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    Fw: 00-279r0 Large SCSI Device Identifiers


    • To: <ips@ece.cmu.edu>
    • Subject: Fw: 00-279r0 Large SCSI Device Identifiers
    • From: "Edward A. Gardner" <eag@ophidian.com>
    • Date: Sat, 8 Jul 2000 16:29:00 -0600
    • Content-Transfer-Encoding: quoted-printable
    • Content-Type: text/plain;charset="iso-8859-1"
    • Sender: owner-ips@ece.cmu.edu

    FYI, the following will be discussed at the T10 working group next Wednesday
    (July 12).  I'm leaving the next day on an overseas trip, so DO NOT try to
    ask me how it went.  However, I do plan to be in Pittsburg.
    
    Edward A. Gardner               eag@ophidian.com
    Ophidian Designs                719 593-8866 voice
    1262 Hofstead Terrace           719 593-8989 fax
    Colorado Springs, CO  80907     719 210-7200 cell
    -----Original Message-----
    From: Edward A. Gardner <eag@ophidian.com>
    To: T10 Reflector <t10@t10.org>
    Date: Saturday, July 08, 2000 4:31 PM
    Subject: 00-279r0 Large SCSI Device Identifiers
    
    
    * From the T10 Reflector (t10@t10.org), posted by:
    * "Edward A. Gardner" <eag@ophidian.com>
    *
    Document: T10/00-279r0  Date: July 8, 2000
    To: T10 Committee Membership
    From: Edward A. Gardner, Ophidian Designs
    Subject: Large SCSI Device Identifiers
    
    The current SCSI standards specify that every SCSI device shall have a
    64-bit device identifier. Quoting from SAM-2 revision 13, page 24:
    
        An Initiator Identifier is a field containing up to 64 bits that is
        a SCSI device identifier for the initiator device.
    
        A Target Identifier is a field containing up to 64 bits that is a
        SCSI device identifier for the target device.
    
    Unfortunately, 64-bits is no longer sufficient to identify a device with
    forthcoming protocols. For example, SVP will need to identify an initiator
    or target by a VI address. Many VI implementations construct a VI address
    from an IPv6 address plus a discriminator. Since an IPv6 address is 128
    bits, this cannot fit within a 64-bit field. For another example consider
    the various proposals for SCSI over IP (Internet Protocol). At least one,
    iSCSI, is proposing to identify devices by URL strings.
    
    There are two approaches to dealing with this problem. One is to have each
    protocol that encounters this problem define a protocol specific device
    identifier mapping. For example, SVP would define an SVP specific 64-bit to
    VI address identifier mapping, with a protocol specific way for initiators
    to load and maintain the mapping table in targets. While that could be done,
    I believe it is fundamentally a kludge and will lead to long-term support
    and compatibility problems.
    
    The other approach is to alter the relevant SCSI standards to allow much
    larger device identifiers. This document sketches how that might be done. If
    the committee agrees that this is the proper approach, I will prepare
    detailed proposals for the affected standards for discussion at this fall’s
    T10 meetings.
    
    So far as I am aware, device identifier size only affects third-party
    operations, those where one target is asked to become a temporary initiator
    to access another target. The two important third-party operations are
    EXTENDED COPY and the third-party disk XOR operations. My impression is that
    other third-party operations (e.g. third-party RESERVATION) are obsolete and
    need not be supported. If anyone disagrees with these assumptions, I trust
    they will let us know.
    
    With these assumptions, the affected documents are SAM-2, SPC-3 and SBC-2.
    The changes I plan to propose to each are summarized below.
    
    
    SAM-2
    
    The existing statements about the size of device identifiers would be
    removed. If SAM-2 says anything at all about device identifier size, it
    should say that it is protocol specific.
    
    
    SPC-3
    
    The EXTENDED COPY command references a lengthy parameter list that controls
    its operation. The parameter list contains a header, a list of target
    descriptors, a list of segment descriptors and an inline data area.
    
    Target descriptors are fixed length, 32-byte constructs. They include a
    24-byte field available to identify the target and LUN. Various formats of
    target descriptors are defined that include Fibre Channel and parallel SCSI
    device identifiers.
    
    Segment descriptors define a data transfer to be performed by the EXTENDED
    COPY command. They identify source and destination target devices with
    offsets into the list of target descriptors. Various formats define various
    kinds of data transfer operations. One format copies data from the inline
    data area to the destination device, presumably for writing labels and other
    control information. The inline data to be copied is identified by a 4-byte
    offset and a 4-byte length.
    
    I plan to propose one or more new target descriptor formats. The actual
    target identifier would be stored in the inline data area. The new target
    descriptor format would contain a LUN (8 bytes), offset to the target
    identifier (4 bytes), length of the target identifier (2 or 4 bytes) and a
    target identifier format code (2 bytes). Target identifier format codes
    would include values for unspecified, FC-VI, Infiniband, etc. The format
    codes serve much the same purpose as the current multiple formats of Fibre
    Channel target descriptors, one for port identifier, one for port WWN, one
    for both, etc.
    
    I am also tempted to allow the LUN’s device identification VPD data to be
    specified (e.g. the LUN’s WWN) within the inline data area, analogous to the
    current Identification descriptor target descriptor format. I welcome
    comments on whether this would be useful.
    
    
    SBC-2
    
    The third-party XOR commands identify the secondary target using a one byte
    SECONDARY ADDRESS field. For parallel SCSI this is the target identifier.
    For Fibre Channel it specifies the low byte of the second target’s port
    identifier; the other bytes of the port identifier are the same as the
    primary target’s.
    
    The third-party XOR commands also contain a TABLE ADDRESS flag. When that
    flag is zero, the commands operate as described above. When that flag is
    one, SBC / SBC-2 specify:
    
        A TABLE ADDRESS bit of one indicates that the SECONDARY ADDRESS
        field contains a pointer to a look up table of ANSI X3.270 SAM
        compliant target identifiers. The look up table is reserved for
        future definition.
    
    I plan to propose that we define the format of the lookup table for SBC-2. I
    plan to propose a table format that is a strict subset of that used by
    EXTENDED COPY. The header, target descriptors and inline data area would be
    identical. Segment descriptors would be prohibited. I believe that all the
    target descriptor formats defined by EXTENDED COPY should be supported.
    
    I would like to have SBC-2 cross-reference SPC-3 for the table format,
    rather than redefining it within SBC-2. Is that legitimate? Does it require
    any re-structuring of either standard?
    
    I believe the table should be volatile, reset on power-up or anything that
    resets the LUN. There should be exactly one copy per LUN, shared by all
    initiators. The maximum table length is close to 256*32 or 8192 bytes plus
    the inline data, perhaps about 64Kbytes total. That’s too large to fit
    easily into our mode page structure. We could segment the table, but I don’t
    like that, I think it should be loaded by a single command, The other
    choices are a new command or to define one of the reserved bytes in MODE
    SELECT to indicate the type of mode information provided. I have no
    preference between these last two choices. I would also welcome other
    suggestions.
    
    Edward A. Gardner               eag@ophidian.com
    Ophidian Designs                719 593-8866 voice
    1262 Hofstead Terrace           719 593-8989 fax
    Colorado Springs, CO  80907     719 210-7200 cell
    
    *
    * For T10 Reflector information, send a message with
    * 'info t10' (no quotes) in the message body to majordomo@t10.org
    
    
    


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