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    Re: iSCSI: Question on StatRN usage



    
    
    Mallikarjun,
    
    Thanks for your comments.
    
    Initiator scoreboarding is not considered. I will try to emphasize this
    even more in the new draft.
    The party responsible for reporting length is the target.  As overlapping
    ranges are not explicitly
    forbidden this would be a harder task than apparent. Reporting counts
    becomes entirely a question of faith!
    
    Regards,
    Julo
    
    "Mallikarjun C." <cbm@rose.hp.com> on 20/10/2000 05:40:16
    
    Please respond to cbm@rose.hp.com
    
    To:   ips@ece.cmu.edu
    cc:
    Subject:  Re: iSCSI: Question on StatRN usage
    
    
    
    
    Julian,
    
    Thanks for the clarifications, I am pleased to understand that
    there's no overloading of any reference #s - the usage of new
    term "DataRN" in your new draft makes it a lot clearer.
    
    Some comments.
    
    >Mallikarjun and Prasanjit,
    >
    >Sorry for the confusion.
    >
    >The text is confusing and I have corrected it the new text. StatRN is
    >mandatory (it is the only way we have to ACK status and is not related to
    >ordering).
    
    Eventhough StatRN itself may not be used by an initiator for ordering
    (unless it
    wants to order completions, for whatever reason), StatRNs are themseleves
    are in a monotonically increasing order.  It is helpful to state this
    explicitly.
    
    >
    >As for the data the intent was to use StatRN to just number data packets
    >for a given command (start with whatever you want) and have them acked
    with
    >a NOP with the same task tag (this is important for input data for which
    we
    >have no other way of acking them). Those numbers are not related to the
    >Status numbers. No ordering or recovery is required up to command restart.
    >I assume that numbers will not wrap unless a target sends more blocks than
    >bytes (and it can!) but even then
    >no harm is done.
    >At recovery the restarted command will be followed by a NOP with the same
    >initiator tag indicating what is the
    >the block expected. The initiator does not have to do any scoreboardong
    >only keep the counters.
    >The target can free early resources and iSCSI can recover eve long reads.
    >For writes evidently R2T does the job but it means that write data can be
    >recovered only with R2T.
    
    This implies that in case an iSCSI implementation is counting the # of
    bytes transferred in/out during a task, it shall not assume an error if
    the count is the less than expected transfer size - if the retry bit
    was set (This is especially true for writes, where the initiator doesn't
    know from which point target starts issuing R2Ts).  I would suggest adding
    this comment as well to enable better interoperability.
    
    >Should we overload on CmdRN/ExpCmdRN to shorten recovery? I don't see a
    >need.
    
    NO, I don't see either.  My concern was that overloading these RNs for
    data would become a scalability bottleneck, when a session spans mulitple
    NICs.  I am glad that it's not what was intended.
    
    Comments on your next email:
       >The NOP message PDUs are not associated with a task, are meant for
       >immediate delivery, and their only purpose is synchronizing the
    ordering
       >registers of the target and initiator.
    
    I would like to point out that NOP PDUs are indeed associated with a task!
    They are associated with a task whose read data they are ack'ing (given
    that the DataRN is only task-unique).  Also, I would like to point out
    that the current definition of NOP payload does not have Initiator Task Tag
    - it needs to be added.
    
    Thanks.
    --
    Mallikarjun
    M/S 5601
    Networked Storage Architecture
    HP Storage Organization
    Hewlett-Packard, Roseville.
    cbm@rose.hp.com
    
    phone: (916) 785-5621
    fax:   (916) 785-2875
    
    
    
    
    
    


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