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    SubjectRe: question about max_readahead for ide devices in 2.4?
    >>>>> "Andrew" == Andrew Morton <> writes:

    Andrew> John Salmon <> wrote:
    >> Several "tuning" recommendations suggest that sequential accesses of
    >> large files, and hence the performance of busy web servers, can be improved
    >> by changing the maximum readahead value with, e.g.,
    >> echo 511 > /proc/sys/vm/max-readahead
    >> But it looks to me like get_max_readahead in filemap.c ignores the
    >> value set by /proc/sys in favor of max_readahead[major][minor] whenever
    >> max_readahead[major] is non-NULL. And furthermore that
    >> max_readahead[major] IS initialized to non-NULL for ide devices in
    >> init_gendisk. (N.B. I'm looking at 2.4 sources).
    >> Conclusion: echoing a value into /proc/sys/vm/max-readahead won't change the
    >> readahead behavior for already-probed IDE devices.
    >> Is this correct, or am I missing something?

    Andrew> That's correct - it's all a bit weird. You should use

    Andrew> blockdev --setra 511 /dev/hda

    Andrew> for IDE devices. Not sure about scsi. You may as well set
    Andrew> /proc/sys/vm/max-readahead to the same thing.

    Are you sure? It looks like that invokes the BLKRASET ioctl, which
    sets an entry in the read_ahead[MAJOR(dev)] array. But the only code
    that uses the read_ahead[] array is in fs/hfs/file.c, and I'm definitely
    not using an hfs filesystem.

    Or am I missing something else??

    John Salmon

    P.S. The weird thing is that I tried the blockdev --setra suggestoin
    and it *seemed to* improve performance. This is why drug tests are
    double-blind ... the patient (the computer) may not be susceptible to
    the placebo effect, but the doctor (me) may be :-(.

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