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    iSCSI: Immediate Delivery Behavior



    Hi:
    
    The behavior for immediate commands seems ambiguous and possibly needlessly
    complex.
    
    Rev 06 says the following regarding ordered delivery to the SCSI layer:
    
       "Except for the commands marked for immediate delivery the iSCSI 
       target layer MUST deliver the commands to the SCSI target layer in 
       the order specified by CmdSN. Commands marked for immediate delivery 
       may be handed over by the iSCSI target layer to the SCSI target layer 
       as soon as detected. iSCSI may avoid delivering some command to the 
       SCSI layer if so required by some prior SCSI or iSCSI action (e.g., 
       clear task set Task Management request received before all the 
       commands it was supposed to act on)."
    
    In a non-striped session consisting of one TCP/IP connection, the above
    could be interpreted to allow the delivery of an immediate command before
    other partly received commands that were previously issued. As a result, an
    operation, such as an abort task, might bypass the command to be aborted --
    even if both were sent on the same connection.
    
    Assuming that's true, I believe a useful simplification is to require that
    all traffic flowing over a given TCP/IP connection be delivered to the SCSI
    layer in the order received over that connection.  In a striped session, an
    immediate command might therefore leapfrog commands on other connections but
    would never bypass commands on the same connection.  In my opinion, that
    simplifies the problem of properly purging commands and stale PDUs in the
    wake of a task management operation. 
    
    Charles
    Charles Monia
    Senior Technology Consultant
    Nishan Systems
    email: cmonia@nishansystems.com
    voice: (408) 519-3986
    fax:   (408) 435-8385
    


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Last updated: Tue Sep 04 01:04:52 2001
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