12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Dave Chinner]

On Sat, Dec 13, 2008 at 11:40:11AM -0600, Eric Sandeen wrote:
> Martin Steigerwald wrote:
> 
> > At the moment it appears to me that disabling write cache may often give 
> > more performance than using barriers. And this doesn't match my 
> > expectation of write barriers as a feature that enhances performance. 
> 
> Why do you have that expectation?  I've never seen barriers advertised
> as enhancing performance.  :)
> 
> I do wonder why barriers on, write cache off is so slow; I'd have
> thought the barriers were a no-op.  Maybe I'm missing something.

Barriers still enforce ordering in this case, so it affects the
elevator algorithm....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com