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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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