A/B Port bit in Inquiry Data

Bob Snively Bob.Snively at Eng.Sun.COM
Tue Feb 24 16:41:11 PST 1998


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

Larry, 

I am sorry, but I do not see the logic of your example.  If you have
two paths to a SCSI node at two different levels of the hierarchy,
the REPORT LUNS command will identify their presence.  A subsequent
INQUIRY command will provide a common node name, but different port
names.  You will then know everything that is knowable about the devices.

What may be missing is performance information for the two paths, but that
should be known from vendor specific information about the configuration
and structure of the first level entity.

In this case, the a/b distinction is totally meaningless, since it is
fully enclosed in the vendor specific properties of the fully identified
paths of the first level device and the second level device.

So what is the problem?

Bob


> Date: Tue,  3 Feb 98 11:23:13 PST
> Subject: Re: A/B Port bit in Inquiry Data 
> To: T10 at symbios.com, Bob Snively <Bob.Snively at Eng>
> MIME-Version: 1.0
> 
> 
> 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.
> 

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