FW: iSCSI: Out Of Sequence due to null sequence with multiple connections.

[Robert Snively <rsnively@Brocade.COM>]

This apparently did not get out on the ips reflector.  Trying again.

-----Original Message-----
From: Robert Snively 
Sent: Tuesday, April 03, 2001 3:11 PM
To: 'Black_David@emc.com'; CBinford@pirus.com; ips@ece.cmu.edu
Subject: RE: iSCSI: Out Of Sequence due to null sequence with multiple
connections.


I support David's conclusions that task management should be sent
once.  Fortunately, the ordered/unordered question need not even
be asked, because SAM-2 compliant SCSI applications already know 
what will happen.

At present, SCSI applications do not have a clear guarantee of
the order between task management functions and the processing or
delivery of any particular task.  In fact, the concept of 
an ORDERED attribute applied to a task management function is
unknown.  As a result, SCSI drivers have to be aware of 
the implications.  Those implications include the possibility that
a command may be in any state of delivery or completion as a
task management function is executed and therefore may or may not
be included in the scope of the task management function.  This
has been partially clarified by the recently created status value
developed by Charles Binford, "TASK ABORTED", which at least indicates
which tasks for other initiators have been cleared by a task 
management function.

In general, the approach is as follows:

	When a task management operation is received by a target,
	tasks are treated in one of three ways, depending on where
	they are with respect to the timing of the execution of
	the command.

   a)	If the command has been completed, it is perfectly possible that
	completion status for the command is transmitted in spite of the
	fact that the task management operation has been received.

   b)	If the command is in progress or enqueued, it is likely that
	the command will be managed according to the rules of the
	task management command unless a) or c) happen.

   c) If the command has not been received by the time the task 
	management function is completed, the command will be received
	and treated as a command that has occurred after the task
	management function, even if it were sent before that.  That
	typically involves presentation of a UNIT ATTENTION condition
	or some other notification.

	When a task management operation passes through an initiator,
	the initiator has the option of acting on the commands 
	whose state it knows, including the possibility of discarding
	or presenting both expected and unexpected returned status,
	in a vendor specific manner.

It is possible that all this work of creating a synchronous behavior
for actions that are designed to be asynchronous may be unnecessary.
The present drivers know what to do to clean up the mess.  All you
have done is statistically increase the number of tasks that may 
be involved in behaviors a) and c) above.

References in support of this include:

>From SAM-2, Rev 15, 4.6.2:

	For convenience, the SCSI architecture model assumes 
	in-order delivery to be a property of the service delivery
	subsystem. This assumption is made to simplify the 
	description of behavior and does not constitute a requirement.
	In addition, this specification makes no assumption about, 
	or places any requirement on the ordering of requests or
	responses between one sending device and several receiving devices.

>From SAM-2, Rev 15, 4.7.4

	The order in which task management requests are 
	executed is not specified by this standard. In particular, this
	standard does not require in-order delivery of such 
	requests, as defined in 4.6.2, or execution by the task manager
	in the order received. To guarantee the execution order 
	of task management requests referencing a specific logical
	unit, an initiator should, therefore, not have more than 
	one such request pending to that logical unit.

>  -----Original Message-----
>  From: Black_David@emc.com [mailto:Black_David@emc.com]
>  Sent: Tuesday, April 03, 2001 12:47 PM
>  To: CBinford@pirus.com; ips@ece.cmu.edu
>  Subject: RE: iSCSI: Out Of Sequence due to null sequence 
>  with multiple
>  con nections.
>  
>  
>  > I would state this much stronger.  Applications had better 
>  not have to
>  know
>  > that it is iSCSI underneath vs. FCP or parallel SCSI else 
>  I believe we
>  > missed the objective (granted, some things such as target 
>  address space
>  are
>  > unavoidably different, but I believe task management 
>  functions should be
>  the
>  > same).  The transport needs to handle the transport issues without
>  exposing
>  > quirks to the SCSI or application layer.
>  
>  Unfortunately, I think we have an impossible situation.  It 
>  appears to me
>  that
>  we have to pick at most two of the following three goals, as 
>  I have yet to
>  see
>  any way to achieve all three for a single task management 
>  command on a
>  multiple connection session:
>  
>  (1) The command takes effect immediately and its status/response
>  	is available immediately.
>  (2) The command affects all commands in flight, and its 
>  status/response
>  	is delayed until all such effects are complete.
>  (3) There is no significant visible departure from existing SCSI task
>  	management behavior.
>  
>  The problem is that trying to do both (1) and (2) either 
>  requires SCSI to
>  "execute" the task management command twice or requires that iSCSI do
>  some task management (e.g., on the in-flight commands) on 
>  SCSI's behalf
>  (or worse like having SCSI prolong the execution of the task 
>  management
>  command until everything in flight in iSCSI arrives).  All 
>  of these appear
>  to lead to problems with (3) in one form or another - two executions
>  result in two SCSI status/responses that have to be merged, and iSCSI
>  task management will sooner or later do something different from SCSI
>  (e.g., I sincerely doubt that a Target in a bridge will ever 
>  get this 100%
>  identical to the devices that are being bridged).
>  
>  The current iSCSI draft provides the choice of  [(1)] XOR [(2), (3)];
>  the reason for not getting (3) with (1) is the possibility 
>  of the task
>  management command bypassing commands that it's supposed to
>  affect.  Charles' original proposal is [(2), (3)] because it 
>  has to time out
>  a stuck connection before executing the command, and is roughly
>  equivalent to sending the command for ordered delivery and having
>  the implementation treat any queue between iSCSI and SCSI as
>  being on the SCSI side of the line.  Doug Otis's counter-proposal
>  falls into the category of iSCSI doing task management on SCSI's
>  behalf and provides an example of how this results in visible changes
>  in behavior -- for the CLEAR ACA task management command,
>  aborting all tasks that are queued or in flight is generally 
>  incorrect.
>  
>  I would note that this issue does not arise on single 
>  connection sessions,
>  because sending the command for immediate delivery plus some care not
>  to reorder things in the iSCSI Target (i.e., consider the 
>  iSCSI to SCSI
>  queue
>  to be in "SCSI" and hence subject to the task management command)
>  obtains all of (1) through (3).
>  
>  Going out on a limb, I suspect applications will generally 
>  want [(2), (3)]
>  -- send for ordered delivery and wait for the dust to settle 
>  because that
>  provides the best odds of having some weird device get into a known
>  state from which further progress is possible.  This allows 
>  the application
>  to not know whether parallel SCSI, FCP or iSCSI is underneath and
>  relies on other iSCSI recovery procedures to make sure that the task
>  management command is delivered and executed (e.g., unstick and/or
>  close "stuck" connections).  There will be cases in which (1) is
>  needed (e.g., observe tape robot doing something obviously wrong,
>  and get it to stop immediately), but those may involve fairly blunt
>  instruments (e.g., LUN RESET) and the need to clean up any collateral
>  damage.
>  
>  Sandeep's proposal to create state in the target either 
>  fails to achieve
>  (1) [if the response is delayed until the state is removed] 
>  or violates SAM2
>  [returns the response to the task management command before the task
>  management command is complete].  Having state linger after 
>  a completed
>  LUN or TARGET RESET is almost certainly wrong.
>  
>  So, I think I'm down to sending task management functions 
>  once, usually
>  for ordered delivery with the application making the ordered 
>  vs. immediate
>  delivery choice (and sending the task management function twice if it
>  so chooses).  I think apps will generally choose ordered 
>  delivery, choosing
>  predictable behavior over immediacy concerns.  Aside from a longer
>  discussion of this issue, I still don't see the need for additional
>  mechanism(s) to task management - what have I missed in the above
>  discussion?
>  
>  --David
>  
>  ---------------------------------------------------
>  David L. Black, Senior Technologist
>  EMC Corporation, 42 South St., Hopkinton, MA  01748
>  +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
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>  
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