What is Disk Scheduling Algorithm?
A Process makes the I/O requests to the operating system to access the disk. Disk Scheduling Algorithm manages those requests and decides the order of the disk access given to the requests.
Why Disk Scheduling Algorithm is needed?
Disk Scheduling Algorithms are needed because a process can make multiple I/O requests and multiple processes run at the same time. The requests made by a process may be located at different sectors on different tracks. Due to this, the seek time may increase more. These algorithms help in minimizing the seek time by ordering the requests made by the processes.
Important Terms related to Disk Scheduling Algorithms
- Seek Time - It is the time taken by the disk arm to locate the desired track.
- Rotational Latency - The time taken by a desired sector of the disk to rotate itself to the position where it can access the Read/Write heads is called Rotational Latency.
- Transfer Time - It is the time taken to transfer the data requested by the processes.
- Disk Access Time - Disk Access time is the sum of the Seek Time, Rotational Latency, and Transfer Time.
Disk Scheduling Algorithms
First Come First Serve (FCFS)
In this algorithm, the requests are served in the order they come. Those who come first are served first. This is the simplest algorithm.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60.
Seek Time = Distance Moved by the disk arm = (140-70)+(140-50)+(125-50)+(125-30)+(30-25)+(160-25)=480
Shortest Seek Time First (SSTF)
In this algorithm, the shortest seek time is checked from the current position and those requests which have the shortest seek time is served first. In simple words, the closest request from the disk arm is served first.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60.
Seek Time = Distance Moved by the disk arm = (60-50)+(50-30)+(30-25)+(70-25)+(125-70)+(140-125)+(160-125)=270
SCAN
In this algorithm, the disk arm moves in a particular direction till the end and serves all the requests in its path, then it returns to the opposite direction and moves till the last request is found in that direction and serves all of them.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60. And it is given that the disk arm should move towards the larger value.
Seek Time = Distance Moved by the disk arm = (170-60)+(170-25)=255
LOOK
In this algorithm, the disk arm moves in a particular direction till the last request is found in that direction and serves all of them found in the path, and then reverses its direction and serves the requests found in the path again up to the last request found. The only difference between SCAN and LOOK is, it doesn't go to the end it only moves up to which the request is found.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60. And it is given that the disk arm should move towards the larger value.
Seek Time = Distance Moved by the disk arm = (170-60)+(170-25)=235
C-SCAN
This algorithm is the same as the SCAN algorithm. The only difference between SCAN and C-SCAN is, it moves in a particular direction till the last and serves the requests in its path. Then, it returns in the opposite direction till the end and doesn't serve the request while returning. Then, again reverses the direction and serves the requests found in the path. It moves circularly.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60. And it is given that the disk arm should move towards the larger value.
Seek Time = Distance Moved by the disk arm = (170-60)+(170-0)+(50-0)=330
C-LOOK
This algorithm is also the same as the LOOK algorithm. The only difference between LOOK and C-LOOK is, it moves in a particular direction till the last request is found and serves the requests in its path. Then, it returns in the opposite direction till the last request is found in that direction and doesn't serve the request while returning. Then, again reverses the direction and serves the requests found in the path. It also moves circularly.
Eg. Suppose the order of requests are 70, 140, 50, 125, 30, 25, 160 and the initial position of the Read-Write head is 60. And it is given that the disk arm should move towards the larger value.
Seek Time = Distance Moved by the disk arm = (160-60)+(160-25)+(50-25)=260
