C scan scheduling 50 2

1. http://raj-os.blogspot.in/ 1
C-SCAN Scheduling
•C-SCAN Scheduling is a variant of SCAN
Scheduling
•It is designed to provide uniform wait time
•Like SCAN, C-SCAN moves the head from one end
of the disk to other end, servicing requests along the
way

2. http://raj-os.blogspot.in/ 2
C-SCAN Scheduling
•When the head reaches the other end,
–it immediately returns to the beginning of the disk,
–without servicing any requests on the return trip
•Treats the cylinders as a circular list that wraps
around from the last cylinder to the first one

3. http://raj-os.blogspot.in/ 3
C-SCAN Scheduling
•Consider for example an ordered disk queue with
requests involving
–Tracks 98, 183, 37, 122, 14, 124, 65, 67
–Head pointer 53
•The request queue (0-199)

4. http://raj-os.blogspot.in/ 4
C-SCAN Scheduling
•If the disk head is initially at cylinder 53,
–it will first move from 53 to 65, (towards right)
–then to 67, 98, 122, 124, 183,199, 0, 14 and 37 as shown in
the next slides

5. http://raj-os.blogspot.in/ 5
C-SCAN Scheduling
Initial head position

6. http://raj-os.blogspot.in/ 6
C-SCAN Scheduling
Head movement from 53 to 65

7. http://raj-os.blogspot.in/ 7
C-SCAN Scheduling
Head movement from 65
th
to 67

8. http://raj-os.blogspot.in/ 8
C-SCAN Scheduling
Head movement from 67 to 98

9. http://raj-os.blogspot.in/ 9
C-SCAN Scheduling
Head movement from 98 to 122

10. http://raj-os.blogspot.in/ 10
C-SCAN Scheduling
Head movement from 122 to 124

11. http://raj-os.blogspot.in/ 11
C-SCAN Scheduling
Head movement from 124 to 183

12. http://raj-os.blogspot.in/ 12
C-SCAN Scheduling
Head moved to the extreme track

13. http://raj-os.blogspot.in/ 13
C-SCAN Scheduling
Head is moved from one extreme end to the other

14. http://raj-os.blogspot.in/ 14
C-SCAN Scheduling
Head is moving from 0 to 14
th
track

15. http://raj-os.blogspot.in/ 15
C-SCAN Scheduling
Head is moved from 14
th
to 37
th
track

16. http://raj-os.blogspot.in/ 16
C-SCAN Scheduling
•Advantages
–Better than SCAN Scheduling in terms of waiting
time
–i.e., provides uniform waiting time
•Disadvantages
–It is not optimal