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    Subjectquestion about max_readahead for ide devices in 2.4?


    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?

    Thanks,
    John Salmon
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