SCAN (elevator algorithm) in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does the SCAN (elevator) algorithm decide the direction of disk head movement?

The SCAN algorithm moves the disk head in one direction servicing requests until it reaches the end, then reverses direction. What determines the initial direction of the disk head movement?

A It starts moving in the direction of the closest pending request from the current head position.

B It always starts moving towards the lowest track number first.

C It moves randomly either towards the highest or lowest track number.

D It always starts moving towards the highest track number first.

💡 Hint

Think about how the algorithm minimizes seek time by choosing the nearest request direction.

The SCAN algorithm chooses the direction based on the closest pending request to minimize seek time, moving the head towards that direction first before reversing.

🚀 Application

intermediate

Calculate total head movement using SCAN algorithm

Given a disk with 200 tracks (0-199), the disk head is currently at track 50, and the pending requests are at tracks 20, 90, 150, and 180. If the head moves towards the higher track numbers first, what is the total head movement (in tracks) to service all requests using the SCAN algorithm?

A 280 tracks

B 230 tracks

C 260 tracks

D 290 tracks

💡 Hint

Remember the head moves up to the highest request or end, then reverses to service remaining requests.

The head moves from 50 to 180 (130 tracks), then reverses to 20 (160 tracks), totaling 290 tracks.

🔍 Analysis

advanced

Compare SCAN and LOOK algorithms in disk scheduling

Which of the following statements correctly describes a key difference between the SCAN and LOOK disk scheduling algorithms?

A LOOK moves the head to the end of the disk regardless of requests, while SCAN only moves as far as the last request in each direction.

B Both SCAN and LOOK always move the head to the end of the disk before reversing direction.

C SCAN moves the head to the end of the disk regardless of requests, while LOOK only moves as far as the last request in each direction.

D Neither SCAN nor LOOK reverses direction; they both move in a circular pattern.

💡 Hint

Consider how far the head travels in each algorithm relative to pending requests.

SCAN moves the head all the way to the disk's end before reversing, even if no requests exist there. LOOK moves only as far as the last request in the current direction before reversing.

❓ Reasoning

advanced

Why does SCAN algorithm reduce starvation compared to SSTF?

Starvation happens when some requests wait indefinitely. Why does the SCAN algorithm reduce starvation compared to the Shortest Seek Time First (SSTF) algorithm?

A Because SCAN always picks the closest request next, like SSTF, so starvation is the same.

B Because SCAN services requests in a fixed direction order, ensuring all requests are eventually reached.

C Because SCAN ignores requests at the ends of the disk, focusing only on the middle tracks.

D Because SCAN randomly selects requests to service, preventing starvation.

💡 Hint

Think about how SCAN moves the head systematically across the disk.

SCAN moves the head in one direction servicing all requests along the way, so no request is skipped indefinitely, reducing starvation compared to SSTF which can keep picking closer requests and ignore far ones.

📋 Factual

expert

Identify the output of SCAN algorithm with given requests and head position

Consider a disk with tracks numbered 0 to 199. The disk head is at track 100. Pending requests are at tracks 95, 180, 34, 119, and 11. The SCAN algorithm moves towards the lower track numbers first. What is the order in which the requests will be serviced?

A 95, 34, 11, 119, 180

B 95, 34, 11, 180, 119

C 11, 34, 95, 119, 180

D 180, 119, 95, 34, 11

💡 Hint

Remember SCAN moves in one direction servicing requests, then reverses.

The head moves down servicing 95, 34, 11, then reverses and moves up servicing 119, 180.

Practice

1. What is the main idea behind the SCAN (elevator) disk scheduling algorithm?

easy

A. Requests are served in the exact order they arrive without reordering.

B. The disk arm moves in one direction, servicing requests until it reaches the end, then reverses direction.

C. The disk arm jumps directly to the closest request regardless of direction.

D. The disk arm moves randomly to service requests.

Solution

1. Step 1: Understand SCAN movement

   SCAN moves the disk arm in one direction, serving all requests along the way until it reaches the last track in that direction.

2. Step 2: Direction reversal

   After reaching the end, the arm reverses direction and serves requests on the way back, similar to an elevator moving up and down.

3. Final Answer:

   The disk arm moves in one direction, servicing requests until it reaches the end, then reverses direction. -> Option B

4. Quick Check:

   SCAN = moves one way then reverses [OK]

Hint: Think of SCAN like an elevator moving up and down [OK]

Common Mistakes:

• Confusing SCAN with FCFS (first-come, first-served)
• Thinking SCAN jumps directly to closest request
• Assuming SCAN moves randomly

2. Which of the following correctly describes the order in which SCAN services disk requests?

easy

A. It services the closest request next, regardless of direction.

B. It services requests in the order they arrive without reordering.

C. It services requests by moving the arm in one direction, then reverses at the end.

D. It randomly selects requests to service next.

Solution

1. Step 1: Identify SCAN's servicing order

   SCAN moves the disk arm in one direction, servicing all requests in that path.

2. Step 2: Direction reversal at end

   When the arm reaches the last request or the disk end, it reverses direction and services requests on the return path.

3. Final Answer:

   It services requests by moving the arm in one direction, then reverses at the end. -> Option C

4. Quick Check:

   SCAN = one direction then reverse [OK]

Hint: Remember SCAN moves like an elevator, up then down [OK]

Common Mistakes:

• Confusing SCAN with SSTF (closest request first)
• Assuming SCAN ignores direction
• Thinking SCAN services requests randomly

3. Given the disk requests at tracks [10, 22, 20, 2, 40, 6] and the disk arm currently at track 15 moving towards higher tracks, which request will SCAN service first?

medium

A. Track 20

B. Track 10

C. Track 6

D. Track 2

Solution

1. Step 1: Identify current arm position and direction

   The arm is at track 15 and moving towards higher tracks (upwards).

2. Step 2: Find the closest request in the moving direction

   Requests above 15 are 20, 22, and 40. The closest is track 20.

3. Final Answer:

   Track 20 -> Option A

4. Quick Check:

   SCAN services requests ahead in current direction first [OK]

Hint: Pick closest request ahead in current direction [OK]

Common Mistakes:

• Choosing requests behind current arm position
• Ignoring arm's moving direction
• Picking the closest request overall instead of in direction

4. A disk arm is at track 50 moving towards track 0. The requests are at tracks [10, 20, 30, 40, 60]. Which request will SCAN service next? Identify the error in this reasoning: "Since the arm is moving towards 0, it will service track 60 next because it is the closest."

medium

A. The arm will service track 60 next; no error in reasoning.

B. The arm will service track 20 next; the error is ignoring request order.

C. The arm will service track 10 next; the error is assuming direction matters.

D. The arm will service track 40 next; the error is ignoring the arm's direction.

Solution

1. Step 1: Understand arm direction and position

   The arm is at track 50 moving towards 0 (downwards).

2. Step 2: Identify requests in arm's direction

   Requests below 50 are 10, 20, 30, 40; above 50 is 60.

3. Step 3: Find next request in direction

   The closest request in the moving direction is track 40, not 60.

4. Final Answer:

   The arm will service track 40 next; the error is ignoring the arm's direction. -> Option D

5. Quick Check:

   SCAN respects arm direction when choosing requests [OK]

Hint: Always consider arm direction before picking requests [OK]

Common Mistakes:

• Ignoring arm direction and picking closest request overall
• Assuming SCAN jumps to farthest request
• Confusing SCAN with SSTF

5. A disk arm is at track 50 on a disk with tracks numbered 0 to 199. Requests are at tracks [95, 180, 34, 119, 11, 123, 62, 64]. The arm is moving towards higher tracks. Using the SCAN algorithm, what is the order in which the requests will be serviced?

hard

A. [62, 64, 95, 119, 123, 180, 199, 34, 11]

B. [62, 64, 95, 119, 123, 180, 34, 11]

C. [95, 119, 123, 180, 199, 64, 62, 34, 11]

D. [95, 119, 123, 180, 64, 62, 34, 11]

Solution

1. Step 1: Identify requests above current arm position

   Arm at 50 moving up; requests above 50 are 62, 64, 95, 119, 123, 180.

2. Step 2: Service requests upwards in order

   Requests are serviced in ascending order: 62, 64, 95, 119, 123, 180.

3. Step 3: Reach end of disk and reverse

   SCAN continues to the last track (199) before reversing direction.

4. Step 4: Service remaining requests on return

   On return, service requests below 50 in descending order: 34, then 11.

5. Final Answer:

   [62, 64, 95, 119, 123, 180, 199, 34, 11] -> Option A

6. Quick Check:

   SCAN = up servicing, end, then down servicing [OK]

Hint: Service all requests up, go to end, then service down [OK]

Common Mistakes:

• Skipping the last track before reversing
• Not servicing requests in sorted order
• Ignoring requests on return path