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    Re: iSCSI: frame formats



    
    
    Steph,
    
    Read10 and Write10 can still be accomodated with several data PDUs as we
    maintained the total read/write count (expected data) as a 32 bit field.
    It is only the individual PDU that went down to 16M.
    
    It is worth noticing also that the CRCs we are contemplating have a
    guaranteed haming distance of 4 for blocklengths of less than 2^31 bits.
    
    Julo
    
    Stephen Bailey <steph@cs.uchicago.edu> on 09/04/2001 18:24:09
    
    Please respond to Stephen Bailey <steph@cs.uchicago.edu>
    
    To:   ips@ece.cmu.edu
    cc:
    Subject:  Re: iSCSI: frame formats
    
    
    
    
    > No one felt that a max data size of 16 megs in an iSCSI PDU was an
    > issue
    
    I did, I was just exhausted.  Furthermore, the size the PDU could
    handle shrank (from 64M to 16M) right there before our ears in a split
    second.
    
    The `question' was also not asked in a form which I could effectively
    answer or discuss.  There were two proposals presented, and they had
    very different characteristics.  I figured I would refrain from
    comment until I a) understood the proposals better b) got a sense of
    which way we might be going.
    
    I have a hard time swallowing a PDU that won't accomodate the data
    from even a READ10 or WRITE10.
    
    If people expect to tile each TCP segment with an iSCSI PDU, I think
    that's a mistake.  The headers are too bulky for that.  If they think
    they're going to gain something meaningful in hardware implementation
    complexity (e.g. bounding the amount of reassembly buffering) by
    assuming small multiples of segment size PDUs, I think that's a
    mistake too.  I'd like to see a show of hands of people who have
    actually implemented this approach in hardware.  Anybody?
    
    If you (eventually) accept this, then you're back to the model that
    there's a lower layer which is providing your data steering on a
    per-segment basis, and your iSCSI PDU is the granularity at which the
    iSCSI implementation delivers data to this layer.  It is clearly a
    granularity at which software (transfer scheduling) occurs.
    
    At 10 Gbit/s, a 16 MB PDU is only 16 MS worth of data.
    
    Steph
    
    
    
    


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