In _______ disk scheduling algorithm, the disk head moves from one end to other end of the disk, serving the requests along the way. When the head reaches the other end, it immediately returns to the beginning of the disk without serving any requests on the return trip.
C - SCAN
Understanding Disk Scheduling Algorithms
Disk scheduling algorithms are essential for managing the requests to read from or write to a disk. The goal is often to minimize the total head movement of the disk arm, which reduces seek time and improves the overall performance of the computer system.
Different algorithms employ various strategies for determining the order in which disk I/O requests are serviced. The question describes a specific pattern of disk head movement: moving from one end to the other, servicing requests, and then returning to the beginning without servicing requests on the return trip.
Analyzing the Disk Head Movement Pattern
The question details a disk head movement strategy where:
- The head starts at one end (or near it).
- It sweeps across the disk towards the other end.
- Requests are served sequentially as the head passes their cylinders.
- Once the head reaches the far end, it immediately jumps back to the starting end of the disk.
- No requests are served during this return trip.
This specific pattern ensures that all requests are serviced in a single pass across the disk cylinders in one direction, providing a more uniform waiting time compared to algorithms that might oscillate back and forth serving requests on both trips.
Identifying the Correct Disk Scheduling Algorithm: C-SCAN
Let's examine the characteristics of the given disk scheduling algorithms in relation to the described pattern:
| Algorithm | Head Movement Pattern | Servicing on Return Trip? |
|---|---|---|
| LOOK | Moves towards the farthest request in one direction, then reverses direction when no more requests exist in that direction. | Yes, serves requests on the return trip. |
| SCAN | Moves towards one end (e.g., the highest cylinder), serving requests. When it reaches the end, it reverses direction and moves towards the other end, serving requests. | Yes, serves requests on the return trip. |
| C-LOOK | Similar to LOOK, but when it reaches the farthest request in one direction, it jumps directly back to the farthest request in the opposite direction without going all the way to the end. | No, only serves requests in one direction of sweep. Jumps back. |
| C-SCAN | Moves towards one end (e.g., the highest cylinder), serving requests. When it reaches the end, it jumps directly back to the other end (e.g., the lowest cylinder) without serving requests on the return trip. | No, only serves requests in one direction of sweep. Jumps back. |
Comparing the described movement pattern with the characteristics of the algorithms:
- LOOK and SCAN both serve requests on the return trip, which contradicts the description ("without serving any requests on the return trip").
- C-LOOK jumps back from the last request served to the first request in the other direction, not necessarily going all the way to the disk ends. The description specifies movement "from one end to other end" and return "to the beginning of the disk".
- C-SCAN matches the description exactly: it sweeps from one end to the other, serving requests, and then jumps back to the beginning without serving requests on the return trip. This circular movement pattern is the defining characteristic of C-SCAN.
Therefore, the disk scheduling algorithm described, where the head moves from one end to the other serving requests and then immediately returns to the beginning without serving requests on the return trip, is C-SCAN.
Revision Table: Disk Scheduling Concepts
| Term | Description | Relevance to Disk Scheduling |
|---|---|---|
| Disk Scheduling | Method for determining the order of disk I/O requests. | Minimizes head movement, improves performance. |
| Seek Time | Time taken for the disk head to move to the correct cylinder. | Primary target for optimization by scheduling algorithms. |
| Latency Time | Time taken for the disk sector to rotate under the head. | Also contributes to total access time, but less affected by scheduling order than seek time. |
| Transfer Time | Time taken to read or write the data once the head is positioned. | Not directly affected by scheduling order. |
Additional Information: Variations and Performance
While C-SCAN provides a fairer distribution of waiting times compared to SCAN by sweeping in only one direction, other algorithms like Shortest Seek Time First (SSTF) aim purely to minimize total head movement by always choosing the request closest to the current head position. However, SSTF can lead to starvation for requests far from the frequently accessed areas.
The choice of disk scheduling algorithm can impact system throughput, response time, and variance in response time. C-SCAN is often used when a more predictable and uniform waiting time is desired, making it suitable for systems where fairness is important.
Important Questions from Disk Scheduling - Teaching
Disc is divided into several concentric circles called _______.
The total storage capacity of a floppy disk having 80 tracks and storing 128 bytes/sector is 163.840 bytes. How many sectors does this disk have?
Consider a disk system with 100 cylinders. The requests to access the cylinders occur in the following sequence:
4, 34, 10, 7, 19, 73, 2, 15, 6, 20
Assuming that the head is current at cylinder 50, what is the time taken to satisfy all requests if it takes 1ms to move from the cylinder to adjacent one and the shortest seek time first policy is used?Consider a disk system having 60 cylinders. Disk requests are received by a disk drive for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. Assuming the disk head is currently at cylinder 20, what is the time taken to satisfy all the requests if it takes 2 milliseconds to move from one cylinder to an adjacent one and Shortest Seek Time First (SSTF) algorithm is used ?
Consider a disk queue with requests for I/O to blocks on cylinders 98, 183, 37, 122, 14, 124, 65, 67. Suppose SSTF disk scheduling algorithm implemented to meet the requests then the total number of head movements are ______ if the disk head is initially at 53.
