C-SCAN Disk Scheduling (Circular SCAN)

🔷 What is C-SCAN?

C-SCAN (Circular SCAN) is a variation of the SCAN algorithm designed to provide more uniform waiting time.

👉 Key idea:

• The disk head moves in one direction only

• After reaching the end, it jumps back to the beginning

• No requests are serviced during the return

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1. How C-SCAN Works

🔷 Step-by-Step

1. Head starts at current position

2. Moves in one direction (e.g., toward higher cylinders)

3. Services all requests along the way

4. Reaches the end of the disk

5. Immediately jumps back to the start (0)

6. Continues servicing 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 from 0 → 199

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🔷 Step-by-Step Movement

Move Forward (servicing requests ≥ 53):

53 → 65 → 67 → 98 → 122 → 124 → 183 → 199

Jump to Beginning:

199 → 0   (no servicing)

Continue Forward:

0 → 14 → 37

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🔷 Final Order of Service

65 → 67 → 98 → 122 → 124 → 183 → (jump) → 14 → 37

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3. Key Characteristics

    1. Uniform Waiting Time

• All requests are treated more equally

• Avoids bias toward middle cylinders

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     2. Single Direction Movement

• Head moves in only one direction

• More predictable performance

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     3. Circular Behavior

• Disk is treated like a circular queue

• End connects back to beginning

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4. Limitations

     1. Extra Movement

• Head travels from end → beginning without servicing

• Adds overhead

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    2. Slightly Less Efficient than SCAN

• Due to jump movement

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5. Key Insight

👉 Unlike SCAN:

• SCAN → services requests in both directions

• C-SCAN → services in one direction only

👉 Result:

• More uniform waiting time

• Less variance in response time

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6. Comparison: SCAN vs C-SCAN

Feature	SCAN	C-SCAN
Movement	Back and forth	One direction only
Return path	Services requests	No servicing
Waiting time	Uneven	More uniform
Complexity	Moderate	Moderate

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7. Key Takeaways 

• C-SCAN = Circular version of SCAN

• Head moves in one direction only

• Jumps back to start without servicing

• Provides uniform wait time

• Slight overhead due to jump