Path: utzoo!attcan!telly!lethe!torsqnt!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!rutgers!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: comp.sys.mac
Subject: Re: Parity vs. non-Parity RAM in the Mac IIci
Message-ID: <4005@phri.UUCP>
Date: 22 Sep 89 14:07:07 GMT
References: <0xe23@deimos.cis.ksu.edu> <34935@apple.Apple.COM>
Reply-To: roy@phri.UUCP (Roy Smith)
Organization: Public Health Research Inst. (NY, NY)
Lines: 40

In article <34935@apple.Apple.COM> chuq@Apple.COM (Chuq Von Rospach) writes:
> RAM reliability is high enough these days that unless you're writing RFC's
> for government contracts or totally paranoid, not terribly useful. [...]
> There's no reason why a 'normal' user needs parity unless their applications
> require extremely high reliability.

	This is a matter of opinion, and clearly Chuq is entitled to his, but
I disagree.  We've gone through about 200 Mbyte-years of Sun-3/50 (parity)
memory here.  I've seen 3 memory failures which the parity systems caught.
That's one failure every 65 or so Mbyte-years.  The usual mode of failure is
an isolated parity error, then another one a few months or weeks later, then
occuring with increasing frequency until eventually it becomes a hard error
at which time it becmes easy to track down using memory diagnostics.

	Lets assume that Mac memory is about as reliable as Sun memory
(although this may be apples and oranges, we're also comparing 1989
technology to 1986 technology).  We've got about 30 Mbytes of installed Mac
memory (not quite true; 8 of those are sitting on a shelf in front of me, but
should be installed this afternoon, but then again, I'm also currently
writing budget requests for Macs with another 10-20 Mbytes worth of ram)
here, so we should expect to see on the order of one memory failure every
other year.  Without parity to detect it, all we'll see is random unexplained
crashes (or worse, silent data corruption).

	So, I conclude that while Chuq's opinion is not totally off the wall,
he is erring on the side of risk, while I'm taking a more conservative point
of view.  A typical user with a 1 Meg MacPlus at home might very well not see
a parity error for decades (i.e. never).  On the other hand, for any place
with an installed base of more than a couple of Macs, the odds that they will
see a parity error in the course of a few years is reasonably high.  And a
large shop, with dozens or even hundreds of Macs with lots of memory in each,
might be seeing many memory errors a year.  And, even if ram technology has
gotten better, there are other reasons why memory errors might occur (for
example, I suspect the snap-in SIMMs in Mac-IIs are more likely to suffer
mechanical connection problems than the soldered-in chips in our Sun-3/50s).
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"