Disk scheduling algorithms

Disk Scheduling Algorithms:

Disk scheduling is done by operating systems to schedule I/O requests arriving for the disk. Disk scheduling is also known as I/O scheduling.

Different disk scheduling algorithms are:

1. FCFS disk scheduling algorithm
2. SSTF disk scheduling algorithm
3. SCAN disk scheduling algorithm
4. CSCAN disk scheduling algorithm
5. LOOK disk scheduling algorithm
6. CLOOK disk scheduling algorithm

1. FCFS disk scheduling algorithm:
FCFS is the simplest disk scheduling algorithm. As the name suggests, this algorithm entertains requests in the order they arrive in the disk queue.


2. SSTF disk scheduling algorithm:
SSTF stands for Shortest Seek Time First. This algorithm services that request next which requires least number of head movements from its current position regardless of the direction.


3. SCAN disk scheduling algorithm:
In SCAN algorithm the disk arm moves into a particular direction and services the requests coming in its path and after reaching the end of disk, it reverses its direction and again services the request arriving in its path.


4. CSCAN disk scheduling algorithm:
Circular SCAN disk scheduling algorithm is a modified version of SCAN disk scheduling algorithm that deals with the inefficiency of SCAN algorithm by servicing the requests more uniformly. Like SCAN, C-SCAN moves the head from one end servicing all the requests to the other end.


5. LOOK disk scheduling algorithm:
LOOK Algorithm is an improved version of the SCAN Algorithm.Head starts from the first request at one end of the disk and moves towards the last request at the other end servicing all the requests in between.After reaching the last request at the other end, head reverses its direction.It then returns to the first request at the starting end servicing all the requests in between. The same process repeats.


6. CLOOK disk scheduling algorithm:
Circular-LOOK Algorithm is an improved version of the LOOK Algorithm. Head starts from the first request at one end of the disk and moves towards the last request at the other end servicing all the requests in between. After reaching the last request at the other end, head reverses its direction. It then returns to the first request at the starting end without servicing any request in between. The same process repeats.

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