Re: [BUG] BUG_ON in I/O scheduler, bugme # 288

[Andrew Morton <akpm@digeo.com>]

Dave Olien <dmo@osdl.org> wrote:
>
> 
> Jens, Andrew
> 
> The group here doing dbt2 workload measurements have hit a couple of
> problems APPARENTLY in the block I/O scheduler when doing write-intensive
> raw disk I/O through a DAC960 extremeraid 2000 controller.
> This wasn't a problem in 2.5.49.  It has appeared since then.
> 
> I've filed a bug on the OSDL bugme database.  You can read it at:
> 
> 	http://bugme.osdl.org/show_bug.cgi?id=288

The title is "2.5.59 and 2.5.50-mm2".  I assume it should be 2.5.59-mm2??


> I've also put a more complete report in my web site:
> 
> 	http://www.osdl.org/archive/dmo/deadline_bugon.

oooh, goody.  A new stresstest tool.

> Begin with the README file.
> 
> For same reason, the README file isn't appearing on my web page.
> I'll look into that. In the mean time, I've included the contests
> of the README file below.
> 
> I'm about to try reproducing the problem on a smaller hardware
> configuration.  Then, I'll test whether the same problem occurs with
> read intensive I/O.

OK, thanks.

The important thing about direct-io is that it will frequently cause multiple
I/Os to be in flight against the same disk sector.  That will never happen
with regular I/O because the pagecache acts as a synchronisation point.

Probably, this has tickled a bug in the I/O scheduler.  Possibly in
direct-io, too - that code's fairly fresh, and quite complex.