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    Re: iSCSI: draft 7: remove S bit and Status field from the Data-in PD U?


    • To: ips@ece.cmu.edu
    • Subject: Re: iSCSI: draft 7: remove S bit and Status field from the Data-in PD U?
    • From: "John Hufferd" <hufferd@us.ibm.com>
    • Date: Fri, 10 Aug 2001 07:35:39 -0700
    • Content-type: text/plain; charset=us-ascii
    • Importance: Normal
    • Sender: owner-ips@ece.cmu.edu

    
    Folks,
    I might have this all wrong, but it seems to me that we are jumping on the
    simplification horse from all directions, whether or not it is right, just
    to say we are trying to simplify.  I am afraid that some of this may be at
    the expense of "at Distance" installations.
    
    First, immediate and unsolicited separate Data PDUs are very valuable for
    situations where the Target is at distance from the Initiator.  (A good
    example is writing to tape.  Most tape are only written and hardly ever
    read.  Further, many folks would like to have tapes somewhere else -- at a
    distance.)  The ability not to have to have R2T exchanges over distance is
    an important feature.  Let us not quickly throw that out.
    
    Second, thoughts of removing the immediate data seem not to be
    simplification, since all the information to tie the data to the command is
    right there with the command.  That has got to be easier then matching up
    separate PDUs of data with the approprate commands.
    
    So I am concerned that one of the important strengths of iSCSI -- that
    being the ability to efficiently operate at distances where Fibre Channel
    would fear to tread -- is being too quickly put aside.
    
    Therefore, if you do not eliminate immediate Data (since that is the most
    efficient, especially for the smaller data sizes), you are faced with
    removing unsolicited Data Packets and only permitting R2T -- which has a
    round trip handshake that is needed to get the data.  From a latency
    standpoint this does not seem good.
    
    I can imagine some vendor making a claim that their Storage Unit is the
    Industry's best "at Distance" storage controller, but it has more memory
    and might be a bit more expensive then the typical R2T based Storage Units
    (since they accept unsolicited Data PDUs), but I think that is good.  We
    need vendors to offer those types of solutions.
    
    If you do not want unsolicited or immediate Data PDUs, don't accept them at
    Login.  But, they were put into the protocol for a reason, and I hope we
    continue to have the features that keep iSCSI a distance compatible
    protocol.
    
    .
    .
    .
    John L. Hufferd
    Senior Technical Staff Member (STSM)
    IBM/SSG San Jose Ca
    Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
    Home Office (408) 997-6136
    Internet address: hufferd@us.ibm.com
    
    


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Last updated: Tue Sep 04 01:04:03 2001
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