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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
SCAN Scheduling


• The head continuously scans back and forth across
  the disk
• Sometimes called the elevator algorithm




                     http://raj-os.blogspot.in/       2
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
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
SCAN Scheduling




Initial Head Position

    http://raj-os.blogspot.in/   5
SCAN Scheduling




Head movement from 53 to 37th track

            http://raj-os.blogspot.in/   6
SCAN Scheduling




Head movement from 37th to 14th track

           http://raj-os.blogspot.in/   7
SCAN Scheduling




Head movement to ‘0’th track

        http://raj-os.blogspot.in/   8
SCAN Scheduling




Head movement to 65th track

      http://raj-os.blogspot.in/   9
SCAN Scheduling




Head movement from 65 to 67th track

           http://raj-os.blogspot.in/   10
SCAN Scheduling




Head movement from 67th to 98th track

           http://raj-os.blogspot.in/   11
SCAN Scheduling




Head movement 98 to 122 track

        http://raj-os.blogspot.in/   12
SCAN Scheduling




Head movement from 122 to 124th track

           http://raj-os.blogspot.in/   13
SCAN Scheduling




Head movement from 124th to 183rd track

             http://raj-os.blogspot.in/   14
SCAN Scheduling


• Disadvantages

  – Doesn't provide uniform waiting time




                       http://raj-os.blogspot.in/   15

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