98-145r1 -- Discovering If This Is SES Port A or Port B

Ralph O. Weber roweber at acm.org
Sun Jul 5 18:12:01 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Ralph O. Weber" <roweber at acm.org>

Date:	  4 July, 1998
To:	  T10 Membership
From:	  Ralph Weber, T10 Alternate member from Symbios, Inc.
Subject:  Discovering If This Is SES Port A or Port B

FC-AL drives are being implemented with the ability to have redundant
loops.  To utilize this redundancy systems are configured with
redundant host adapters or adapters with dual ports.  Let's label the
redundant loops from the host's point of view as loop X and loop Y.

Using the Device element entry in the SES Enclosure Control page, an
application client may independently manage the bypass circuits found
on dual port disk drives by setting the ENABLE BYP A (enable bypass
on port A) and/or the ENABLE BYP B bits. The physical connection from
the drive's port A (or B) to the host's loop X (or Y) is an FC cable,
often installed by the user.  So, the association between A and B and
X and Y is unpredictable and must be detected automatically by the
application client.  If the association is not detected correctly,
use of the SES Enclosure Control page ENABLE BYP X bits could easily
render device access impossible.

An application client might detect a configuration by trial and
error.  It could enable the bypass on port A and scan loops X and Y
to discover where the path to the device disappeared.  This is
unacceptable due to its disruptive nature, especially in multiple-
host environments.

The goal of this proposal is the definition of a mechanism for
identifying the port being used for communications.  The proposed
definition is intended to meet the requirements described by the SCSI
Working Group in May 1998.  All clause and table numbers reference
SPC-2 revision 3.

The Proposal

It is proposed that the Device Identification VPD page (SPC-2 clause
8.4.3) be enhanced to allow reporting of a port number that has a
defined relationship to the A and B identifiers used in SES.  The
specific proposed changes are as follows.

In Table 121 (Identifier Type), add code value 4h with the following

"If the Association value is 1h, the identifier value contains a
four-byte binary number identifying the port relative to other ports
in the device using the values shown Table x.  In this case, the Code
set field shall be set to 1h and the Identifier length field shall be
set to 4.  If the Association value is not 1h, use of this identifier
type is reserved."

Add Table x, titled "Relative Port Identifier Values", with the
following contents:

 Value       Description
   0h        Reserved
   1h        Relative port 1, also known as port A
   2h        Relative port 2, also known as port B
 3h-7FFFh    Relative port 3 through 7FFFh
8000h-FFFFh  Reserved

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