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    Re: Unsolicited Data.



    Sanjeev,
    
    > why would initiator choose not to send unsolicited immediate data when
    > it has negotiated for IntialR2T= Yes??
    
    I assume you mean InitialR2T=no.
    
    I would do this because I think that the real benefit of unsolicited
    data is for writes where ALL the write data can be sent unsolicited,
    and I'm happy to have the target schedule the transfer of (ALL) data
    for larger writes.  Of course, others would say I'm a moron.
    
    Assuming the overwhelming majority of storage use is file access,
    small writes are exclusively file system metadata writes.  Unsolicited
    data is a huge win for metadata writes because the latency of these
    metadata writes is frequently not covered by transferring other data
    (within a single access thread).  On the other hand, the file system
    (usually) generates enough concurrent demand for real data writes that
    the flow control (R2T) latency is well hidden.
    
    You might argue that if you're running a file system optimized fors
    today's SANs over a link with a HUGE bandwidth * delay product
    (greater than the expected demand created by the file system), sending
    large amounts of unsolicited write data will substantially improve the
    link utilization.  That is true, but I don't think many targets are
    going to offer unsolicited data bursts remotely that large.
    
    In a hardware accelerate implementation, unsolicited data will be
    handled differently than solicited data.  The target can chose where
    the solicited data is delivered, but unsolicited data arrives through
    a general-delivery path.  Therefore, if the offered unsolicited data
    burst size approaches the regular burst size, an implementation will
    need TWICE as much buffer memory (half in the general-delivery area
    and half in the flow-controlled buffer area) to support the same
    operation load.  Maybe this tradeoff is OK, but given the cost
    sensitivity of storage targets, I doubt it.
    
    As always, I welcome comments from those who see things differently.
    
    Steph
    


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Last updated: Tue Sep 04 01:04:33 2001
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