Path: utzoo!utgpu!attcan!uunet!peregrine!elroy!gryphon!cadovax!mitchell
From: mitchell@cadovax.UUCP (Mitchell Lerner)
Newsgroups: comp.sys.next
Subject: Re: NeXT Memory - No Error Checking or Parity !
Keywords: addendum to my last posting...
Message-ID: <2304@cadovax.UUCP>
Date: 2 Nov 88 23:38:36 GMT
References: <549@gt-eedsp.UUCP> <2302@cadovax.UUCP>
Reply-To: mitchell@cadovax.UUCP (Mitchell Lerner)
Organization: Contel Business Systems, Torrance, CA
Lines: 43

I apologize for the half truths (which are often more harmful than lies) 
in my last posting.

What I now understand is this:

In the old days, memory wasn't that reliable and parity checking was 
implemented to bring the system down quickly so that damage was minimal 
due to corrupt data.

Parity used to be implemented on buses between processors and memory
but the logic and the technology got so refined that harware people 
eventualy found out that they never had an error across these channels, so 
they removed parity in that area of the system.

Today's memory is VERY reliable and (he said) it virtualy never fails one
cell at a time; usually the entire bank or group of banks fail.  I suppose 
that memory errors like this make failures much more obvious these days and 
the system will come down pretty quickly in the case of a memory failure.

The logic used for parity checking can introduce more errors into the
system if it should fail.

Implementing parity on a system slowes the system down.  With 100ns memories
and 200ns to compute parity, one cannot run a system as fast as without
parity.

When I told him that the Next computer was implemented without parity,
he said: "Well, I guess that guy is smarter than I give him credit for" :-)

We build multi-user business systems that are used for on-line accounting, 
order-entry, billing and such.  People's businesses depend on our systems and 
from what I understand, our systems are very reliable (software and hardware).

I talked to some of our field support people and they said that memories
just don't fail that often these days.  "We just don't see data disasters 
caused by memory failing these days".

Just one man's thoughts, not the opinion of Contel Business Systems.
-- 
Mitchell Lerner -- UUCP:  {ucbvax, decvax}!trwrb!cadovax!mitchell
	Don't speak to him of your heartache, for he is speaking.
	He feels the touch of an ants foot.  If a stone moves under the 
	water he knows it.