SAS - Protocol-Specific Port log page

Elliott, Robert (Server Storage) Elliott at hp.com
Wed Dec 12 12:19:20 PST 2007


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________________________________
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Mike Berhan
Sent: Wednesday, December 12, 2007 12:49 PM
To: T10 at t10.org
Subject: SAS - Protocol-Specific Port log page
I am currently working with a SAS 1.1 device and reviewing the SAS-1.1 and
SAS-2 specification.  I have some questions / comments regarding the protocol
specific port log page for SAS.  Before I get to that, I need to provide some
simple screenshots of an actual SAS 1.1 device to help describe my question.
The first eight bytes of the "Protocol-Specific Port log parameter for SAS"
are similar between SAS-1.1 and SAS-2.	The SAS-1.1 decoding is below.	SAS-2
marks the "DS" bit as Obsolete, uses "Format and Linking" instead of "LBIN"
and "LP", and defines offset 6 as the "Generation Code."
[cid:003105519 at 12122007-2BCF]
An array of "SAS Phy Log Desriptor" entries follows this 8 byte header.  The
number of entries in the array is given to us in the "Number of Phys" entry
seen above.  OK, so far so good.
In reviewing SAS-1.1, the SAS phy log descriptor is a fixed 48 byte structure
as you see here:
[cid:003105519 at 12122007-2BD6]
Traversing through the array of SAS phy log descriptors is rather simple as
you know the number of Phys and you know that each descriptor is 48 bytes in
length.
The complexity comes with SAS-2.  This specification defines variable length
SAS phy log descriptors.  Offset 3 of the above structure is now defined as
the "SAS Phy Log Descriptor Length".  The structure is	larger than 48 bytes
and contains a variable number of Phy Event Descriptors.
In my task, I do not have the luxury of enumerating various information from
the device so I do not know if it's a SAS-1.1 or SAS-2 device.	All I have is
the log sense data to analyze.	I need to know if I'm looking at a SAS-1.1
descriptor or a SAS-2 descriptor.  The first method, or perhaps I should call
it a hack, that comes to mind is:
If ( DescriptorOffset[3]==0 )
   // It's a fixed 48 byte descriptor (i.e. SAS-1.1)
else
   // it's a variable length descriptor, total len = DescriptorOffset[3]+4
RE: That's the intended algorithm.  Always consider that byte to be a LENGTH
field, but special-case the length of 00h to mean 48 total bytes.
Now finally to my question.  If all I have access to is this log page, would
the above pseudo code be the preferred method of knowing which descriptor the
device is returning?  If not, is there another field in the descriptor that
would be preferred?  Should a "Version" field be introduced so that the
application layer knows which version of the structure they are looking at
and which fields are valid?
RE: T10 usually rejects Version fields; witness the response to 07-403r0.
Another question.  If I could enumerate more information from the device,
what would be the preferred way to detect a SAS-1.1 vs SAS-2 device from the
application layer?  Would the "Version Descriptors" in the Inquiry page be
the recommended method?
Thinking out loud, should SAS-2 introduce a new "cleaner" sub-page instead of
expanding the current page?  I am not a big fan of variable sized arrays
nested within other variable sized arrays.
RE: The page needs to report a variable number of events (vendor-specific)
for a variable number of phys (fixed by the hardware design) configured as a
variable number of ports (determined by what the phys happen to be attached
to); I don't think any other solution is going to be much cleaner.  The event
information could be given its own subpage, leaving the old page alone, but
log page subpages didn't even exist in SPC-4 until June 2006; the event
descriptors were added to SAS-2 in January 2006.
-------
Mike Berhan
busTRACE Technologies
9700 Village Center Drive
Suite 50-F
Granite Bay, CA  95746
916.773.4554 phone
916.218.6283 fax
http://www.bustrace.com



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