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.