RE: iSCSI: Out Of Sequence due to null sequence with multipleconn ections.

[julian_satran@il.ibm.com]

David,

Your summary is correct. And (except for a minor point) it is all a matter
of target implementation.
SCSI is not a "completely layered" stack and others have gone so far as to
do task cancellation by an action at link layer (parallel and fiber).
There might be 2 funny side-effects though if an implementer chooses to
cancel "holes" (commands in flight on other connections):

1)the cancelled command is a another task management command (there can be
only one active but what it the one active gets stucked?)
2)(academic I admit) the cancelled command arrives after a wrap around in
command sequencing; this is a bit harder (although not impossible) to fix
in the implementation

Implementers should be aware of those side effects.

Regards,
Julo



Black_David@emc.com on 26/03/2001 18:57:48

Please respond to Black_David@emc.com

To:   dotis@sanlight.net
cc:   ips@ece.cmu.edu
Subject:  RE: iSCSI: Out Of Sequence due to null sequence with multipleconn
      ections.




Let me apologize for unintentionally stepping on Doug
in the meeting.  Due to the time squeeze, I neglected
to ask for other issues at the end of the iSCSI discussion
- sorry about that.

I'm going to back off and try to take a high level view of
this and see what sort of observations emerge.  When a SCSI
task management command pops out of an iSCSI TCP connection
at the target, there are four places that the SCSI operations
it affects could be:

(1) Executing in SCSI.
(2) Queued to SCSI for execution, but not executing.
(3) Queued in iSCSI waiting for command sequencing.
(4) In-flight.

(2) includes the "resource limitations between the
sequencer and the target that may lead to a stall or
a long term delay".

(1) and (2) are the easy cases - the SCSI implementation
must apply the task management command to executing tasks,
and should perform the obvious "peephole optimization" to
the commands queued for execution (i.e., if they're to be
aborted, abort them and send the response(s) without starting
execution).  In essence, this models a command as crossing
the boundary from iSCSI to SCSI the moment that iSCSI is
prepared to give it to SCSI (i.e., any queue and related
resource limitations are on SCSI's side of the line).

(4) is hard.  One SCSI task management command generates one
response.  That response can either be generated immediately
(command arrives, is passed to SCSI, SCSI does its thing) or
at the right point in the sequence (command arrives, is
sequenced by iSCSI, passed to SCSI at the right point in the
sequence, and SCSI does its thing), but NOT both.  As things
currently stand, having a task management command apply to
in-flight commands requires sending the task management
command for ordered delivery - so if it's desired to have
the task management command take immediate effect and also
catch everything in flight, it's going to have to be sent
twice.  I'm not enthusiastic about the idea of the task
management command taking immediate effect but delaying the
response until everything in flight that might be affected
arrives, as I suspect the Initiator would like to know what
happened sooner rather than later.

(3) is "interesting".  The results of applying a SCSI task
management command to a SCSI operation are known only to
SCSI, and hence asking that a command stuck in the iSCSI
sequencer be affected immediately by a task management
command is asking that the task management command have
the side effect of changing some of the commands it affects
to immediate delivery so that it can immediately do its
(SCSI) thing to them.  I wouldn't want to mandate this,
nor would I want to prohibit it, BUT ... if the above
discussion of in-flight commands is correct, I would
observe that the application on the Initiator side
can't tell the difference between commands that are in-flight
vs. waiting for something in-flight on another connection,
and hence is going to have to issue the task management
command for ordered delivery if it wants to affect operations
in either place (and issue a second copy if it wants
immediate action).

The upshot is that, aside from a longer discussion of this
issue, I'm not sure anything needs to be changed.  Comments?

Thanks,
--David

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