A/B Port bit in Inquiry Data

Larry Chen larryc at maxstrat.com
Mon Feb 2 17:19:13 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Larry Chen <larryc at maxstrat.com>
*

On Mon, 2 Feb 1998 15:51:09 -0800 (PST)  Bob Snively wrote:
>* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
>* Bob Snively <Bob.Snively at Eng.Sun.COM>
>*
>

<cut>

>	
>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.
>
Hi Bob,

In the future, some RAID device may have multiple paths to a LUN device
thru a fabric. In this case, one path may be much faster than the other
path and the system driver may want to take advantage of this extra
bit of information that the LUN can provide about its paths.

Larry
>Bob
>
>
>> >
>> >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.
>
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-------------------------------------------------------
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/
-------------------------------------------------------


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