From owner-cvs-all  Sun Oct 28  3:58:43 2001
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Date: Sun, 28 Oct 2001 22:57:33 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Greg Lehey <grog@FreeBSD.org>
Cc: Matthew Jacob <mjacob@feral.com>,
	Doug Rabson <dfr@nlsystems.com>, Luigi Rizzo <rizzo@aciri.org>,
	John Baldwin <jhb@FreeBSD.org>, Jonathan Lemon <jlemon@FreeBSD.org>,
	<cvs-all@FreeBSD.org>, <cvs-committers@FreeBSD.org>
Subject: Re: RAID-5 parity calculations (was: cvs commit: src/sys/dev/fxp
 if_fx)
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On Sat, 27 Oct 2001, Greg Lehey wrote:

> On Thursday, 25 October 2001 at 15:24:06 -0700, Matt Jacob wrote:
> >
> > And the fastest software RAID-V I've known was at NASA/Ames on the
> > Convex 3280s- they used the otherwise unused vector units for parity
> > calculations- this gave write performance for a 22 wide stripe on a
> > terabyte fileystem to be at about 88% of theoretical maximum, which
> > sure aint' bad.
>
> The parity calculations for RAID-5 are several orders of magnitude
> faster than the disk accesses.  Even on a 486, they took hardly any
> time.

Actually, a 486 can't possibly have been more than about one order of
magnitude faster than the disk accesses, since main memory was only
that much faster (usually less).  My 486DX2/66 has 15MB/sec main memory
and a 2MB/sec disk.  It would be possible to upgrade the disk (but not
the memory).  Then the disk would want to transfer at about half an
order of magnitude faster then the memory.

Bruce


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