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Scan scheduling 50 1

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  • why the disk head moves from 14 to zero(0) & why not 14 to 65. As earlier no other scheduling reaches to zero. And it it touches 0 to define scan aka elevator concept why doesn't it reaches to 199.
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Scan scheduling 50 1

  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. 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. 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. 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. 5. SCAN SchedulingInitial Head Position http://raj-os.blogspot.in/ 5
  6. 6. SCAN SchedulingHead movement from 53 to 37th track http://raj-os.blogspot.in/ 6
  7. 7. SCAN SchedulingHead movement from 37th to 14th track http://raj-os.blogspot.in/ 7
  8. 8. SCAN SchedulingHead movement to ‘0’th track http://raj-os.blogspot.in/ 8
  9. 9. SCAN SchedulingHead movement to 65th track http://raj-os.blogspot.in/ 9
  10. 10. SCAN SchedulingHead movement from 65 to 67th track http://raj-os.blogspot.in/ 10
  11. 11. SCAN SchedulingHead movement from 67th to 98th track http://raj-os.blogspot.in/ 11
  12. 12. SCAN SchedulingHead movement 98 to 122 track http://raj-os.blogspot.in/ 12
  13. 13. SCAN SchedulingHead movement from 122 to 124th track http://raj-os.blogspot.in/ 13
  14. 14. SCAN SchedulingHead movement from 124th to 183rd track http://raj-os.blogspot.in/ 14
  15. 15. SCAN Scheduling• Disadvantages – Doesnt provide uniform waiting time http://raj-os.blogspot.in/ 15

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