On Wed, Jan 19, 2005 at 08:36:52PM +1100, Daniel Carosone wrote:
> On Wed, Jan 19, 2005 at 10:55:27AM +0200, Mihai CHELARU wrote:
> 
> > START disks
> > /dev/wd0e
> > /dev/wd1e
> > /dev/wd2e
> > /dev/wd3e
> > START layout
> > 32 1 1 5
> 
> A 4-unit raid-5 is almost always going to suck for filesystem
> performance. There is virtually no layout that will easily map a
> useful filesystem blocksize onto the underlying 3-unit stripe (plus
> parity).
> 
> Therefore, for almost every write you do, you wind up doing a
> read-modify-write cycle; that's what kills your performance.

Note that, with this hardware configuration, you can't even paralelize
accesses to the hardware, because you have 2 disks per IDE channel (so
some access have to be serialized). Things would be a bit better if you could
have one disk per channel (for each write, you have to write data and
partity, at last if those can be paralelized, you get soome performance
boost).
But anyway, if you want good write performances, go with raid1.

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--