From owner-freebsd-hackers  Tue Jul 14 12:50:48 1998
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From: Wilko Bulte <wilko@yedi.iaf.nl>
Message-Id: <199807141805.UAA03774@yedi.iaf.nl>
Subject: Re: Software RAID-5 performance
In-Reply-To: <19980714122952.L754@freebie.lemis.com> from Greg Lehey at "Jul 14, 98 12:29:52 pm"
To: grog@lemis.com (Greg Lehey)
Date: Tue, 14 Jul 1998 20:05:16 +0200 (CEST)
Cc: tlambert@primenet.com, gibbs@plutotech.com, andre@pipeline.ch,
        Matthew.Alton@anheuser-busch.com, Hackers@FreeBSD.ORG
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As Greg Lehey wrote...
> (trimming -fs)

> > Software RAID is a data integrity issue, not a performance one,
> > and I think making the performance argument for whatever reason
> > (protection domain crossing, interleaved I/O, SMP scalability,
> > etc.) is a strawman at best.
> 
> I'm not sure that I understand what you're saying here.  Obviously
> offloading the checksum calculation (or anything else, for that
> matter) to an external box will offload the CPU.  And I can't see any
> particular difference in data integrity between the two approaches.

Software raid without stable (battery backed) storage is flawed:

assume a write that had 2 disks actually write the data onto the platter
and (e.g.) the parity data did not make it out on it's platter because your
system crashed. Or the power went bang. Or...

You now have an inconsistent raid5 set.

Wilko
_     ______________________________________________________________________
 |   / o / /  _  Bulte 				  email: wilko @ yedi.iaf.nl 
 |/|/ / / /( (_) Arnhem, The Netherlands          WWW:   http://www.tcja.nl
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