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    Re: iSCSI ERT: data SACK/replay buffer/"semi-transport"



    > Exactly, I've worked in this context (though its been some years now).
    > It was true (at one time) that tape had a tractability limit, e.g.,
    > a tape backup of a terabyte was out of the question.  Has that changed?
    
    I think this is precisely the point.  Existing, off-the-shelf SCSI
    solutions DO NOT presently solve this problem.  Both ||SCSI an FCP
    burp the operation on a expectable, O(days) failure rate.  The rate of
    adoption for the FCP-2 command recovery feature is overwhelming to the
    point that the tape guys have been talking about end-running the
    problem with explicitly addressed commands.
    
    What we have running iSCSI on TCP is such a drastic improvement in
    what you can expect from your SCSI service that we can eventually
    expect a disruptive change.  Trying to engineer it to the point where
    its 2^100 times more disruptive, when we don't really know where it's
    taking us in the first place is meaningless.
    
    [Warning: repetition ahead]
    
    TCP + link layer error detection is engineered precisely to ensure
    reliable data delivery.  It's clear from an engineering stand point
    that it is likely (not guaranteed, what is?) to do this quite well.
    In spite of much research, it seems like nobody here has come up with
    a strong indication that TCP + link layer error detection does NOT do
    its job well.  I do not think this is because nobody has ever looked
    at the problem.
    
    The lack of concrete information to support the case that TCP + link
    layer error detection is inadequate has us chasing our tails.
    
    Given the layer iSCSI occupies in the protocol layer cake, if we don't
    try to solve which is presently assigned to a lower layer, it seems
    quite comfortable to shim additional checks or recovery, or even a completely
    different transport substrate underneath if we do discover TCP + link
    layer error detection is not doing the trick, but it really seems like
    folly to engineer based upon an assumption that nobody has done a good
    job documenting.
    
    Steph
    
    


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Last updated: Tue Sep 04 01:05:08 2001
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