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    RE: iSCSI: ISID and the New Persistent Reservations Proposals



    Jim,
    
    > My strongest discomfort is this:  in essence you are removing the SCSI
    > Initiator Port Name from the iSCSI implementation of the model.  You're
    > hoping to fill that gap later with a text key whose semantics have yet to
    > be defined.
    
    Having the text key semantics be "yet to be defined" seems reasonable
    because the behavior required by T10 is in the same state.  Horse first,
    then cart :-).
    
    > My expectation is that once you attempt to put the name back
    > in you're recreated the OptionA/OptionB debate and will never 
    > be able to solve that satisfactorially.
    
    I disagree.  ISIDs aren't negotiable, text keys are.  In the text key
    situation analogous to the ISID situation that causes the OptionA/OptionB
    mess, the Target can set the text key to some sort of "Port Identity
    Conflict Detected" value and ship everything back to the Initiator
    with no harm done.  Trying to do that sort of thing with ISIDs would
    be a mess, as indicated by the amount of email on OptionA/OptionB.
    
    > I'm concerned that removing the notion of SCSI Initiator Port Name
    > from the model today, you will have strongly hindered iSCSI's ability
    > to take advantage of a number of things that T10
    > will be able to do, now that Names are part of the lexicon.
    
    And I have a strong discomfort with designing for functionality that isn't
    specified.  We already have a mismatch where the current T10 proposal #1
    does not match current ISID specification (see below on conservative reuse).
    As T10 does things with the Port Name, we can put in the text key(s)
    required
    to make them work.
    
    > I also believe as I've stated that the OS's are going to prefer to see
    > a more stable SCSI port view of the initiator's ports than we're currently
    > anticipating in iSCSI.
    
    As long as the TSID is not used to provide this view, there is no problem
    here (e.g., the enumeration of interface cards on boot can be used).
    
    > Finally, I think
    >    - the reason to do this rather than the ISID is that it cleanly splits
    >    persistent reservation management away from iSCSI session management.
    > is a major violation of layering.  You're going to burden the reservation
    > management with having to coordinate iSCSI actions.
    
    I disagree. The interaction required is something that will have to happen
    anyway if T10 adopts any proposal that has reservations span sessions at the
    Target.  Suppose that port X opens a session to port A, places a persistent
    reservation that spans target ports, and then opens a session to port B -
    as part of setting up that second session, port B has to interact with the
    persistent reservation logic to discover that the reservation exists and
    applies to the second session.  This is all that's required, and it's true
    for any transport - in each cases the transport-specific port identification
    has to be passed from the transport to the SCSI reservation logic.  The
    only wrinkle is that iSCSI gets this name from a text key instead of a WWN
    or a parallel SCSI bus ID - at the SCSI level, this shouldn't matter (the
    IDs are opaque and are being checked for equality).  In other words, if
    there's a layering problem here, it's in SAM2 ...
    
    > [I've argued that conservative reuse of ISIDs to different target portal
    groups
    > provides a clean consistent view of SCSI Initiator Ports to the upper
    layers and
    > that's all that's needed.]
    
    But that requirement is NOT in the current draft, and inserting it would
    impact implementation approaches such as that originally described by Nick
    Martin in which an iSCSI implementation has to cons up an ISID in the
    potential absence of coordination with other implementations that share
    the same iSCSI Initiator Node Name.  The nasty problems result when there
    are multiple Initiators connected to multiple Targets, but not all
    Initiators
    are connected to all Targets - an Initiator could set up several sessions
    and only then discover that the ISID it used is taken and have to tear down
    those sessions and start over.  This is why ISIDs as currently specified
    aren't sufficient to deal with proposal #1 and changing them to do so would
    be a visible technical change.
    
    > None of those are strong technical arguments however.
    > 
    > I think we agree that the problem comes up because the belief that
    > coordination of ISIDs on the initiator side is going to be difficult.  I'm
    > not convinced that's the case, but let me try a different approach, which
    > may make the implementation easier.
    
    The summary is to have the OS enumeration provide the portal group tag,
    and make that part of the ISID either explicitly or via a text key that
    implicitly extends the ISID.  It's a good try, but I think it runs into
    problems because device enumeration order in an operating system can
    change between reboots for all sorts of reasons ranging from the
    unavoidable (interface card fails in a fashion that causes it not to
    be discovered on boot - this is discovery by plug-and-play logic or the
    like, not iSCSI discovery) to the inane (stupid plug and play tricks,
    new driver revisions).  This may wind up adding the Initiator Portal
    Group number to the list of things that an Initiator has to remember
    in order to get its persistent reservation state restored.
    
    --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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Last updated: Sun Oct 07 22:17:26 2001
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