RE: Response Fence Flag

The problem is that the definition of "previous" in this context is most
likely flawed.
 
The "Response Fence" is merely an internal and untestable state machine
input that
attempts to order the transmission of responses in the target port's
queue.
It  does nothing to improve the overall ordering of the responses at the
application
client.
 
Drivers for generic SCSI transports must always be aware that requested
operations
report completion with a more or less arbitrary time relationship to
each other and must be
designed to tolerate that.  
 
Using LOGICAL UNIT RESET as an example, the operation does not act only
on
the device server.  As it passes down through the initiator stack, it
may clean out
previously offered commands that have not yet been transported.  Then as
it
passes up through the target stack, it may clean out previously offered
commands
that have not yet been operated on.  As it passes through the device
server
execution path, it may clean out previously offered commands that have
begun
execution and some that have completed execution without yet responding.
As its 
response passes back down the target stack, it may clean up commands
that have completed for which the response has not yet been transmitted,
and as
it passes back up the initiator stack, it may clean up commands for
which the
response has been transmitted, but not yet been made available to the
driver or
application.  SAM-4 attempts to define the passage through the target
port,
through the device server, and back to the target port.  However, beyond
the
initiator port is outside the SAM-4 scope, and even where it is defined,
the "shall ensure"s are unlikely to be testable and in some cases not
implementable. 
 
Will I sometimes get command complete indications for commands after
a LOGICAL UNIT RESET response is received?  You bet.  Response Fence
doesn't
change that.
 
Will I sometimes get command aborted indications for commands after a 
LOGICAL UNIT RESET is response is received?  You bet.  Response Fence
doesn't change that.
 
Is this a problem?  It never has been in the past, why is it now?
 
Can you test the corner conditions for a fence operation?  I doubt it.
The
existence of the "Response Fence" argument is internal to the target
implementation and invisible to testing.
 
Bob
 

________________________________

From: Elliott, Robert (Server Storage) [mailto:[email protected]] 
Sent: Wednesday, January 03, 2007 3:35 PM
To: [email protected]
Subject: RE: [Ips] Response Fence Flag



SAM-3 and SAM-4 require all transport protocols to tag the TMF responses
with the requests:

"Each SCSI transport protocol shall allow a Received Task Management
Function Executed confirming completion of the requested task to be
associated with the corresponding Send Task Management Request."

Although iSCSI was based on SAM-2, it complies with that rule. The Fence
concept is asking for more than that - it wants the target to be able to
ensure that all previous commands/TMFs are complete before delivering a
particular TMF response (e.g., for a LOGICAL UNIT RESET).  Since iSCSI
doesn't have ACKs, the target must wait for the next PDU from the
initiator with an updated ExpStatSN.

06-341r0 discussed an alternative - add a Delivery Result output to the
Send Command Complete and Task Management Function Executed
confirmations (as previously proposed in 04-072r0 for a different
purpose). This would let the device server/task manager wait for all the
previous commands/TMF responses to complete (be ACKed) before proceeding
to make additional calls (e.g., Task Management Function Executed for a
LOGICAL UNIT RESET).

However, iSCSI allows the target port to send a NOP-IN to force delivery
of a NOP-OUT with ExpStatSN, rather than passively wait for a PDU to
show up.  The device server/task manager must instruct the target port
when to do this, and the Request Fence argument serves that purpose.
Target ports using protocols without such a force mechanism are still OK
- they will just wait for protocol-specific delivery confirmations (e.g.
ACKs).

-- 
Rob Elliott, [email protected] 
Hewlett-Packard Industry Standard Server Storage Advanced Technology 
https://ecardfile.com/id/RobElliott 


 

________________________________

	From: Robert Snively [mailto:[email protected]] 
	Sent: Wednesday, January 03, 2007 4:46 PM
	To: Elliott, Robert (Server Storage); [email protected]
	Subject: RE: [Ips] Response Fence Flag
	
	
	I need a little tutorial here.  Note that this is an
architectural hack that
	is very "un-SCSI" and is not required if the response is
qualified with identification
	information associated with the original request.  I know that
such qualification
	exists for commands, but does it exist for task management
requests in
	iSCSI?  
	 
	I know that it does exist for parallel SCSI (which is strictly
interlocked)
	and for SAS and Fibre Channel SCSI, making such "fences"
unrequired by
	those technologies.
	 
	Note that Rob's previous revision of 06-341 is available on
www.t10.org.
	 
	I would hate to see such a hack creep into the SCSI
architecture.
	 
	Bob
	 

________________________________

	From: Elliott, Robert (Server Storage) [mailto:[email protected]] 
	Sent: Wednesday, January 03, 2007 1:56 PM
	To: [email protected]
	Subject: RE: [Ips] Response Fence Flag
	
	
	If T10 agrees, T10 proposal 06-341 will add it to SAM-4.
	 
	-- 
	Rob Elliott, [email protected] 
	Hewlett-Packard Industry Standard Server Storage Advanced
Technology 
	https://ecardfile.com/id/RobElliott 



________________________________

		From: Eddy Quicksall
[mailto:[email protected]] 
		Sent: Wednesday, January 03, 2007 2:00 PM
		To: [email protected]
		Subject: [Ips] Response Fence Flag
		
		
		Section 3.3.1 talks about a Response Fence Flag:
		 
		SCSI protocol layer instructs the SCSI transport layer
of a 
		"Response Fence" associated with the response in
question when 
		the "Send Command Complete" protocol data service
(SAM-2, clause 
		5.4.2) ...
		 
		But I don't see any reference to that in SAM-2.
		 
		Is this strictly an iSCSI flag? Where is the flag
specified?
		 
		Eddy

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