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    Re: Third party naming vis-a-vis iSCSI



    
    
    Jim,
    
    Your first solution is what we already did in the working draft (0706).
    The only element on which we in fact differ is that we decomposed the
    action
    in two elements - the address - that is given out by the initiator
    and what you call an alias (in the draft it is a SAR) that is set by the
    target.
    This way the target can manage by himself the small name server in the best
    way he sees fit.  This can be extended to several other protocol elements.
    
    The disassociation between mapping and command is not a major impediment as
    we
    are talking about long and complex commands anyhow and can be even an
    advantage
    if the commands are repeated as the decoding logic in the target gets
    simpler and more
    regular.
    
    The one point in which I would like us to interact sometime with T10 is the
    logical
    unit name and mapping - this messier than the target address mapping - your
    access
    control merging with LU identification does not make things simpler.
    
    In the current draft we allude to the fact that the proxy mapping has to be
    discussed
    with T10 but I think the issue is larger than that.
    
    Regards,
    Julo
    
    hafner@almaden.ibm.com on 06/07/2000 19:56:45
    
    Please respond to hafner@almaden.ibm.com
    
    To:   "IPS (E-mail)" <ips@ece.cmu.edu>
    cc:    (bcc: Julian Satran/Haifa/IBM)
    Subject:  Third party naming vis-a-vis iSCSI
    
    
    
    
    
    
    Here's my promised suggestion for third-party naming.
    
    There are two approaches, one fully integrated into SCSI (and so
    independent of iSCSI, though iSCSI can best take advantage of it). The
    other approach is more attuned to iSCSI, though could be used in other
    (probably future) transports.
    
    The real problem in this space is not in the higher application layers
    which can do naming in anyway they want, so long as there is an application
    that can translate from one form to another.  (So a URL can get "resolved"
    by different application layers into an ethernet MAC and IP port, etc.) The
    problem is that an arbitrary naming scheme doesn't map well into existing
    SCSI paramter data where third party addresses are used (e.g., third party
    reservations, RAID commands like XDWRITE, extended copy, etc.).   The
    biggest problem in that space is the fact that SCSI has only (at the
    moment) set aside fixed sized fields for this purpose (and we know iSCSI is
    going to need much longer names).  In some cases, this field is 8 bytes
    (e.g., third party reservations) and in others 16 bytes (bytes 12-27 in
    extended copy target descriptors; I'm referring here to the identifier of
    the "target device" and not the logical unit within that device).
    
    The first suggestion, then is to use the existing fixed field (and only 8
    bytes of that) of third party addressing as an "alias" field.  The
    requesting initiator will (through a mechanism described later) define for
    the target a translation mapping of this alias to the transport specific
    identifier of the target (e.g., the <hostname> portion of the url, the FC
    WWN, the parallel SCSI address).  A flag bit or field (TBD for each usage)
    will indicate that this is an alias field and not a specific address
    identifier.  Using aliases in this way, we don't need to change the size of
    the fields currently dedicated to third party addresses and we don't need
    to change their existing usage.
    
    Here are the two approaches for an initiator to define the
    alias<->thirdparty mapping. Note, in all cases, (without a completely
    different protocol) the mapping would be initiator specific, not a global
    aliasing.
    
    1) Within iSCSI, a Text command could define the mapping.
    Advantages:
    a) you'd only need to change SPC-2 to allow for alias-type usage, and leave
    the mapping to "the transport".
    b) it would apply to all third-party addressing contexts.
    Disadvantages:
    a) there's a disassociation between the actual mapping and the use of the
    mapping (that is, state must be maintained) and so all the issues about
    state (persistence, etc) have to be addressed.
    b) it's not clear how other transports could take advantage of this.
    
    2)an additional "chunk" of the parameter data could be used to define the
    alias.  For example, in extended copy, an additional "page" of parameter
    data can define the mapping for that instance of the command.
    Advantages:
    a) can be used in all transports and future transports
    b) is a general extensible mechanism
    c) has state only for the context of the command itself
    d) can handle whatever size of device identifier you want (e.g.,
    arbitrarily long <hostname>) - that's because you can define the parameter
    data in this way.
    Disadvantages
    a) would require more changes to SPC-2 (both alias usage and the definition
    of mapping), and would (might?) affect any command where third-party
    addressing is used.
    
    As for how logical units are referenced within a target, the existing
    mechanisms of LUN and Proxy Token (from access controls) are sufficient.
    In all cases, the harder part is finding the device.  SCSI already knows
    how to find the logical unit within the device.  But I don't see a
    fundamental problem with adding this logical unit identifier (in whatever
    form you like, LUN, Proxy Token, EVPD data) as part of the alias mapping.
    The subtle issue is that in some cases there is no logical unit to
    reference (e.g., third party reservations reference an initiator, not a
    target).
    
    Political comment:  After having worked with the T10 guys over the last
    year or so, I find them very amenable to additions/changes to SCSI so long
    as it doesn't break anything and that it meets someones perceived need.  I
    think the issues raised here meet both those requirements.  So, I wouldn't
    be reluctant (if I were more active in this iSCSI thing) to take specific
    proposals to T10 to enhance what iSCSI can do.
    
    Jim Hafner
    
    
    
    
    
    


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