Disk scheduling algorithms are crucial for optimizing the performance of computer systems by managing how disk I/O requests are processed. Two commonly discussed algorithms are FCFS (First-Come, First-Served) and SCAN. 

What is FCFS Disk Scheduling?

FCFS (First-Come, First-Served) is the simplest disk scheduling algorithm. It processes disk requests in the order they arrive, without prioritizing any specific request. This approach is straightforward but may lead to inefficiencies, particularly in scenarios with varied request arrival times and locations.

Characteristics of FCFS:

1. Order of Requests: Processes requests based on the sequence of their arrival.
2. Complexity: Simple to implement and understand.
3. Performance: Can suffer from high seek times and delays due to the potential for long movements between requests.

Example of FCFS: 

If requests for disk access arrive in the order of 50, 10, 70, and 30, FCFS will handle them sequentially: first 50, then 10, followed by 70, and finally 30.

What is SCAN Disk Scheduling?

SCAN (also known as the Elevator Algorithm) improves upon FCFS by minimizing disk arm movement. The disk arm moves in one direction, serving all requests along the way, and then reverses direction when it reaches the end, servicing requests on its return trip.

Characteristics of SCAN:

1. Order of Requests: Serves requests in a sweeping motion from one end of the disk to the other.
2. Complexity: More complex than FCFS but reduces seek time and improves efficiency.
3. Performance: Generally provides better performance by reducing the average seek time compared to FCFS.

Example of SCAN: 

If requests for disk access are at 10, 30, 50, and 70, and the disk arm starts at 20 and moves to the end at 80, SCAN will process requests in the following sequence: 30, 50, 70, and then move back to 10.

Difference Between FCFS and SCAN Disk Scheduling Algorithms: 

Basis	FCFS (First-Come, First-Served)	SCAN (Elevator Algorithm)
Definition	Processes disk requests in the order they arrive.	Moves disk arm in one direction to serve requests and reverses direction at the end.
Order of Processing	Sequential based on arrival time.	Serves requests in a sweeping motion from one end to the other.
Complexity	Simple and easy to implement.	More complex but more efficient in reducing seek time.
Seek Time	May result in high seek times due to non-optimal movement.	Generally reduces seek time by minimizing long movements.
Efficiency	Can lead to increased waiting time and decreased throughput.	Provides better throughput and reduced waiting time.
Performance	Performance can degrade with high request volume and scattered request locations.	Improved performance with reduced disk arm movement and less waiting time.
Example Scenario	Requests arrive in the order 50, 10, 70, 30, processed in the same sequence.	Requests at 10, 30, 50, 70 are processed as the disk arm sweeps from 20 to 80 and back.