Mode Parameter changes and when they take effect

Gerry.Houlder at Gerry.Houlder at
Wed Aug 6 14:50:03 PDT 2008

* From the T10 Reflector (t10 at, posted by:
* Gerry.Houlder at
The general principles developed over the years aren't described very much.
The principles are:
*  current values are updated and take effect when GOOD status of a MODE
SELECT command occurs. A SCSI device should always use the current values
to control its operations. Don't know why we don't have a statement to this
* default and saved values only matter when a reset event occurs, at which
time the device must choose which set of values to copy to current (based
on what it supports and what it can get access to).
The Mode Select wording does have some conflicts.
(a) This statement "Logical units shall maintain current and saved values
of each mode page based on any of the policies listed in table
151" seems to require all devices to support current and saved values. A
more accurate statement would be "shall maintain current and default values
(b) This statement "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"
clearly allows saved values to be optional [conflicts with statement (a)].
(c) This statement "If the logical unit implements no distinction between
current and saved mode pages and the SP bit is set to zero, ..." seems be
weasel words to allow a device to implement saved values and always require
them to be the same as the current values [sort of like not having current
values -- some might think this conflicts with (a)]. Perhaps this is for
your vendors that require a POR to make the new values effective.
Somewhere in all these words each of your vendors can find justification
for their implementation. This is why companies have invented procurement
specs ...
	     Kevin D Butt						   
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				       Mode Parameter changes and when	   
				       they take effect 		   
	     08/06/2008 02:09						   
I have a question related to Mode Parameters.  I have searched SPC-4 (and
earlier SPC versions) and cannot find guidance about when the changes to
mode parameters take effect.  This concern is related to configuration
settings and differences in expectations of various vendors.
Vendor 1 claims that any change to a mode parameter shall become effective
immediately and should not cause a device to reboot.
Vendor 2 claims that changes to some mode parameters require a reboot
before they can be accomplished.  As such the device automatically performs
a POR on completion of the Mode Select that changes one or more of these
Vendor 3 also claims that changes to some mode parameters require a reboot
before they can be accomplished.  This device,however, requires that an
outside entity send a command to force the device to perform a POR.  The
new values of the mode parameters are not in effect until after the POR.
In another form of when do parameters take effect, the requested change to
the format (or density) does not occur until a subsequent command (e.g., a
Format, a Write from BOT, etc.).
Is there any guidance in SPC?
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
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