question about max_readahead for ide devices in 2.4?

[John Salmon]

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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