SCAN (elevator algorithm) in Operating Systems - Mini Project: Build & Apply

SCAN (Elevator Algorithm) Step-by-Step

📖 Scenario: You are managing a disk drive that needs to service several read/write requests at different track positions. To reduce the total movement of the disk arm, you will simulate the SCAN (elevator) algorithm, which moves the arm in one direction servicing requests until it reaches the end, then reverses direction.

🎯 Goal: Build a simple step-by-step simulation of the SCAN algorithm by creating the request list, setting the initial head position and direction, processing the requests in order, and completing the scan cycle.

📋 What You'll Learn

Create a list of disk requests with exact track numbers

Set the initial head position and direction

Implement the SCAN algorithm logic to order requests

Complete the simulation by showing the final order of serviced requests

💡 Why This Matters

🌍 Real World

Disk scheduling algorithms like SCAN help operating systems reduce the time it takes to read or write data by minimizing the movement of the disk arm.

💼 Career

Understanding SCAN is important for roles in system programming, operating system development, and performance optimization.

1

Create the list of disk requests

Create a list called requests with these exact track numbers: 95, 180, 34, 119, 11, 123, 62, and 64.

Operating Systems

# Create the list of disk requests
# Your code here

Hint

Use square brackets and separate the numbers with commas.

2

Set the initial head position and direction

Create a variable called head and set it to 50. Then create a variable called direction and set it to the string 'up' to indicate the disk arm moves towards higher track numbers first.

Operating Systems

requests = [95, 180, 34, 119, 11, 123, 62, 64]
# Set the initial head position and direction
# Your code here

Hint

Use an integer for head and a string for direction.

3

Implement the SCAN algorithm logic

Create a list called up_list containing requests greater than or equal to head, sorted in ascending order. Create another list called down_list containing requests less than head, sorted in descending order.

Operating Systems

requests = [95, 180, 34, 119, 11, 123, 62, 64]
head = 50
direction = 'up'
# Separate requests into up_list and down_list
# Your code here

Hint

Use list comprehensions with conditions and the sorted() function.

4

Complete the SCAN sequence of serviced requests

Create a list called scan_sequence. If direction is 'up', set scan_sequence to up_list followed by down_list. Otherwise, set it to down_list followed by up_list.

Operating Systems

requests = [95, 180, 34, 119, 11, 123, 62, 64]
head = 50
direction = 'up'
up_list = sorted([r for r in requests if r >= head])
down_list = sorted([r for r in requests if r < head], reverse=True)
# Create scan_sequence based on direction
# Your code here

Hint

Use an if statement to check direction and combine the lists accordingly.

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