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

[Black_David@emc.com]

It still appears to me that the behavior that
Charles wants is available by sending the task management
function twice - once for immediate delivery and once for
ordered delivery.  Duplication on all connections would
not be necessary in the normal case because the ordered
instance would naturally come behind the unordered ones.
The timer seems to be something that ought to be taken
up under error recovery in general (and in particular,
we should consider letting TCP time out the connection
rather than adding yet another recovery timer).  IMHO,
the bottom line is that the logic to duplicate the task
management commands at the Initiator and coordinate
them at the Target costs implementation complexity,
and I have not seen a convincing statement of what
we're buying with that added complexity.

It may be the case that applications don't want to be
bothered with sending task management commands twice,
in which case we have a coordination problem of some
form that has to be addressed in iSCSI.

For further discussion,
--David

> -----Original Message-----
> From:	Binford, Charles [SMTP:CBinford@pirus.com]
> Sent:	Monday, April 02, 2001 9:20 AM
> To:	iSCSI (E-mail)
> Subject:	FW: iSCSI: Out Of Sequence due to null sequence with
> multipleconn ections.
> 
> I sent this Friday, but never saw it come back, so I'm resending.....
> 
> Charles Binford
> Pirus Networks
> 316.315.0382 x222
> 
> 
> Doug, If I'm hearing you correct, you are saying process the task
> managment
> command immediately, but toss any commands waiting for a CmdSN hole to be
> filled.   The problem with this approach, is there are many different
> flavors of task managemet commands and not all effect all commands.  The
> iSCSI layer would have to duplicate the SCSI layer effort of parsing the
> task managment request and the LU/tags of all commands it comtemplates
> aborting.  I don't like that solution.
> 
> David,  I don't think the current state of things is acceptable as you
> imply
> with your final statement of 'I'm not sure anything needs to be changed.'
> 
> FC solves this problem by requiring the initiator FCP layer (note: FCP,
> not
> SCSI)to send an ABTS for every exchange 'in an ambiguous state' after any
> task management functin.  It is ugly, but is covers the hole of the
> initiator not knowing for sure which commands were properly aborted and
> which were not.
> 
> I have suggested in the past (although, from the response I've gotten, I
> think I have been misunderstood) that the iSCSI initiator send all task
> management functions down all connections of a session.  I'm NOT saying
> duplicate the task management function.  The distinction being only one
> instance of the task management function should be passed up to the SCSI
> layer on the target side.  
> 
> To address the issue of the task management function being delayed because
> of a slow connection I suggest the following:  upon receipt of the first
> task management function request on any connection, the iSCSI target shall
> start a relatively short timer (I'm thinking of a range from a few 100 ms
> to
> a couple of seconds).  As soon as the task management function is received
> on all connections, cancel the timer and pass a single instance of the
> request up to the SCSI layer.  If the timer expires, close the
> connection(s)
> the task management function(s) did not arrive on and pass up a single
> instance of the request to the SCSI layer.
> 
> Charles Binford
> Pirus Networks
> 316.315.0382 x222
> 
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Douglas Otis
> Sent: Tuesday, March 27, 2001 9:42 AM
> To: julian_satran@il.ibm.com; ips@ece.cmu.edu
> Subject: RE: iSCSI: Out Of Sequence due to null sequence with
> multipleconnections.
> 
> 
> Julian,
> 
> It could be that a "stuck" command in flight or at the sequencer is the
> reason for the task management which is made problematic by the null
> sequence.  Even if implementers are aware of this problem, there is not a
> good solution until null sequence is removed.  Making a flag to indicate
> "immediate" or perhaps "reject prior pending iSCSI commands" is a means of
> ensuring the initiator remains in control with respect to all situations.
> This ensures the initiator is aware of the state of the target and
> sequence
> of commands can be maintained if required.  Be careful about being too
> disk
> centric.  Treating these SN numbers as unsigned allows a simple means of
> tracking.  Here is an example explaination:
> 
>    Comparisons and arithmetic on SNs in this document SHOULD use Serial
>    Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.
> 
> Doug
> 
> > 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.
> >
> >
> [stuff deleted]
> >
> > (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
> >
> > ---------------------------------------------------
> > David L. Black, Senior Technologist
> > EMC Corporation, 42 South St., Hopkinton, MA  01748
> > +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
> > black_david@emc.com       Mobile: +1 (978) 394-7754
> > ---------------------------------------------------
> >
> >
> >
> >
> >