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From: donnh@ziggy.SanDiego.NCR.COM (Donn Holtzman)
Newsgroups: comp.arch
Subject: Re: Fault Tolerant Micros
Message-ID: <2484@ncr-sd.SanDiego.NCR.COM>
Date: 6 Feb 90 01:17:52 GMT
References: <13910004@hpisod2.HP.COM> <13910010@hpisod2.HP.COM>
Sender: news@ncr-sd.SanDiego.NCR.COM
Reply-To: donnh@ziggy.SanDiego.NCR.COM (Donn Holtzman)
Organization: NCR Corporation, Rancho Bernardo
Lines: 37

In article <13910010@hpisod2.HP.COM> dhepner@hpisod2.HP.COM (Dan Hepner) writes:
>
>Ideally FT would exist completely in the hardware, and present
>a platform to the OS which looks like a non-FT machine.
>
From my perspective one problem that exists with a "HW only" solution
to FT is the issue of SW failures. As the gentleman from Tandem
pointed out there are a class of faults (Heisen Bugs) which are very
timing and case dependant. A loosely coupled approach, such as
Tandem's, will recover from many SW based faults simply because the
timing and load characteristics are different on another processor. If
the bug causes your kernel to hang, TMR or pair-and-spare approaches
won't succeed. Performance is certainly and issue but one can trade
check pointing overhead for recovery speed (at least in the OLTP
arena).

On the other hand the "HW only" approaches are easier to explain and
sell. They certainly are conceptually simpler, if not actually simpler
to implement correctly. 

>The reality is that this can't be quite true.  FT vendors will
>always be required to supply whatever kernel support that their 
>idiosyncratic implementation requires, and an OS port to such a 
>machine will always be more difficult than on a non-FT platform.
>
This is a good point. I would be surprised if Tandem didn't have to
make kernel changes to make their machine work. But in this day of
standards and narrow market windows it was probably easier to sell
this approach to management then a large SW effort (kernel changes or
no kernel changes) to support a loosely coupled approach.

Interesting stuff.

Donn Holtzman
NCR E&M San Diego

Donn.Holtzman@SanDiego.NCR.COM