Despite theoretical advantages, in practice RAID 5 nearly always results in
worse performance.  Even with a smart frame like an EMC this is
true.  We've done extensive benchmarking on customer's systems and maintain
that, while RAID 5 has significant cost advantages over other high
availability options, a performance penalty will have to be paid.

Bill

At 03:37 AM 12/8/99 -0800, Steve Dirickson wrote:
>That sounds like software RAID5, where the host computer has to do the
>parity calculations and the extra disk I/O in addition to its normal
>workload. With a dedicated RAID controller running the array, I don't see
>how a RAID5 array could have overall worse performance than a non-RAID setup
>except in a very write-intensive system. Well, unless the RAID controller
>was running on a Z80B or something....
>
> > I have to disagree.  I've never seen RAID 5 increase anyone's HP 3000
> > system performance.  In *many* cases I've seen RAID 5 seriously hurt
> > performance.  In many of those cases, going back to RAID 1 or
> > JBOD in turn increased performance.
> >
> > Bill
> >
> > At 04:29 PM 12/7/99 -0800, Steve Dirickson wrote:
> > > > over RAID level in these arrays.  If you use RAID 5 you'll
> > > > pay a price in performance.
> > >
> > >Probably not. RAID5 has slightly worse write performance
> > than a single
> > >spindle (due to the need to update the parity information), but
> > >substantially better read performance. Since reads outnumber
> > writes by an
> > >order of magnitude or two (or three...) in "normal"
> > installations, RAID5
> > >should *increase* system performance.
> > >
> > >Simple explanations/discussions:
> > >  http://www.digidata.com/raiddesc.html
> > >  http://www.acc-sd.com/site/raidlevels.htm
> > >  http://www.adaptec.com/technology/whitepapers/raid.html
> > (note the comment
> > >that "Database servers are an example" of where RAID5 works well)
> > >
> > >More extensive discussions:
> > >  http://www.eurologic.com/tn/tnwp2.htm
> > >  http://www.acnc.com/raid.html
> > >
> > >
> > >Steve Dirickson   WestWin Consulting
> > >[log in to unmask]   (360) 598-6111
> >