Question

Disk Scheduling Algorithms:
Summary of each algorithm researched.
Algorithm Explanation (first served disk scheduling) The FCFS algorithm processes requests in the order they arrive. This approach is straightforward: the first request that comes is the first one to be serviced. There’s no prioritization based on the position of the requests relative to the current head position. For example, consider a disk with the following request queue where each number represents a track number.

Algorithm Explanation (shortest seek time first disk scheduling) selects the request that is closest to the current head position, minimizing the seek time and aiming to reduce the wait time for all requests. The advantages are that shortest seek time generally provides a shorter average waiting time compared to FCFS since it prioritizes closer requests. But by servicing the nearest request first, the movement of the disk arm is minimized, saving time.
For example, 45,10,23,70,50 the starting position will be 35

Algorithm Explanation SCAN (also known as the Elevator Algorithm) moves the disk arm in one direction let's say from the outer tracks to the inner tracks and services all requests in that path until it reaches the end. Once it reaches the end, it reverses direction and services requests on the way back. This method resembles how an elevator would operate, hence the name. for example, 45.10,23,70,50
Starting postion will be 35 but assume that the disk has 200 track

Answer 1

Here’s a summary and explanation of three popular disk scheduling algorithms: FCFS, SSTF, and SCAN.

### 1. First-Come, First-Served (FCFS) Disk Scheduling

**Summary:**
FCFS is the simplest disk scheduling algorithm. It processes requests in the order they arrive, without any prioritization.

**Explanation:**
- **Order of Processing**: When requests come in, they are queued and the disk arm services them in that order.
- **Example**: If the requests are [45, 10, 23, 70, 50] and the starting head position is at 35:
- The orders of service would be from the head moving to 45, then to 10, 23, 70, and finally 50.
- **Drawbacks**: This method doesn’t take into account the distance of requests from the current head position. As a result, it can lead to longer seek times, especially if the requests are spread out over a large range of tracks.

### 2. Shortest Seek Time First (SSTF) Disk Scheduling

**Summary:**
SSTF selects the request that is closest to the current head position, aiming to minimize seek time.

**Explanation:**
- **Order of Processing**: The algorithm chooses the nearest request in terms of track number from the current head position, reducing the movement of the disk arm.
- **Example**:
- Given requests [45, 10, 23, 70, 50] and starting position at 35:
- The nearest request is 23 (distance 12), so it is served first.
- Next would be 10 (distance 13), then 45 (distance 12), followed by 50 (distance 5), and finally 70 (distance 20).
- **Advantages**: SSTF generally offers a shorter average waiting time compared to FCFS.
- **Drawbacks**: It can lead to starvation for requests that are far from the current head position if there are always closer requests being made.

### 3. SCAN (Elevator Algorithm)

**Summary:**
SCAN moves the disk arm in one direction, servicing all requests in that path until it reaches the end, then reverses direction.

**Explanation:**
- **Order of Processing**: The disk arm moves in a linear fashion, like an elevator, servicing requests sequentially until it reaches the outermost or innermost track, and then it changes direction to service the requests on the way back.
- **Example**:
- Given requests [45, 10, 23, 70, 50] with a starting position of 35 and assuming 200 tracks:
- The disk arm will move towards the direction of 70, servicing as it goes: first it would service requests 45, then 50, then 70. After reaching 70, it will reverse direction and service 23, 10 (if they are still in range).
- If it reaches the outer track (let's say track 199), it would go back to service the requests on the way back.
- **Advantages**: SCAN can reduce the overall seek time compared to FCFS by systematically servicing requests in an organized manner.
- **Drawbacks**: However, it may still result in long waiting times for requests that are located in the opposite direction of the current head movement.

### Conclusion:
Each disk scheduling algorithm has its strengths and weaknesses, ranging from simplicity to efficiency. The choice of algorithm often depends on the specific workload and the patterns of disk access in a computing environment.