Re: regression: 100% io-wait with 2.6.24-rcX

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, 18 Jan 2008, Martin Knoblauch wrote:
>
>  just to make one thing clear - I am not so much concerned about the
> performance of AACRAID. It is OK with or without Mel's patch. It is
> better with Mel's patch. The regression in DIO compared to 2.6.19.2 is
> completely independent of Mel's stuff.
> 
>  What interests me much more is the behaviour of the CCISS+LVM based
> system. Here I see a huge benefit of reverting Mel's patch.

Ok, I just got your usage cases confused.

The argument stays the same: some controllers/drivers may have subtle 
behavioural differences that come from the IO limits themselves.

So it wasn't AACRAID, it was CCISS+LVM. The argument is the same: it may 
well be that the *bigger* IO sizes are actually what hurts, even if the 
conventional wisdom is traditionally that bigger submissions are better.

>  At least, rc1-rc5 have shown that the CCISS system can do well. Now
> the question is which part of the system does not cope well with the
> larger IO sizes? Is it the CCISS controller, LVM or both. I am open to
> suggestions on how to debug that. 

I think you need to ask the MD/DM people for suggestions.. Who aren't cc'd 
here.

		Linus