Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!zaphod.mps.ohio-state.edu!rpi!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <2399@crdos1.crd.ge.COM>
Date: 9 Aug 90 14:30:37 GMT
References: <1990Aug3.204358.330@portia.Stanford.EDU> <40694@mips.mips.COM>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 32

In article <40694@mips.mips.COM> cprice@mips.COM (Charlie Price) writes:
| >A 1Mbit DRAM chip may have a typical soft error rate of 
| >.001-.005 PPM/KPOH/bit. Suppose we have a workstation with
| >16 Megabytes of memory ( = approx 1.34 * 10^ 8 bits). This
| >yields a memory system error rate of .671 errors/KPOH, a non-negligible
| >number. Servers may have even more memory than this, and may
| >be running continually, so some errors are bound to occur. What
| >happens if a bit flips, and then the data is paged out or written to
| >a file ? The error is now permanent and can propagate. 
| >Why does no one worry about this ?

  The answer is that at those error rates the chances of a two bit error
(to avoid parity checking) are so low that it is not worth worrying
about. Not that paranoids like myself don't validate their files with an
external 32 bit CRC program on a regular basis.

| Most PCs (including the MACs I've seen) don't have or at least
| don't use parity.
| They silently accept occasional wrong computations rather than
| stop a computation that gets a transient memory error.
| Cost seems to be extremely important for PCs.

  The IBM PC, AT, and PS/2 models use per-byte parity, as do all of the
clone machines built by other vendors. This provides adequate
protection. The Mac and Amiga don't use parity (at least the older ones
don't). The term PC includes both business PCs, with minicomputer
features, and machines intended primarily for games and home use, which
are built as cheaply as possible for a customer base which doesn't
understand or care about data security, and which is highly price concious.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
            "Stupidity, like virtue, is its own reward" -me