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    Re: iSCSI: Out Of Sequence due to null sequence with multipleconnections.



    
    
    That might interfere with ITT reuse.  Julo
    
    Sandeep Joshi <sandeepj@research.bell-labs.com> on 03/04/2001 19:45:44
    
    Please respond to Sandeep Joshi <sandeepj@research.bell-labs.com>
    
    To:   Black_David@emc.com
    cc:   dotis@sanlight.net, ips@ece.cmu.edu
    Subject:  Re: iSCSI: Out Of Sequence due to null sequence with
          multipleconnections.
    
    
    
    
    
    There is probably more to this than meets the eye, in which
    case do please let me know where I err..
    
    Is it not possible to use the refTaskTag in task management
    command to introduce "state" at the target ?
    
    Specifically,
    1) send task management command with immediate delivery(cmdSN=0).
    2) if iSCSI target sees a non-existing refTaskTag,
       it uses that fact to create some "state" at the target.
       (NOTE: we dont know if the task had already completed ??)
    3) When actual task arrives, it gets dropped since iSCSI sees
       that for that refTaskTag, state=must abort.
    
    But there is still the question..
    1) when do we delete that target "state" ?  there is no
       endCmdSN or refCmdSN.
    
    -sandeep
    
    Black_David@emc.com wrote:
    >
    > 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
    >
    > ---------------------------------------------------
    > 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
    > ---------------------------------------------------
    
    
    
    


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