C.S.C.A.N
Theory :
Circular-SCAN Algorithm is an improved version of the SCAN Algorithm.
Head starts from one end of the disk and move towards the other end servicing all the requests in between. After reaching the other end, head reverses its direction. It then returns to the starting end without servicing any request in between. The same process repeats.
And current position of Read/Write head is : 54, moving in the left direction
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So, Number of Cylinders = 40 + 14 + 65 + 13 + 20 + 24 + 11 + 4 + 46 = 237
Head starts from one end of the disk and move towards the other end servicing all the requests in between. After reaching the other end, head reverses its direction. It then returns to the starting end without servicing any request in between. The same process repeats.
Advantages:
- The waiting time for the cylinders just visited by the head is reduced as compared to the SCAN Algorithm.
- CSCAN provides uniform waiting time.
- CSCAN provides better response time.
Disadvantages:
- CSCAN causes more seek movements as compared to SCAN Algorithm.
- It causes the head to move till the end of the disk even if there are no requests to be serviced..
Example:
Consider the following disk request sequence for a disk with 100 tracks : 98, 137, 122, 183, 14, 133, 65, 78And current position of Read/Write head is : 54, moving in the left direction
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So, Number of Cylinders = 40 + 14 + 65 + 13 + 20 + 24 + 11 + 4 + 46 = 237