A/B Port bit in Inquiry Data

Bob Snively Bob.Snively at Eng.Sun.COM
Mon Feb 2 15:51:09 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Bob Snively <Bob.Snively at Eng.Sun.COM>

As a reminder of the mechanism that was selected to handle port management,
the dual port stuff was essentially thrown out some time ago.  The 
mechanism that was put in place to address the questions that were raised
by dual ports were two:

	Persistent reservation allowed establishment of reservations for
	multiple initiator/port environments and allowed those initiators
	in charge of the configuration to eliminate failing initiators from
	the permitted group.  While there is still some work going on in this
	area, the principles are sound.
	The Device ID "page" in the Inquiry Data included a node identifier.
	After SPC, but before SPC-2, this node identifier was extended to allow
	multiple identifiers in a single field.  One of the identifiers was
	intended to identify the port uniquely, providing all the tools that
	were necessary to complete the management of multiple port devices.
For that reason, it is certainly unclear to me that any additional
flag information is required to properly identify and manage multi-port
devices, including large RAID devices.  It is even less clear to me that
we want to drag in either an "a/b" concept or an "active/alternate" concept.


> >
> >Several of the drafts of SPC identified bit 5 in byte 6 and the MultiP
> >bit.  Its function was as you describe, 0=A-port 1=B-port.  The bit has
> >since been marked vendor-specific, so that implementations that used
> >the MultiP bit are still good.
> >
> >Although I cannot speak for the T10 committee, my understanding is that
> >the desire is to allow implementations with more than two ports.  Hence,
> >a single bit is insufficient to identify which port, and thus the fate
> >of the MultiP bit.  Since there is no agreement on the proper upper
> >limit for the number of ports, no field has been defined in the 
> >Inquiry data to take the place of MultiP.
> >
> >All in all, you may wish to consider using the vendor-specific bit
> >at bit position 5 in byte 6 to implement the port identifier flag
> >described in your mail.
> >
> >Thanks.
> >
> >Ralph...
> -------------------------------------------------------
> Larry Chen             Tel: 408.383.1600 (x116)
> MAXSTRAT Corporation   Fax: 408.383.1616
> 801 Buckeye Court      E-mail: larryc at maxstrat.com
> Milpitas, CA 95035     URL: http://www.maxstrat.com/
> -------------------------------------------------------

> > >
> > >I was wondering whether I could get something into SPC-2 ??
> > >
> > >I want to use bit 5 of Byte 6 of the Standard Inquiry Data
> > >for the following purposes:
> > >
> > >0	- port A or Primary Port
> > >1	- port B or Alternate Port
> > >
> > >Note:
> > >The use of Port A or Primary Port maybe preferred over Port B or
> > >Alternate Port for performance reasons by the target device.

* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at symbios.com

More information about the T10 mailing list