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    SubjectRe: Random file I/O regressions in 2.6
    Andrew Morton wrote:
    > Peter Zaitsev <> wrote:
    >>On Mon, 2004-05-03 at 13:57, Andrew Morton wrote:
    >>>Ram Pai <> wrote:
    >>>>>The place which needs attention is handle_ra_miss(). But first I'd like to
    >>>>>reacquaint myself with the intent behind the lazy-readahead patch. Was
    >>>>>never happy with the complexity and special-cases which that introduced.
    >>>>lazy-readahead has no role to play here.
    >>Could you please clarify how this things become to be dependent on
    >>read-ahead at all.
    > readahead is currently the only means by which we build up nice large
    > multi-page BIOs.
    >>At my understanding read-ahead it to catch sequential (or other) access
    >>pattern and do some advance reading, so instead of 16K request we do
    >>128K request, or something similar.
    > That's one of its usage patterns. It's also supposed to detect the
    > fixed-sized-reads-seeking-all-over-the-place situation. In which case it's
    > supposed to submit correctly-sized multi-page BIOs. But it's not working
    > right for this workload.
    > A naive solution would be to add special-case code which always does the
    > fixed-size readahead after a seek. Basically that's
    > if (ra->next_size == -1UL)
    > force_page_cache_readahead(...)

    I think a better solution to this case would be to ensure the
    readahead window is always min(size of read, some large number);

    The size of the read is basically a free and accurate "hint" to
    the minimum size of the required readahead.

    Either that or do a simple "preread" while you're still in the
    read request window, and run readahead when that completes.
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