Scheduling algorithm (SCAN, LOOK) in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the SCAN scheduling algorithm?

SCAN is a disk scheduling algorithm 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, like an elevator.

beginner

How does the LOOK algorithm differ from SCAN?

LOOK is similar to SCAN but the disk arm only goes as far as the last request in each direction before reversing, instead of going to the physical end of the disk.

beginner

Why is SCAN called the elevator algorithm?

Because it moves the disk arm back and forth across the disk like an elevator moving up and down floors, servicing requests in one direction before reversing.

intermediate

What is the main advantage of LOOK over SCAN?

LOOK reduces unnecessary movement by not going to the disk's physical end if there are no requests there, improving efficiency and reducing seek time.

intermediate

In what scenario would SCAN or LOOK scheduling be preferred?

They are preferred when we want to reduce the average seek time and avoid starvation by servicing requests in a fair order, especially in systems with many disk I/O requests.

What does the SCAN algorithm do when it reaches the end of the disk?

A Skips requests at the end and continues forward

B Reverses direction and services requests on the way back

C Jumps back to the start of the disk immediately

D Stops and waits for new requests

SCAN moves the disk arm to the end, then reverses direction to service requests on the return trip.

How does LOOK improve efficiency compared to SCAN?

A By moving faster across the disk

B By skipping requests at the disk edges

C By servicing requests randomly

D By only moving as far as the last request in each direction

LOOK stops at the last request in each direction, avoiding unnecessary travel to the disk's physical end.

Which scheduling algorithm is also known as the elevator algorithm?

A SCAN

B LOOK

C FCFS

D SSTF

SCAN is called the elevator algorithm because it moves back and forth like an elevator.

What is a key benefit of using SCAN or LOOK over FCFS in disk scheduling?

A They reduce average seek time and avoid starvation

B They guarantee the shortest seek time for every request

C They process requests in random order

D They only service requests at the disk edges

SCAN and LOOK reduce average seek time and prevent starvation by servicing requests in a fair order.

If there are no requests beyond a certain point on the disk, which algorithm avoids unnecessary travel?

A SCAN

B FCFS

C LOOK

D SSTF

LOOK avoids traveling to the disk's physical end if no requests exist beyond the last request.

Explain how the SCAN scheduling algorithm works and why it is called the elevator algorithm.

Think about how an elevator moves up and down floors.

Describe the difference between SCAN and LOOK algorithms and the advantage of LOOK.

Consider how far the disk arm travels in each algorithm.