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    Re: iSCSI: header digest error at initiator



    Julian,
    
    I think Matt clarified earlier that the operation I was referring to is not
    score-boarding. I was referring to a check along the following lines to verify
    if there was I/O underrun :
    
    (total bytes xfer'ed as indicated by the chip ) =
    (no. of bytes of data xfer specified by ULP) - (resid reported by the
    target in
    SCSI Response PDU).
    
    This is a standard check in an un-reliable scsi transport protocol like FC.
    One can argue that it is not required with reliable SCSI transports like
    iSCSI. More importantly, if iSCSI allows overlapping data transfers (which is
    the case, unlike FC), this type of operation is not possible, as doug pointed
    out.
    
    Thanks All.
    
    Regards,
    Santosh
    
    julian_satran@il.ibm.com wrote:
    
    > Santosh,
    >
    > In the SCSI world there is no scoreboarding at the initiator.
    >
    > The whole operation is master-slave with the target being the master.
    > The status, counters etc. are determined by the target and the initiator
    > has propagate them to the application client.
    >
    > Julo
    >
    > Santosh Rao <santoshr@cup.hp.com> on 16/01/2001 22:42:47
    >
    > Please respond to Santosh Rao <santoshr@cup.hp.com>
    >
    > To:   IPS Reflector <ips@ece.cmu.edu>
    > cc:
    > Subject:  Re: iSCSI: header digest error at initiator
    >
    > Matt,
    >
    > My replies inline.
    >
    > Regards,
    > Santosh
    >
    > Matt Wakeley wrote:
    >
    > > Santosh Rao wrote:
    > > >
    > > > If this is the intention of the recommended error recovery, it is the
    > > > result of not allowing score-boarding. By score-boarding an initiator
    > > > would detect an underrun and would just error the affected I/O back.
    > >
    > > Two comments here.  First, in your example, the initiator is inventing an
    > > error that really didn't occur in the target.  The target completed the
    > I/O
    > > successfully, it was the transport that experienced an error, but you're
    > > treating it like a target error.
    >
    > The LLP (initiator) would return an error to ULP indicating a transport
    > error
    > (service response of "service delivery or target failure") occurred. [due
    > to the
    > data underrun.]
    >
    > > Second, you say that initiators and targets routinely perform
    > scoreboarding.
    > > How is this done today?  Buffer(s) are provided to an I/O chip.  The I/O
    > chip
    > > writes the data into the buffers.  It does not have the memory to
    > determine
    > > that each and every byte has been written to. So how is the
    > initiator/target
    > > supposed to be absolutely sure that every byte was written to?
    >
    > The initiator would use a check along the following lines  :
    >
    > (total bytes xfer'ed as indicated by the chip ) =
    > (no. of bytes of data xfer specified by ULP) - (resid reported by the
    > target in
    > SCSI Response PDU).
    >
    > to verify that all the data the target sent is accounted for at the
    > initiator
    > end.
    
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    n:Rao;Santosh 
    tel;work:408-447-3751
    x-mozilla-html:FALSE
    org:Hewlett Packard, Cupertino.;SISL
    adr:;;19420, Homestead Road, M\S 43LN,	;Cupertino.;CA.;95014.;USA.
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    email;internet:santoshr@cup.hp.com
    title:Software Design Engineer
    x-mozilla-cpt:;21088
    fn:Santosh Rao
    end:vcard
    


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Last updated: Tue Sep 04 01:05:48 2001
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