C-SCAN (circular SCAN) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does C-SCAN stand for in disk scheduling?

C-SCAN stands for Circular SCAN. It is a disk scheduling algorithm that moves the disk arm in one direction servicing requests, then quickly returns to the start without servicing requests on the return.

intermediate

How does C-SCAN differ from the SCAN disk scheduling algorithm?

Unlike SCAN, which moves the disk arm back and forth servicing requests in both directions, C-SCAN only services requests in one direction and jumps back to the beginning without servicing on the return, providing a more uniform wait time.

intermediate

Why is C-SCAN considered fairer than SCAN for servicing disk requests?

C-SCAN treats the disk as a circular list and services requests in one direction only, so all requests get a more uniform wait time because the arm does not service requests on the return trip, avoiding starvation of requests at the edges.

beginner

Describe the movement of the disk arm in C-SCAN scheduling.

The disk arm moves from the lowest to the highest track servicing requests along the way. After reaching the highest track, it quickly returns to the lowest track without servicing any requests during the return, then repeats the process.

beginner

What is a key advantage of using C-SCAN over FCFS (First-Come, First-Served) in disk scheduling?

C-SCAN reduces the average waiting time and provides more predictable service by moving the disk arm in a single direction and avoiding long delays caused by random request order in FCFS.

In C-SCAN, what happens when the disk arm reaches the highest track?

A It reverses direction and services requests on the way back.

B It services requests randomly.

C It stops until new requests arrive.

D It quickly returns to the lowest track without servicing requests.

In C-SCAN, the disk arm moves in one direction and, upon reaching the highest track, jumps back to the lowest track without servicing requests during the return.

Which of the following is a benefit of C-SCAN over SCAN?

A It ignores requests at the edges.

B It services requests in both directions.

C It provides a more uniform wait time for requests.

D It always services the closest request first.

C-SCAN provides a more uniform wait time by servicing requests in one direction only and jumping back without servicing on the return.

What type of scheduling algorithm is C-SCAN?

A CPU scheduling algorithm

B Disk scheduling algorithm

C Memory management algorithm

D Network scheduling algorithm

C-SCAN is a disk scheduling algorithm used to decide the order of servicing disk I/O requests.

How does C-SCAN treat the disk tracks conceptually?

A As a circular list where the arm moves in one direction only.

B As a stack with last-in, first-out order.

C As random points with no order.

D As a linear list with back and forth movement.

C-SCAN treats disk tracks as a circular list, moving the arm in one direction and jumping back to the start.

Which problem does C-SCAN help to reduce compared to FCFS scheduling?

A Long average waiting time due to random request order

B Disk arm starvation

C CPU overload

D Memory fragmentation

C-SCAN reduces long average waiting times caused by random request order in FCFS by servicing requests in a structured, circular manner.

Explain how the C-SCAN disk scheduling algorithm works and why it is used.

Think about how the disk arm moves and how requests are handled in one direction only.

Compare C-SCAN with SCAN disk scheduling in terms of disk arm movement and fairness.

Focus on the direction of movement and how requests are serviced.

Practice

1. What is the main characteristic of the C-SCAN disk scheduling algorithm?

easy

A. The disk arm moves back and forth serving requests in both directions.

B. The disk arm moves in one direction and jumps back to the start without serving requests on return.

C. The disk arm serves requests randomly without any specific order.

D. The disk arm serves only the nearest request to its current position.

Solution

1. Step 1: Understand C-SCAN movement

   C-SCAN moves the disk arm in one direction only, serving requests along the way.

2. Step 2: Identify behavior at the end of the disk

   After reaching the last track, it jumps back to the first track without serving requests during the return.

3. Final Answer:

   The disk arm moves in one direction and jumps back to the start without serving requests on return. -> Option B

4. Quick Check:

   C-SCAN moves one way and jumps back = A [OK]

Hint: Remember: C-SCAN moves one way, then jumps back [OK]

Common Mistakes:

• Thinking C-SCAN moves back and forth like SCAN
• Assuming requests are served during the return jump
• Confusing C-SCAN with random or nearest request scheduling

2. Which of the following is the correct way to describe the jump in C-SCAN?

easy

A. The disk arm jumps back to the first track without serving any requests during the jump.

B. The disk arm stops and waits for new requests before moving.

C. The disk arm reverses direction and serves requests immediately.

D. The disk arm jumps back to the first track and serves requests on the way.

Solution

1. Step 1: Analyze the jump behavior in C-SCAN

   C-SCAN jumps back to the first track after reaching the last track.

2. Step 2: Check if requests are served during the jump

   Requests are not served during this jump; the arm moves quickly without processing.

3. Final Answer:

   The disk arm jumps back to the first track without serving any requests during the jump. -> Option A

4. Quick Check:

   Jump without serving requests = D [OK]

Hint: Jump skips requests, no service during return [OK]

Common Mistakes:

• Assuming requests are served during the jump
• Confusing jump with reversing direction
• Thinking the arm waits before jumping

3. Consider a disk with tracks numbered 0 to 199. The disk arm is currently at track 50 and requests are at tracks 95, 180, 34, and 119. Using C-SCAN, which track will the arm serve immediately after 50?

medium

A. 95

B. 34

C. 119

D. 180

Solution

1. Step 1: Identify requests ahead of current position

   Requests are at 34, 95, 119, 180. Current position is 50. C-SCAN moves forward only, so it serves requests with track numbers greater than 50 first: 95, 119, 180.

2. Step 2: Find the nearest request ahead

   The nearest request ahead is at track 95, so it will be served immediately after 50.

3. Final Answer:

   95 -> Option A

4. Quick Check:

   Next request after 50 going forward = 95 [OK]

Hint: Serve next higher track number after current position [OK]

Common Mistakes:

• Serving requests behind current position before forward ones
• Choosing the closest track number regardless of direction
• Confusing C-SCAN with SCAN which moves back and forth

4. A student wrote that in C-SCAN, the disk arm serves requests while moving back to the first track after reaching the last track. What is wrong with this statement?

medium

A. C-SCAN does not move back to the first track; it reverses direction instead.

B. C-SCAN serves requests randomly, so direction does not matter.

C. C-SCAN serves requests only while moving forward, not during the jump back.

D. C-SCAN serves requests only at the last track.

Solution

1. Step 1: Review C-SCAN jump behavior

   C-SCAN moves forward serving requests, then jumps back to the first track without serving requests during the jump.

2. Step 2: Identify the error in the student's statement

   The student incorrectly said requests are served during the jump back, which is false.

3. Final Answer:

   C-SCAN serves requests only while moving forward, not during the jump back. -> Option C

4. Quick Check:

   Requests served only forward, not on jump = C [OK]

Hint: Requests served only forward, jump is skip [OK]

Common Mistakes:

• Thinking C-SCAN reverses direction instead of jumping
• Assuming requests are served during the jump
• Confusing C-SCAN with SCAN algorithm

5. Given a disk with tracks 0 to 199, the disk arm is at track 150. Requests are at tracks 10, 170, 15, 190, and 30. Using C-SCAN, what is the order in which the disk arm will serve these requests?

hard

A. 30, 15, 10, 190, 170

B. 10, 15, 30, 170, 190

C. 190, 170, 30, 15, 10

D. 170, 190, 10, 15, 30

Solution

1. Step 1: Identify requests ahead of current position 150

   Requests ahead are 170 and 190. C-SCAN serves these first moving forward.

2. Step 2: After reaching last track, jump to start and serve remaining requests

   After 190, the arm jumps to track 0 and serves requests at 10, 15, and 30 in ascending order.

3. Final Answer:

   170, 190, 10, 15, 30 -> Option D

4. Quick Check:

   Serve forward requests, jump, then serve from start = A [OK]

Hint: Serve forward requests first, then jump and serve from start [OK]

Common Mistakes:

• Serving requests behind current position before forward ones
• Serving requests in descending order
• Ignoring the jump and serving requests in wrong order