A/B Port bit in Inquiry Data

Binford, Charles cbinford at ppdpost.ks.symbios.com
Tue Feb 3 10:16:00 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Binford, Charles" <cbinford at ppdpost.ks.symbios.com>

Hello Bob,

I agree with your comments in general.  However, I don't see how I can use 
the mechanisms you mention below to solve the problem of mapping a path to a 
device to either port A or B in SES.  Consider the following:

       |     Host     |
       |              |  Host with 2 FC adapters - hba1 and hba2
       |              |
       | hba1    hba2 |
           |      |
          /        \
  Loop 1 /          \ Loop 2
        /            \
  |  esm device            |
  |                        |   FC JBOD with 5 dual-ported drives
  | d1   d2   d3   d4   d5 |   (d1 - d5), plus a dual ported enclosure 
  |------------------------|   device (esm).

The SES definition of the Device element page (rev 8a, page 39) provides the 
ability to enable and disable the bypass for port A and port B 
independently.  In my example above, I'd like to use SES to bypass ports 
connected to loop 2 (say a port's receiver broke and brought the loop down). 
 Given the picture, the host would issue an SES command via hba1, across 
loop 1, to the esm device.  In the SES data, the host would indicate that 
the esm should bypass the 'B' ports of the drives.

Now, lets consider the assumptions I just made:

 - The "loop 2" connector on the JBOD box is connected to the 'B' port of the 
drives.  Valid assumption, manufacture can ensure that.

 - The ESM device interprets the 'ENBALE BYP B' bit in the SES data as being 
associated with loop 2.  Again, a valid assumption because it is under 
control of the JBOD manufacturer.

 - Loop 2 is connected to hba2. INVALID assumption.  The *user* ran the 
cables from the JBOD to the HBAs, software has no way to know if loop 2 
(port B on the drives) is connected to hba1 or hba2.

Another way to look at the same problem is this.  Assume that loop 2 goes 
down for the same reason (broken receiver in a port).  The host knows that 
the remaining good path is loop 1, **but he doesn't know if he should tell 
the esm device to bypass loop A or loop B ports**.  There must be some 
reliable way to know if hba1 is connected to port A or port B of the drive.

The bit in the Inquiry seems to be a handy way to accomplish this.  Granted, 
it is limited to only two port devices, but then again so is SES's Device 
element definition.

If you have another way to solve the problem I'd love to here it.

Charles Binford

>From: Bob Snively
>To: T10 at symbios.com
>Subject: Re: A/B Port bit in Inquiry Data
>Date: Monday, February 02, 1998 5:51PM
>* 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" 
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