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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

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SCAN Scheduling

• The head continuously scans back and forth across

the disk

• Sometimes called the elevator algorithm

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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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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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SCAN Scheduling

Initial Head Position

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SCAN Scheduling

Head movement from 53 to 37th track

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SCAN Scheduling

Head movement from 37th to 14th track

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SCAN Scheduling

Head movement to ‘0’th track

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SCAN Scheduling

Head movement to 65th track

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SCAN Scheduling

Head movement from 65 to 67th track

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SCAN Scheduling

Head movement from 67th to 98th track

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SCAN Scheduling

Head movement 98 to 122 track

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SCAN Scheduling

Head movement from 122 to 124th track

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SCAN Scheduling

Head movement from 124th to 183rd track

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SCAN Scheduling

• Disadvantages

– Doesn't provide uniform waiting time