Question: Deferred Sense reporting priority

Rodney Van Meter Rodney.Vanmeter at QNTM.COM
Mon May 4 15:20:39 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Rodney Van Meter <Rodney.Vanmeter at qntm.com>
*


> -----Original Message-----
> From:	Jeff Stai [SMTP:stai at Brocade.COM]
> Sent:	Monday, May 04, 1998 11:03 AM
> To:	'Joseph C. Nemeth'; t10 at Symbios.COM
> Cc:	Jeff Stai
> Subject:	RE: Question: Deferred Sense reporting priority
> 
> * From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
> * Jeff Stai <stai at Brocade.COM>
> *
> Switching from my fabric guy role to my former life of writing SCSI
> target firmware, it occurs to me that you may already have decided how
> many Initiators you can actively support at one time, based on memory
> limitations in storing data structures for each Initiator. Within each
> data structure you could keep some sort of unit attention info. Let's
> try the following:
> 
> - An N_Port login (PLOGI) causes you to allocate an initiator data
> structure to that port (or maybe WWN). You at that time set up the UA
> info to indicate the appropriate condition. 
> 
> - Subsequent commands would cause the UA to be reported at the
> appropriate time.
> 
> - If you ran out of initiator data structures, you would have to
> reject
> additional PLOGIs because you have no place to keep the PLOGI
> parameters. You would never get to the point where you would need to
> report UA, and therefore would not need to keep 2^24 UA bits!
> 
> Hope this helps a little. I also hope someone makes any necessary
> corrections regarding the target ramblings above. I *am* a little
> rusty
> in that department...;-)
> 
> If you have any more questions about FL_Ports, please feel free to
> write
> or call!
> 
	Okay, I'll follow up with a related question.  Forgive me, too,
if I'm rusty on target implementations, it's been several years since I
did one.

	How many Unit Attention conditions should the target keep?

	If I recall correctly (and my memory is notoriously bad), for
the device I worked on, we could keep a small fixed number, so that we
could report both Power On and Media Changed, or something to that
effect.  When that number overflowed, we reused 'em, and anybody who
came in late gets the N most recent ones.

	Note that this applies regardless of whether it's Fibre Channel
or SCSI -- when you first come up, you're uncertain of how many
initiators may access you, so you keep Power On for all of them. In FC,
though, this is especially important -- your suggestion above reads like
it's okay to keep only UAs that occur *after* the PLOGI, but I don't
think that's what you meant.  That would mean that initiators could miss
that Power On or Reset condition, and the equivalent on a physical SCSI
bus would be inappropriate.

	Or have I gone senile, and N=1, keep only the most severe or the
most recent?  It's amazing how quickly you forget the details when
you're not doing this stuff day in and day out, I tell ya...

				--Rod


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