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Homework-10 Version 0
👤 Author: by 390645143qqcom 2019-12-25 14:43:50
review  various Disk scheduling algorithms
FCFS、SSTF、SCAN、CSCAN

FCFS:
Algorithm idea: Serve according to the order in which access requests arrive.
Pros: Simple and fair.
Disadvantages: The efficiency is not high. Two adjacent requests may cause the innermost to outermost cylinder seek, which makes the magnetic head move repeatedly, which increases the service time and is not good for the machine.

SSTF:
Algorithm idea: The access request closest to the current magnetic head is preferentially selected for service, and the seek priority is mainly considered.
Pros: Improved average disk service time.
Disadvantages: cause some access requests to wait for long periods of time without being served.

SCAN:
Algorithm idea: when the device has no access request, the magnetic head does not move; when there is an access request, the magnetic head moves in one direction, and the access request encountered during the movement is serviced, and then whether there is an access request in that direction, If there is, continue scanning; otherwise, change the direction of movement and service the access requests that have passed, and so on.
Advantages: better seek performance, avoiding "hunger"
Disadvantage: It is not good for access requests far from the head

C-SCAN:
Pros: Eliminates unfair requests for tracks at both ends

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