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    SubjectRe: question about max_readahead for ide devices in 2.4?
    John Salmon <jsalmon@thesalmons.org> 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?

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

    blockdev --setra 511 /dev/hda

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