Scan scheduling 50 1
- 1. SCAN Scheduling • The disk arm starts at one end of the disk, and moves toward the other end • Servicing requests until it gets to the other end of the disk • Where the head movement is reversed and servicing continues http://raj-os.blogspot.in/ 1
- 2. SCAN Scheduling • The head continuously scans back and forth across the disk • Sometimes called the elevator algorithm http://raj-os.blogspot.in/ 2
- 3. 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) http://raj-os.blogspot.in/ 3
- 4. SCAN Scheduling • If the disk head is initially at cylinder 53, – it will first move from 53 to 37, (towards left) – then to 14, 0, 65, 67, 98,122, 124 and 183 as shown in the next slides • The total head movement of 208 tracks http://raj-os.blogspot.in/ 4
- 5. SCAN Scheduling Initial Head Position http://raj-os.blogspot.in/ 5
- 6. SCAN Scheduling Head movement from 53 to 37th track http://raj-os.blogspot.in/ 6
- 7. SCAN Scheduling Head movement from 37th to 14th track http://raj-os.blogspot.in/ 7
- 8. SCAN Scheduling Head movement to ‘0’th track http://raj-os.blogspot.in/ 8
- 9. SCAN Scheduling Head movement to 65th track http://raj-os.blogspot.in/ 9
- 10. SCAN Scheduling Head movement from 65 to 67th track http://raj-os.blogspot.in/ 10
- 11. SCAN Scheduling Head movement from 67th to 98th track http://raj-os.blogspot.in/ 11
- 12. SCAN Scheduling Head movement 98 to 122 track http://raj-os.blogspot.in/ 12
- 13. SCAN Scheduling Head movement from 122 to 124th track http://raj-os.blogspot.in/ 13
- 14. SCAN Scheduling Head movement from 124th to 183rd track http://raj-os.blogspot.in/ 14
- 15. SCAN Scheduling • Disadvantages – Doesn't provide uniform waiting time http://raj-os.blogspot.in/ 15