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C-SCAN Scheduling

• C-SCAN Scheduling is a variant of SCAN

Scheduling

• It is designed to provide uniform wait time

• Like SCAN, C-SCAN moves the head from one end

of the disk to other end, servicing requests along the

way

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C-SCAN Scheduling

• When the head reaches the other end,

– it immediately returns to the beginning of the disk,

– without servicing any requests on the return trip

• Treats the cylinders as a circular list that wraps

around from the last cylinder to the first one

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C-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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C-SCAN Scheduling

• If the disk head is initially at cylinder 53,

– it will first move from 53 to 65, (towards right)

– then to 67, 98, 122, 124, 183,199, 0, 14 and 37 as shown in

the next slides

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C-SCAN Scheduling

Initial head position

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C-SCAN Scheduling

Head movement from 53 to 65

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C-SCAN Scheduling

Head movement from 65th to 67

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C-SCAN Scheduling

Head movement from 67 to 98

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C-SCAN Scheduling

Head movement from 98 to 122

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C-SCAN Scheduling

Head movement from 122 to 124

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C-SCAN Scheduling

Head movement from 124 to 183

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C-SCAN Scheduling

Head moved to the extreme track

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C-SCAN Scheduling

Head is moved from one extreme end to the other

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C-SCAN Scheduling

Head is moving from 0 to 14th track

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C-SCAN Scheduling

Head is moved from 14th to 37th track

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C-SCAN Scheduling

• Advantages

– Better than SCAN Scheduling in terms of waiting

time

– i.e., provides uniform waiting time

• Disadvantages

– It is not optimal