Newsgroups: comp.unix.ultrix
Path: utzoo!utgpu!watserv1!watcgl!idallen
From: idallen@watcgl.waterloo.edu (Ian! D. Allen [CGL])
Subject: Re: Pair of DS5400's with cross-mounted RA90's in AB mode
Message-ID: <1990Aug26.162359.9371@watcgl.waterloo.edu>
Organization: University of Waterloo
References: <1990Aug23.021922.13346@watcgl.waterloo.edu> <1578@shodha.dec.com>
Date: Sun, 26 Aug 90 16:23:59 GMT
Lines: 40

>	finishes coming up.  There is though a period of time when both 
>	systems have equal access to the drives without either having 
>	them mounted.
[...]
>	"How safe" is the real question?  Having two systems that can get
>	to a disk doubles the chance that something can go wrong.  What
>	if the hardware that makes A and B exclusive breaks at the same
>	time one of the controllers also breaks and starts writting random
>	bits?  Not very likely, but it could happen.  Actually you don't
>	even have to have the disk break.  If both controllers break at
>	about the same, one allows the disk to go offline letting the other
>	have access to it you get the same result.

Yes, one 5400 usually ends up knowing about its own two disks and the
two on the second machine (which it sees when the second machine goes
down or reboots).  The second 5400 only knows about its own two disks
(because the first has its own disks mounted, preventing the second
machine from even finding them).

Since even when a kernel knows about all four disks, it only mounts
its local two, I think I'm pretty safe from having the same disk mounted
on two machines simultaneously by mistake.

I've had some mysterious "hang" situations with the machines.  I'm
running them now with only the A ports selected, to see if the hangs
recur.  I have the nasty feeling that having two kernels recognize the
same disk is causing problems, even if only one system actually mounts
the disk.  Perhaps there are things that kernels do even to unmounted
disks that would interfere with those disks while they are being mounted
and used by another kernel.  I can imagine that when a kernel goes to
find out if a disk is there, it might do something that would interfere
with the concurrent use of that disk by another kernel.  Or, a disk
might generate some message or interrupt to the kernel that would end
up being fielded by *both* kernels, and funny things might happen.

Oh well.  It would have been so convenient.
I'll try A+B mode again in a few days.
-- 
-IAN! (Ian! D. Allen) idallen@watcgl.uwaterloo.ca idallen@watcgl.waterloo.edu
 [129.97.128.64]  Computer Graphics Lab/University of Waterloo/Ontario/Canada