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