Disk Scheduling

Disk Scheduling - • Cons – Request at the other end...

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1/31/12 1 Disk Scheduling Which disk request should the OS service first? FCFS Shortest seek time first Elevator (SCAN) LOOK C-SCAN (Circular SCAN) C-LOOK Looks familiar?
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1/31/12 2 FIFO (FCFS) order Method First come first serve Pros Fairness among requests In the order applications expect Cons Arrival may be on random spots on the disk (long seeks) Wild swing can happen Analogy: Can elevator scheduling use FCFS? 0 199 98, 183, 37, 122, 14, 124, 65, 67 53
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1/31/12 3 SSTF ( Shortest Seek Time First ) Method Pick the one closest on disk Rotational delay is in calculation Pros Try to minimize seek time Cons Starvation Question Is SSTF optimal? Can we avoid starvation? Analogy: elevator 0 199 98, 183, 37, 122, 14, 124, 65, 67 (65, 67, 37, 14, 98, 122, 124, 183) 53
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1/31/12 4 Elevator (SCAN) Method Take the closest request in the direction of travel Real implementations do not go to the end (called LOOK) Pros Bounded time for each request
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Unformatted text preview: • Cons – Request at the other end will take a while 199 98, 183, 37, 122, 14, 124, 65, 67 (37, 14, 0, 65, 67, 98, 122, 124, 183) 53 1/31/12 5 C-SCAN (Circular SCAN) • Method – Like SCAN – But, wrap around – Real implementation doesn’t go to the end (C-LOOK) • Pros – Uniform service time • Cons – Do nothing on the return 199 98, 183, 37, 122, 14, 124, 65, 67 (65, 67, 98, 122, 124, 183, 199, 0, 14, 37) 53 1/31/12 6 LOOK and C-LOOK • SCAN and C-SCAN move the disk arm across the full width of the disk • In practice, neither algorithm is implemented this way • More commonly, the arm goes only as far as the final request in each direction. Then, it reverses direction immediately, without first going all the way to the end of the disk. • These versions of SCAN and C-SCAN are called LOOK and C-LOOK...
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This note was uploaded on 01/30/2012 for the course CS 354 taught by Professor Staff during the Fall '08 term at Purdue.

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Disk Scheduling - • Cons – Request at the other end...

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