P1394 Mail Archive: Re: P1394> units and logical units definition

P1394 Mail Archive: Re: P1394> units and logical units definition

Re: P1394> units and logical units definition

Eric Anderson (ewa@apple.com)
16 Mar 98 14:23:47 -0800

> >
> > Greg Shue writes:
> > > The SBP-2 proposal allows this to be modeled as:
> > > 1 unit with:
> > > 2 logins to the Print Job logical unit
> > > N logins to the Printer Status logical unit
> > > 1 login to the Printer control logical unit
> > >
> Eric Anderson:
> > So at face value I would prefer the first model (above) over
> > the second.
> >
Greg Shue:
> The big problem with the first model is that you cannot have
> multiple connections between device status applications and
> device status services when the applications reside on the same
> node. This _requires_ that multiplexing be done somewhere in the
> system.

Hmm, this is an interesting part of SBP-2. No initiator node is
allowed to have more than one login to the same LUN in a target.
I believe it was assumed that any initiator that is so complex
that it might want multiple logins to the same LUN must also be
so smart that it would be able to multiplex a single login in
software.

If this problem only comes up in computers (and similar devices)
then maybe us software folks can do the multiplexing. I didn't
see details of the Status logical unit in the latest profile,
but if it has no side-effects, multiplexing access to it might
not be too hard.

On the other hand, if you forsee devices like printers or scanners
that might have two or more independent functions that both want
status from the same target, then it might be more difficult for
them to share one login, because of the nature of their implemen-
tations. Do you think this second case is likely, and if so, do
you think that the difficulty of multiplexing one connection is
sufficient to warrant using another model of units and logins?

Or should we consider asking SBP-2 to allow multiple logins to one
LUN from the same initiator? It's not clear to me that the limit
of one login per LUN per initiator causes any real savings in cost
or complexity in the target.

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Eric Anderson ewa@apple.com
Apple Computer, Inc. 408-974-8187
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