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    SubjectRe: disk head scheduling
    On Fri, 19 Mar 1999, Mark H. Wood wrote:
    > On Fri, 19 Mar 1999, Ingo Molnar wrote:
    > > On Thu, 18 Mar 1999, Yasushi Saito wrote:
    > > > What I tried to implement was two-way elevator seeking (SCAN). In my
    > > > tiny benchmark that let many threads write on random files, SCAN
    > > > showed a throughput improvement of anywhere between 0 to 20%. But I
    > > > also noticed benefits in the original algorithm (it's fairer), so I
    > > > don't know if my change makes sense.
    > >
    > > the bigger problem is that dumber devices will just execute non-forwards
    > > ordered requests. Most modern harddisks will either cache a full track, or
    > > will reorder the request per-track anyway, but eg. a floppy disk or a
    > > CD-ROM will execute the requests as given, and the 'downwards' queue will
    > > perform badly. Would you mind doing the seek benchmark on your CDROM too,
    > > just to test this theory?
    >
    > Well of course a 2-way elevator should sort by *ascending* sector within
    > descending track. I take it this is difficult?
    Since all modern harddiscs lie faster than a politician in an election
    year when asked about their geometry, the very idea of knowing enough
    about the track boundaries to switch sort direction within each track is
    absurd.

    --
    Henrik Olsen, Dawn Solutions I/S
    URL=http://www.iaeste.dk/~henrik/
    Get the rest there.


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