[dm-devel] [PATCH 1/6] dm raid45 target: export region hash functions and add a needed one

[Dan Williams]

On Sat, Jul 11, 2009 at 5:44 AM, Doug Ledford<dledford at redhat.com> wrote:
> Thanks.  This isn't too far off from what I would expect.  I would say that
> real world loads fall all along a spectrum from "create lots of writes, but
> do little to no work" to "does lots of work, and only sporadically writes".
>  It's the later end of this spectrum that is most likely to be helped by the
> cache avoiding routines, while the former is not.  So, one of the tests I
> had in mind was to use something like timing a complete kernel build, or
> doing a database load/report cycle, or some other things like that.  Things
> that do actual work in the foreground while the raid is kept busy in the
> background.  Of course, testing all the various points along the spectrum is
> needed, so this test gets us the first.

This reminds of the testing I did when quantifying the benefit of
hardware accelerated raid5.  I played with kernel builds while
resyncing to show the cache and cpu-utilization savings, but never got
that to settle on a solid number.  I took a look at Con Kolivas'
"contest" benchmark [1], but ultimately just published plain iozone
data [2].  The interesting bit is that cpu limited random writes saw
more throughput improvement than streaming writes because the i/o
processing did not need to compete with management of the stripe
cache.

--
Dan

[1]: http://users.on.net/~ckolivas/contest/
[2]: http://sourceforge.net/projects/xscaleiop/files/MD RAID
Acceleration/iop-iozone-graphs-20061010.tar.bz2/download