Question

Disk Scheduling Analysis:

Research and analyze the following disk scheduling algorithms:

First Come First Served Disk Scheduling:
Explain the algorithm and provide a simple example of how it operates.
Shortest Seek Time First Disk Scheduling:
Describe this method and discuss its advantages.
SCAN Disk Scheduling:
Define SCAN disk scheduling and illustrate with an example.

Answer 1

### Disk Scheduling Algorithms

Disk scheduling algorithms are essential for managing read/write requests to a disk drive. Various methods exist to optimize the processes for efficiency and performance. Here we’ll analyze three common disk scheduling algorithms: First Come First Served (FCFS), Shortest Seek Time First (SSTF), and SCAN.

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

**Algorithm Explanation:**
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.

**Example:**
Consider a disk with the following request queue, where each number represents a track number:
- Request Queue: [45, 10, 23, 70, 50]
- Assume the disk head starts at track 35.

Here’s how FCFS would operate:
1. Move from 35 to 45 (10) → Total Movement: 10
2. Move from 45 to 10 (35) → Total Movement: 45
3. Move from 10 to 23 (13) → Total Movement: 58
4. Move from 23 to 70 (47) → Total Movement: 105
5. Move from 70 to 50 (20) → Total Movement: 125

**Total head movement with FCFS:**
- 10 + 35 + 13 + 47 + 20 = **125** tracks

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

**Algorithm Explanation:**
SSTF 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.

**Advantages:**
- **Efficiency**: SSTF generally provides a shorter average waiting time compared to FCFS since it prioritizes closer requests.
- **Reduced Seek Time**: By servicing the nearest request first, the movement of the disk arm is minimized, saving time.

**Example:**
Consider the same request queue:
- Request Queue: [45, 10, 23, 70, 50]
- Starting position: 35

SSTF would handle the requests as follows:
1. Closest to 35 is 45 (10) → Move to 45
2. Next closest is 50 (5) → Move to 50
3. Next is 70 (20) → Move to 70
4. Next is 23 (47) → Move to 23
5. Finally, move to 10 (13) → Move to 10

**Total head movement with SSTF:**
- 10 + 5 + 20 + 47 + 13 = **95** tracks

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### 3. SCAN Disk Scheduling

**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.

**Example:**
Consider a request queue:
- Request Queue: [45, 10, 23, 70, 50]
- Starting position: 35
- Assume the disk has a total of 100 tracks.

- The head starts moving towards the outermost track (i.e., track 100).

Here's how SCAN would operate:
1. Move from 35 to 45 (10) → Continue to service requests while moving outward.
2. Move from 45 to 50 (5)
3. Move from 50 to 70 (20)
4. Continue to the end (track 100)
5. Reversing direction, move from 100 to 70 (30), then to 50 (50), and finally to 23 (27), then to 10 (13).

*Note: Only the successful requests will be serviced, as track 23 and 10 will be picked up as the arm returns.*

**Total head movement with SCAN:**
- 10 + 5 + 20 + 30 + 30 + 27 + 13 = **135** tracks

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### Conclusion

Each disk scheduling algorithm has its strengths and weaknesses. FCFS is simple but may lead to longer waiting times, especially in large queues. SSTF reduces average wait time by servicing the closest request, but it may lead to starvation for distant requests. Meanwhile, SCAN effectively balances the workload by ensuring all requests are serviced in an orderly manner and can help mitigate some of the shortcomings present in FCFS and SSTF. Understanding these algorithms is critical for system performance and optimization in contexts where disk access time can become a bottleneck.