FC-TAPE Command Reference Numbers

Bob Snively Bob.Snively at Ebay.Sun.COM
Thu Oct 29 10:44:11 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Bob Snively <Bob.Snively at Ebay.Sun.COM>

While working on the FCP-2 document, I thought long and carefully about
how we use CRN.  I have made the following adjustments in the first revision
of FCP-2.  I hope everybody likes them.

1)  Naming the function

I have arbitrarily named the function provided by CRN as "precise ordering".

2)  Determination and enabling

At present, the FC-TAPE document has two independent mechanisms for
determining the support of precise ordering:

	a)  Process Login negotiates the support of precise ordering using
	    the CRNS (CRN supported) bit.
	b)  The FC Control mode page controls the enabling of precise
	    ordering using the ECRN bit.
Because of the splendors of the SCSI implementation, this dual enabling
is actually redundant.

The precise ordering function is actually a function that is decided upon
and required on a command by command basis for a certain class of devices.
As such, it is really a SCSI level functionality, not a Fibre Channel level
functionality.  And the SCSI level control function, provided by 
MODE SENSE/SELECT, is sufficient to properly identify, control, and
enable precise ordering.

Therefore I propose to drop the PRLI CRNS bit.

What remains is the ECRN bit managed by MODE SENSE/SELECT.

Each initiator can talk to its own applications in a vendor specific manner
to let the application client know that it supports precise ordering.  Of course
this is necessary anyway, since the application client must be capable of
instructing the initiator which commands should have precise ordering 
applied.  The application client then asks the device server of the target 
whether or not the precise ordering capability is supported using the
appropriate MODE SENSE command.  If there is pre-knowledge, it may be
sufficient to ask if the current ECRN bit is one, indicating not only that
it is capable of precise ordering, but presently executing it.  If there is
no pre-knowledge, the bomb-proof mechanism is to ask the device server if
the ECRN bit is changeable.  If it is, the application client can choose to
set it or clear it.  If it is not, the application client knows that the
function is not supported.

That all provides a clean, fool-proof, SCSI only mechanism, making the
PRLI CRNS an unnecessary request across some strange architectural boundaries.

Bob Snively			     Phone:	  (510) 574-9051
Sun Microsystems Computer Company      
Mail Stop NWK 04-104
901 San Antonio Road	             E-mail: bob.snively at sun.com
Palo Alto, CA 94303

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