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    Re: iSCSI: error recovery



    julian_satran@il.ibm.com wrote:
    
    > /<JS>
    >
    > The whole point about data numbering is to allow a target to discard
    > read-data when read-data is hard to recover (as in tapes).  Once data is
    > acked it is discarded. A target getting a command in restart mode will
    > resend whatever data it has still buffered (not acked) and continue from
    > there. The initiator would not e any wiser because it is not supposed to
    > scoreboard.
    >
    > <JS>/
    
    Julian,
    
    The target should not through away data that is hard to recover until the
    target has received notification that the *whole* I/O has successfully
    completed at the initiator.  You must not have read my response (included
    below) showing how complex it would be for an initiator to reliably piece
    together two incomplete I/Os into one good I/O.
    
    > > I assume that a clever target will keep only unacked data (the whole point
    > > of data PDU numbering is to lower the amount of data a target has to keep
    > > for recovery).
    >
    > I don't think there is any advantage to retransmitting only data that was not
    >
    > "acked".  Let's say the data was being sent over a tcp connection to
    > initiator
    > HBA #1 of a multi tcp connection iSCSI session.  I think for simplicity most
    > iSCSI HBAs will inform the host driver of completed I/Os, not "partial" I/Os
    > (especially indicating what was received and what wasn't).  When the
    > connection
    > is "failed over" to another connection, perhaps running on a different HBA,
    > it
    > will be much simpler to retry the whole I/O over the new connection, rather
    > than piece together two partial I/Os.  Simpler is better isn't it?
    >
    > > At command restart it will resent what it has.
    >
    > No, command retry (restart) was meant to retry the whole I/O.  It was not
    > meant to be "send what you think I didn't get".
    
    -Matt
    
    


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Last updated: Tue Sep 04 01:06:35 2001
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