Lecture19

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Unformatted text preview: quot; "precedes" means "comes before" Text should be reversed proceeding request should be backmerged preceding request should be frontmerged LOOK (elevator) The disk arm starts at one end of the disk, and moves toward the other end No starvation Requests are serviced in blocknumber order until the head reaches the highest block number in the request queue Head direction is reversed and servicing continues. However, some critical requests might wait for a long time LOOK Merged LOOK request queue: Updated request queue*: 15205,16000,(2,3,4) suppose previous request was block #15000 new requests: 14398, 18655, 13587, 15206 (15205,15206),16000,18655,14398,13587,(2,3,4) *assumes that no requests were dispatched The head moves from one end of the disk toward the other, servicing requests as it goes When it services the highest block number in the request queue, it returns to the lowest block number in the request queue CLOOK Treats the cylinders as a circular list that wraps around from the last cylinder to the first cylinder. Provides a more uniform wait time than LOOK no service on return trip. No starvation CLOOK Mer...
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This note was uploaded on 06/28/2009 for the course CS 411 taught by Professor Staff during the Spring '08 term at Oregon State.

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