requests

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6. Key Insight

👉 SSTF is like Shortest Job First (SJF) in CPU scheduling:

• Always picks nearest request

• Optimizes performance but may ignore distant requests

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7. Comparison Snapshot

Algorithm	Total Movement	Efficiency
FCFS	640	Poor
SSTF	236	Much better

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8. Simple Analogy

• Like a delivery person:

  • Always goes to the nearest house next

  • Saves fuel, but far houses wait longer

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9. Key Takeaway

• SSTF selects request with minimum seek time

• Improves performance significantly

• May cause starvation