A/B Port bit in Inquiry Data

Larry Chen larryc at maxstrat.com
Tue Feb 3 11:23:13 PST 1998


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

On Tue, 3 Feb 1998 07:24:37 -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>
>*
>
Hi Bob,

There has already been a few documented cases where an association
between a physical layer and ULP entity are needed.

Charles has already mentioned a SES example where the a/b concept
helps. Thanks Charles :)

Now, I'll give you a performance example here where the a/b bit
can help.

Assume a dual-ported RAID LUN device with WWN X.

LUN X can be accessed on port A using first level addressing
of the 8 byte LUN address structure.

LUN X can be accessed on port B using second level addressing
of the 8 byte LUN address structure. 

Assume for the moment that the "performance" on port A is 2x faster
than port B (and that the only device that knows this is the RAID
controller device). In this case, the RAID controller device could
use the a/b bit to pass this info to the host ULP.

Regards,
Larry
>Larry,
>
>I continue to state that you have complete information identifying the
>path.  There is no other information you can obtain through the
>standard SCSI command set.  Quality of service issues are addressed by
>information exchanged with the fabric components, and since you can
>precisely identify which port is where on the fabric, no additional
>information is needed.
>
>The tools are there, and I stand by my statement that there is no need
>for a SCSI device to internally create an a/b concept or an active/alternate
>concept when any QoS fabric can be attached to either port.
>
>Bob
>
>
>
>> >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.
>
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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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