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    SubjectRe: [PATCH 2.6.13 5/14] sas-class: sas_discover.c Discover process (end devices)
    On 09/13/05 18:42, Patrick Mansfield wrote:
    > 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:
    >>>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).
    >>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.

    Notice that Doug is _right_. To convince yourself of this,
    please look up _who_ would execute REPORT LUNS on the target

    >>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.

    Doug is right here too.

    > 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) {
    > do_report_lun();
    > }
    > For only W_LUN support, the code might be something like:
    > if (lun == W_LUN) {
    > if (cmd == REPORT_LUN) {
    > do_report_lun();
    > }
    > }
    > But the first case above already covers even the W_LUN case.

    _Except_, that what the firmware actually does is, it routes
    the tasks by LUN first, _before_ looking up with what the command
    is.* This is crucial.

    You can convince yourlelf of this taking a look at the SCSI Target
    architecture in SAM.

    (*) Notice how according to your code above, the initiator may
    assume that a LUN exists where it actually _does_not_.

    > 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.

    I wonder if the maintainer of the SCSI Core would listen or ignore your
    opinion here.

    I wonder _who_ decides here where speculation ends and industry
    opinion starts?

    As Documentation/ManagamentStyle points out, the Manager does _not_
    have to know everything -- in fact this is encouraged in that document.
    What she/he has to know is _who_ to listen to, and how to make

    > 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.]

    Well, what can I say...
    "No one will ever need more than 64K in their computer."


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