Re: IDE performance drop between 2.4.23 and 2.6.0

[Linus Torvalds]

On Sat, 27 Dec 2003, Jean-Luc Fontaine wrote:
> 
> I solved the problem in a very strange way. Note that the (b) disk
> performance only improves after readahead has been increased on another
> (c) drive! (the (c) drive performance was also increased by to 2.4
> levels but is not shown here). I could reliably repeat this behavior
> after rebooting.
> 
> Can any IDE expert explain it?

Looks like a bug. If you don't access hdc, then the read-ahead on hdc 
shouldn't matter. I wonder if the read-ahead code (either the setting or 
the reading) gets the value from the wrong queue or something.

Jens? This could certainly explain some of the confusion about
performance.. Do you see anything strange?

			Linus

> # hdparm -t /dev/hdb
> ~ Timing buffered disk reads:   34 MB in  3.09 seconds =  11.02 MB/sec
> # hdparm -a 4096 /dev/hdb
> /dev/hdb: readahead = 4096 (on)
> # hdparm -t /dev/hdb
> ~ Timing buffered disk reads:   34 MB in  3.08 seconds =  11.04 MB/sec
> # hdparm -a 4096 /dev/hdc
> /dev/hdc: readahead = 4096 (on)
> # hdparm -t /dev/hdb
> ~ Timing buffered disk reads:   46 MB in  3.12 seconds =  14.76 MB/sec
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