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
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2. SCAN Scheduling
• The head continuously scans back and forth across
the disk
• Sometimes called the elevator algorithm
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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)
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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
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5. SCAN Scheduling
Initial Head Position
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6. SCAN Scheduling
Head movement from 53 to 37th track
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7. SCAN Scheduling
Head movement from 37th to 14th track
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8. SCAN Scheduling
Head movement to ‘0’th track
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9. SCAN Scheduling
Head movement to 65th track
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10. SCAN Scheduling
Head movement from 65 to 67th track
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11. SCAN Scheduling
Head movement from 67th to 98th track
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12. SCAN Scheduling
Head movement 98 to 122 track
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13. SCAN Scheduling
Head movement from 122 to 124th track
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14. SCAN Scheduling
Head movement from 124th to 183rd track
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15. SCAN Scheduling
• Disadvantages
– Doesn't provide uniform waiting time
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