Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!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: <593@zds-ux.UUCP>
Date: 4 Apr 91 16:55:06 GMT
References: <581@zds-ux.UUCP> <1991Apr1.232550.21460@mp.cs.niu.edu> <590@zds-ux.UUCP> <1991Apr3.004833.20688@mp.cs.niu.edu>
Reply-To: gerry@zds-ux.UUCP (Gerry Gleason)
Organization: Zenith Data Systems
Lines: 54

In article <1991Apr3.004833.20688@mp.cs.niu.edu> bennett@mp.cs.niu.edu (Scott Bennett) writes:
>In article <590@zds-ux.UUCP> gerry@zds-ux.UUCP (Gerry Gleason) writes:
>>In article <1991Apr1.232550.21460@mp.cs.niu.edu> bennett@mp.cs.niu.edu (Scott Bennett) writes:
>>Don't you think "intractable" is rather strong?  Of course, it all depends

>     No, and I did say "almost."

I guess I disagree.  It is true that there are a lot of possible approaches
to the problem, many of which are intractable, but there are clean solutions
if you really need this.

>>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

>     True and, if a human were fast enough, a human could make the
>changeover by hand.

What's your point, that this is a trivial use of the capability?  Not
for people who want this.

>>the other hand, you want to share disks NFS style, it gets a bit more
>>complicated since the two initiators must coordinate their activities.

>     I did mean sharing disks, but *not* NFS-style.  NFS does *not*
>share disks.  NFS has a server process running on a machine that has
>a disk dedicated to that machine and the server process can maintain
>integrity and, where necessary, serialization.  There is no sharing
>involved with NFS.  Shared disks, which I thought the earlier writer
>was refering to, pose a very difficult set of *software* problems,
>though the hardware problems aren't usually that bad.

I'm not sure any more if the original posting just asked about two
initiators or sharing disks, nor did I intend to produce an exhaustive
list of applications.  Further, from an application perspective
their is not really an important difference between "NFS-style" and the
type of disk sharing you are talking about.  Performance, yes, but
functionally, not really, particularly if you ignore details like the
two machines having different view and privledges on the shared disk
(really a feature that give more capability, not less).  I'm saying
your distinction is implementational, not functional.

>     In fact, we use something like it in a few cases on our Amdahl
>5890-300, which is running three domains.  However, the support provided
>by MVS/XA is not exactly wonderful at making it easy for the problem
>program to maintain data integrity in shared files.  I guess it has
>some way of keeping the VTOC from turning to spaghetti, but the data
>in the files aren't well protected by the system.  [ ... ]

Sounds like they chose one of the intractable aproaches.

Gerry Gleason