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work queue: list of disk operations requested
- FCFS (First Come, First Served)
- perform operations in order requested
- no reordering of work queue
- no starvation: every request is serviced
- poor performance
- SSTF (Shortest Seek Time First)
- after a request, go to the closest request in the work queue,
regardless of direction
- reduces total seek time compared to FCFS
- Disadvantages
- starvation is possible; stay in one area of the
disk if very busy
- switching directions slows things down
- SCAN
- go from the outside to the inside servicing requests and then back from
the outside to the inside servicing requests.
- repeats this over and over.
- reduces variance compared to SSTF.
- LOOK
- like SCAN but stops moving inwards (or outwards) when no more requests
in that direction exist.
- C-SCAN (circular scan)
- moves inwards servicing requests until it reaches the innermost cylinder;
then jumps to the outside cylinder of the disk without servicing any requests.
- repeats this over and over.
- variant: service requests from inside to outside,
and then skip back to the innermost cylinder.
- C-LOOK
- moves inwards servicing requests until there are no more requests
in that direction, then it jumps to the outermost outstanding requests.
- repeast this over and over.
- variant: service requests from inside to outside, then
skip back to the innermost request.
Howard Hamilton
Wed Feb 2 10:00:00 CST 2000