The PPC bit in the LOG SENSE command

Kevin D Butt kdbutt at
Tue Nov 11 11:20:21 PST 2008

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I believe the intent is not if a new parameter code is added, but if the 
value returned by parameter code x changes but the value returned by 
parameter code y does not change, then a PPC bit set to one specifies that 
parameter code x should be returned but parameter code y should not be 
returned.  I think this is the intent.
The other point you make is related to which I_T nexus this applies to. 
Since the text does not call out a specific I_T nexus the implication is 
that it relates to any I_T nexus as you point out.  The other possibility 
is that it could relate to a change in value returned by a parameter code 
since the last LOG SELECT or LOG SENSE received from the same I_T nexus 
that issued this LOG SENSE.  My suspicion is that the intent would have 
been the latter - that it relates to this I_T nexus only and the 
information needs retained on an I_T nexus basis.
With that said, my devices do not support a PPC=1 so this comment is 
purely from reading the spec and guessing what the intent is.
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-2869 / 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at 
"Mark Evans" <Mark.Evans at>
<t10 at>
11/11/2008 10:57 AM
The PPC bit in the LOG SENSE command
Hello all,
I was looking at the definition for the PPC bit in the LOG SENSE command 
in SPC-4.  My issue is with when the PPC bit is set to one.  List item a 
in the definition for the bit in SPC-4 reads:
A PPC bit set to one specifies that the device server shall return a log 
page with parameter code values that have changed since the last LOG 
SELECT or LOG SENSE command.  The device server shall return only those 
parameter codes that are greater than or equal to the contents of the 
PARAMETER POINTER field in ascending order of parameter codes from the 
specified log page?
These are almost the same words used to define the meaning for the PPC bit 
set to one in SCSI-2.  This definition places no bounds on, ??since the 
last LOG SELECT or LOG SENSE command??.  In addition, as I read it, the 
only way a parameter code can change is if a new parameter code is added 
to a log page.	To me this all means that, if the following sequence 
1)	a LOG SENSE command was received on one I_T nexus 20 years ago;
2)	many parameter codes have been added in the interim, along with 
the occurrence of many power cycles, resets, etc.;
3)	a LOG SENSE command from a different I_T nexus was received ten 
minutes ago;
4)	no parameter codes have changed in the last ten minutes; and
5)	a LOG SENSE command is received on the first I_T nexus, 
then no parameter data would be returned.
Another odious implication of this definition is that context shall always 
be maintained identifying any new parameter code that has been added until 
the device server receives a LOG SENSE command or a LOG SELECT command, 
again, this could be on ANY I_T nexus.
I think that whatever meaning this bit once had was lost a long time ago 
in a galaxy far, far away.  Unless someone can point out something I?m 
missing, I think the PPC bit should be made obsolete, and I will generate 
a proposal to do just that.
Please feel free to call or send an email to me if you have any additional 
questions about this stuff.
Mark Evans
Western Digital Corporation
5863 Rue Ferrari
San Jose,  CA  95138
Email: mark.evans at
Office: 408.363.5257
Fax: 408.363.5139

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