The PPC bit in the LOG SENSE command

Mark Evans Mark.Evans at
Tue Nov 11 09:03:47 PST 2008

Formatted message: <a href="r0811110_f.htm">HTML-formatted message</a>

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
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 occurs:
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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