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    SubjectRe: [PATCH 2.6.13 5/14] sas-class: sas_discover.c Discover process (end devices)
    On Tue, Sep 13, 2005 at 07:05:15PM +1000, Douglas Gilbert wrote:
    > Patrick Mansfield wrote:
    > > On Mon, Sep 12, 2005 at 11:06:37AM -0400, Luben Tuikov wrote:
    > <snip>
    > > IMO adding well known LUNs at this point to the standard added nothing of
    > > value, the target firmware has to check for special paths no matter what,
    > > adding a well known LUN does not change that. And most vendors will
    > > (likely) have support for use without a well known LUN. (This does not
    > > mean we should not support it in linux, I just don't know why this went
    > > into the standard.)
    > >
    > > Using well known LUNs will be another code path that will have to live
    > > alongside existing ones, and will likely require further black listing
    > > (similar to REPORT LUN vs scanning for LUNs).
    > Patrick,
    > The technique of supporting REPORT_LUNS on lun 0 of
    > a target in the case where there is no such device
    > (logical unit) is a pretty ugly. It also indicates what
    > is really happening: the target device intercepts
    > REPORT_LUNS, builds the response and replies on behalf
    > of lun 0.

    It should ignore the lun value for REPORT LUNS.

    > Turns out there are other reasons an application may want
    > to "talk" to a target device rather than one of its logical
    > units (e.g. access controls and log pages specific to
    > the target's transport). Well known lus can be seen with the
    > REPORT_LUNS (select_report=1) but there is no mechanism (that
    > I am aware of) that allows anyone to access them
    > from the user space with linux.

    What I mean is that the target has to intercept the command whether it is
    a REPORT LUN or for the well known (W_LUN).

    The target (firmware) code has to have code today like:

    if (cmd == REPORT_LUN) {

    For only W_LUN support, the code might be something like:

    if (lun == W_LUN) {
    if (cmd == REPORT_LUN) {

    But the first case above already covers even the W_LUN case.

    So adding a W_LUN at this point does not add any value ... maybe it looks
    nice in the spec and in someones firmware, but it does not add anything
    that I can see.

    Kind of like an 8 byte lun, it adds no meaningful functionallity. [I mean,
    who would want 2^64 LUs on one target? Yeh, let's give everyone in the
    world ... no in the universe their own private LUN on a single target. The
    LUN hiearchy is a bad idea, I have not seen a device that supports it,
    kind of like trying to implement network routing inside your storage box.
    Don't let those storage or database experts design your network hardware.]

    -- Patrick Mansfield
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