From: lpc@dickens.com (Luis P Caamano)
Subject: Re: Sharing SCSI devices between machines
Date: Wed, 29 Jul 1992 16:50:39 GMT
Organization: Dickens Data Systems, Inc.

In article <1992Jul29.143619.16834@grace.dsir.govt.nz> srghdxp@grace.dsir.govt.nz writes:
>I have been able to successfully share some SCSI devices
>between a RS6000 320H and two MicroVAXII's by putting all
>the devices and controllers on the same SCSI bus. Although

[ some stuff deleted]

>   
>The purpose for doing this is to allow the use of the tape
>drives from all three computers. I am not attempting to
>share access to the disks and so far the BUS seems ok with
>this load. 
>
>The system will only work if I use the IBM external SCSI
>terminator which I understand is not ANSI standard. Reading
>the info section on SCSI interface under the hardware
>technical reference manual, there is a section on the
>"Highly Available Configuration" which seems relevant. It is
>intended to share SCSI units between RS6000's and states the 
>following limitations among others; 
>
>   o No internal disks
>   o Use Highly Available Software
>
>I would like to know the following;
>
>1. Is the internal SCSI terminator on a 320H an ANSI 
>   standard terminator ?

No.

>2. Are the pass through terminators used in the "Highly 
>   available configuration" ANSI standard?

Big No or maybe better, "no way."

>3. Why the limitation on no internal disks and is it only
>   relevant if the 320H is switched off?

I don't know the exact answer about the limitation but I know
it is relevant even if the 320 is off since the terminator is
part of the SCSI bus even if the controller is off.

>4. What does the "Highly available configuration software"
>   do and is it necessary?

HANFS is a software that requires a specific hardware configuration
to work.  This configuration is a pair of 5xx RS/6000 that share
a group of disks in a common volume group.  Each machine has two
network adapters (TR or eth), and two scsi adapters, one for its
internal disk (boot disk) and another one for the shaed scsi bus.
The SCSI cards are common simple IBM scsi cards. You'll need
a minimum revision level for it to work though. 

These two machines are connected to a network, and clients in that 
network mount filesystems from the shared volume group using NFS.  
The key point is that one of the machines is the active server and 
is the one that has exclusive access to the shared disks and thus, 
services all the NFS requests.

Now, if that machine goes offline the backup will detect that condition
(That's one the functions of the HANFS software), and then the backup
will tell its second network adapter to use the alternate hardware 
address which is configured to be the same hardware address as the 
network card in the server.  By doing this, the backup is going to 
get all the requests addressed to the failing server.  Next,
the backup will get new reservations for the scsi disks and
takeover the disks, fsck the filesystems, logredo, mount them, and
NFS-export them.  This process is known as "takeover".  When the
backup detects that the server is back online, it will start the
"reintegration" part which is an undo of the takeover.

If you're sitting on a client writing to a filesystem on the shared
volume group, you'd see a delay of about 70 seconds for the write or
read to complete.  This delay is the time it takes today for the
takeover and reintegration to complete.

>5. Would SCSI Forced Perfect Terminators (FPT) be better in
>   my application and do IBM sell them (part No)?

Even though you're configuration won't be supported neither by IBM nor
DEC nor the tape manufacturers, I think that the FPT will help you.
External SCSI terminator part No. 00G0968. You DON'T want the old
terminator P/N 70F9761.

>6. What exactly does jumper J1 on the 320H SCSI board do and
>   should I remove it.
>

Sorry, don't know that one.

-- 
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Luis P. Caamano                    |         lpc@dickens.com
Dickens Data Systems, Inc.         |         uunet!ddssuprs!lpc
Norcross, GA                       |         (404) 242-5991
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If I think I know it all, I'll stop learning. -myself
The more I learn, the more I know I know nothing. -somebody else