Mode Sense

Knight, Frederick Frederick.Knight at netapp.com
Thu Jul 10 10:33:37 PDT 2008


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There are 3 sets of mode pages (current, saved, and default).  Consider
the following sentence from the MODE SELECT description in spc4r15:
After a logical unit reset, each mode parameter header, block
descriptor, and mode page shall revert to saved values if supported or
default values if saved values are not supported.
This makes it clear that saved pages are not required.	It also says a
little further down:
If the logical unit does not implement saved mode pages and the SP bit
is set to one, then the command shall be terminated with CHECK CONDITION
status,
Not implementing saved pages is different than making no distinction
between current and saved pages.  Making no distinction means you treat
them as the same.  But not implementing them means you know the
difference, and you know that you don't have the saved ones.
    Fred Knight
________________________________
From: Eddy Quicksall [mailto:Quicksall_SCSI at Bellsouth.net] 
Sent: Thursday, July 10, 2008 9:19 AM
To: t10 at t10.org
Subject: Re: Mode Sense
Thanks, (do you have saved and current reversed in your last sentence?)
Now when I re-read based on your answer I come up with this ... do you
think this is correct?
The LUN always has current and saved pages, even though it is not
allowing the initiator to save changes. If there is no distinction
between current and saved then it would follow that if the current were
changed then the saved must be changed also (in order to maintain "no
distinction"). So the SP bit must be 1 (but if none of them are saveable
then nothing can be changed nor saved).
Is there something that tells the initiator that there is "no
distinction between current and saved mode pages"?
Eddy
	----- Original Message ----- 
	From: Kevin_Marks at Dell.com 
	To: Quicksall_SCSI at Bellsouth.net ; t10 at t10.org 
	Sent: Wednesday, July 09, 2008 9:56 PM
	Subject: RE: Mode Sense
	I would say, it means that current and saved are the same. You
cannot change the saved page without changing the current operating
mode, therefore you must always set the SP bit to 1, changing the
current and saved page. A bit of zero would mean changing the saved page
but not the current and this would not make sense if they were the same,
hence the CC.
	Kevin
	From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of
Eddy Quicksall
	Sent: Wednesday, July 09, 2008 3:08 PM
	To: t10 at t10.org
	Subject: Mode Sense
	6.9 MODE SELECT(6) command
	. . .
	A save pages (SP) bit set to zero specifies that the device
server shall perform the specified MODE SELECT
	operation, and shall not save any mode pages. If the logical
unit implements no distinction between current and
	saved mode pages and the SP bit is set to zero, the command
shall be terminated with CHECK CONDITION status,
	. . .
	If my target is not saving mode pages then I am assuming the
"logical unit implements no distinction between current and saved mode
pages". If that is the case then I don't understand the sentence because
it would seem the initiator would have to set SP to 0; but 0 would cause
a CHECK CONDITION. So I think I don't understand the meaning.
	Can someone explain what this means? 
	Eddy



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