SCAN (elevator algorithm) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the SCAN (elevator) algorithm in operating systems?

The SCAN algorithm is a disk scheduling method where the disk arm moves in one direction servicing requests until it reaches the end, then reverses direction and services requests on the way back, similar to an elevator moving up and down.

beginner

How does the SCAN algorithm differ from the FCFS (First-Come, First-Served) disk scheduling?

Unlike FCFS which services requests in the order they arrive, SCAN moves the disk arm in one direction servicing all requests along the way, reducing the total seek time and avoiding long delays for requests far from the current head position.

beginner

Why is the SCAN algorithm also called the elevator algorithm?

Because it works like an elevator that moves in one direction servicing all floors (requests) on the way, then reverses direction to service requests in the opposite direction, ensuring fair and efficient servicing.

intermediate

What is the main advantage of the SCAN algorithm?

It reduces the average seek time by servicing requests in a sweeping motion, preventing starvation and providing a more uniform wait time compared to simpler algorithms.

beginner

What happens when the disk arm reaches the end of the disk in the SCAN algorithm?

The disk arm reverses direction and starts servicing requests in the opposite direction, continuing this back-and-forth movement like an elevator.

What does the SCAN algorithm do when it reaches the last request in one direction?

A Stops and waits for new requests

B Reverses direction and services requests on the way back

C Jumps back to the start immediately

D Continues moving in the same direction without servicing

The SCAN algorithm reverses direction after reaching the last request in one direction, servicing requests on the return trip.

Why is the SCAN algorithm called the elevator algorithm?

A Because it moves the disk arm back and forth like an elevator

B Because it uses elevators to move disks

C Because it only works in buildings

D Because it randomly selects requests

The disk arm moves back and forth servicing requests like an elevator moving between floors.

Which of these is a key benefit of the SCAN algorithm over FCFS?

A It always services requests in arrival order

B It services only one request per sweep

C It ignores requests at the disk edges

D It reduces average seek time by sweeping through requests

SCAN reduces average seek time by moving the disk arm in one direction servicing all requests along the way.

What problem does the SCAN algorithm help to avoid?

A Disk arm starvation

B Data corruption

C File system crashes

D Power failure

By moving back and forth, SCAN prevents starvation where some requests might never get serviced.

In SCAN, what determines the direction of the disk arm movement?

A User input

B Random choice

C Current position and pending requests

D Time of day

The disk arm moves in one direction servicing requests until the end, then reverses based on current position and pending requests.

Explain how the SCAN (elevator) algorithm works in disk scheduling.

Think about how an elevator moves between floors.

Describe the advantages of using the SCAN algorithm compared to simpler methods like FCFS.

Consider how SCAN organizes request servicing.

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