Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!uunet!zds-ux!gerry
From: gerry@zds-ux.UUCP (Gerry Gleason)
Newsgroups: comp.periphs.scsi
Subject: Re: Two HAs on single SCSI bus
Message-ID: <590@zds-ux.UUCP>
Date: 2 Apr 91 21:37:01 GMT
References: <1991Mar27.175719.3474@seachg.uucp> <1991Mar31.190347.4360@netcom.COM> <581@zds-ux.UUCP> <1991Apr1.232550.21460@mp.cs.niu.edu>
Reply-To: gerry@zds-ux.UUCP (Gerry Gleason)
Organization: Zenith Data Systems
Lines: 44

In article <1991Apr1.232550.21460@mp.cs.niu.edu> bennett@mp.cs.niu.edu (Scott Bennett) writes:
>In article <581@zds-ux.UUCP> gerry@zds-ux.UUCP (Gerry Gleason) writes:
>>In article <1991Mar31.190347.4360@netcom.COM> feustel@netcom.COM (David Feustel) writes:
>>>   [text deleted  --SJB]

>>This is a lunitic fringe feature; not very common.

>     I'm not quite sure what your point is in making this remark.  If
>by "lunitic[sic] fringe feature" you mean that a) it is one that is not
>often supported in SCSI (or anywhere else, for that matter) and/or b)
>you'd have to be nuts to use a feature that is so poorly (in most cases)
>supported, then your comment is more or less correct.  However, if you
>mean that c) there is little use for such a feature and/or d) you'd
>have to be nuts to want the functionality that such a feature *ought*
>to provide, then you are clearly out of touch with how a vast portion
>of computer systems work and are used.

Sorry, I didn't mean to attack the sanity of those who might want such a
feature, but just what you said in a) and/or b).  Actually c) is not that
far off either, if a substantial number of users have a need for the feature,
it would be available (doesn't have to be big percentages either, 10-20%).
The fact that nobody has offered a pointer to somebody doing it is pretty
good evidence that the number that *must* have it is vanishingly small.

>The problem is usually that
>the hardware implementation is almost trivial, while the software design
>and implementation problem is almost intractable.  Even IBM has never
>come up with a very reasonable version of it.  The need, however, is
>*very* great.

Don't you think "intractable" is rather strong?  Of course, it all depends
and why you want to do it; some of the possible applications a quite
tractable.  Say your goal is single-fault-tolerance, you can do this with
a pair of standard machines each with two controllers, one on each of two
SCSI busses, mirrored drives, etc.  In this application, all you want to
do is switch from one initiator to the other, which is simple.  If, on
the other hand, you want to share disks NFS style, it gets a bit more
complicated since the two initiators must coordinate their activities.
This "lack of well defined feature set" issue probably means that it will
remain on the "linatic fringe", at least for a while, although it would
be nice if some of the controler/driver suppliers would at least provide
some hooks for integrators/developers to implement their own support.

Gerry Gleason