Descriptor Sense question

Knight, Frederick Frederick.Knight at netapp.com
Thu Sep 15 04:39:00 PDT 2011


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IN yesterday's T10 meeting, the discussion was that DESC overrides.
Since the host knows that it wants, that request wins over the other
rules.	In addition, we are clarifying the rule in SPC-4.  Here is one
take at what it might look like (this text is not committee approved):
When sense data is returned in the same I_T_L_Q nexus transaction (see
3.1.67) as the status, the response code field shall be set to 70h in
all unit attention condition sense data in which:
a)	the additional sense code field is set to 29h; or
b)	the additional sense code is set to MODE PARAMETERS CHANGED.
So:
1)	The host gets what they ask for (DESC)
2)	If the host doesn't ask (it is autosense, not a REQUEST SENSE
command), then the host gets what D_SENSE says they will get (which
includes the 4.5.1 exception for some UA sense data).
Fred Knight
From: Rajiv Madabhushi [mailto:RMadabhushi at plianttechnology.com] 
Sent: Monday, May 09, 2011 10:16 PM
To: t10 at t10.org
Subject: Descriptor Sense question
Hello,
I have some questions regarding the Descriptor Sense format.  In SPC4
(I'm looking at rev 29), 
4.5.1 Sense data introduction
...
The RESPONSE CODE field shall be set to 70h in all unit attention
condition sense data in which:
a) the ADDITIONAL SENSE CODE field is set to 29h; or
b) the additional sense code is set to MODE PARAMETERS CHANGED.
I interpret this to mean that the various types of resets (POR, bus
reset, etc.) as well as mode parameters changed should be in fixed
format...  
Seems clear enough that sense bytes sent for subsequent host commands
following these particular unit attentions may (or may not) be in
Descriptor Format, and the onus is on the host to understand that.
1.	 I was wondering if the drive (target) were coming ready
(sending 02/04/01 K/C/Qs), say on a POR, may those sense bytes be sent
with Descriptor format?  
During that time, the drive would not be honoring mode sense commands,
so I was curious to know how the host would know... 
I suppose looking at the response code (70h vs 72h, for example)...?
My second set of questions is regarding the request sense command
itself... if the drive (target) were not SPC4 compliant, and thus did
not support the D_SENSE bit in control mode page 0x0A, and thus did not
support the DESC bit 
In the request sense cdb, I see according to table 263 in section 6.29
that it will ignore the DESC bit and continue sending sense bytes in
Fixed Format:
Table 263 - DESC bit
...
Note: Device servers that are compliant with SPC-3 are capable of
ignoring a DESC bit that is set to one.
So this makes sense to me I think... if the D_SENSE bit in mode page
0x0A is 0 (CLEAR), and a request sense command with DESC bit is 1
(SET)... then the SPC-3 compliant drive will return sense bytes in fixed
format... and the host will have to honor that, I assume.
2.	 Now if the drive were SPC-4 compliant, but the D_SENSE bit in
mode page 0x0A is 0 (CLEAR), but the request sense command's DESC bit is
1(SET)... what does that mean?	Does the request sense command's DESC
bit take precedence over the D_SENSE bit in the mode page?
3.	 Next, what about the other configuration... if the D_SENSE bit
in mode page 0x0A is 1 (SET), and a request sense command's DESC bit is
0 (CLEAR)... again, does the command's DESC bit take precedence?
4.	 What about cases where some hosts are SPC-4 compliant, and
others are SPC-3 compliant... If the SPC-4 compliant host were to set
the D_SENSE bit, and expect sense bytes in Descriptor Format... then the
SPC-3 host I assume would not be able to speak with the drive?
Thanks in advance.
-Rajiv Madabhushi
Pliant Technology
(408 321-0320, x108



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