Dual porting UDA50 drives under 4.2

[Joe Kelsey]

There is a point which everyone seems to be missing here.  DSA (Digital
Storage Architecture) drives CANNOT be "dual-ported".  They may be
"dual-pathed", but it is physically impossible to "dula-port" them.
What you need to consider is that a UDA50 really is not intelligent
enough to be able to handle the full MSCP protocol.  If you consider
what happens in an HSC50 environment, then you can see the problems
better.  When you have a VAXcluster, centered around the HSC50, the HSC
is responsible for coordinating ALL read/write access to all drives,
and the VAXen communicate other information about locking files,
records, etc., among themselves via the CI.  In this environment, it is
possible to dual-path the drives attached to a given HSC for physical
redundancy, i.e., if one HSC goes down, the other one already has the
cables connected to the drives and can pick up the traffic by using the
dual-path connection.  Thus, even in the HSC environment, only one
controller can be the master of the drives once the drive goes on-line.
The same thing hold true in the UDA50 case, but in this case,
dual-pathing a drive gains you NOTHING, unless you have two UDA50s on
your computer just in case one of them fails.

In otherwords, if you want to share disks, you have several choices:

1) Wait for the Ultrix people to figure out what they are going to do
about clusters, CIs, and HSCs.  Then, maybe, you can buy an HSC, star
coupler, a bunch of CI780s or CI750, and go to town.

2) Talk to SI about SIMACS - they claim that it runs on UNIX now.

3) Buy Massbus disks and come up with a way of mounting the disk R/W on
one system with caching disabled and R/O on the other system.

/Joe