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    SubjectRe: [PATCH] Move SG_GET_SCSI_ID from sg to scsi
    Linus Torvalds wrote:
    >
    > On Sun, 26 Mar 2006, Bodo Eggert wrote:
    >
    >>Having a SCSI ID is a generic SCSI property
    >
    >
    > No it's not.
    >
    > Havign a SCSI ID is a f*cking idiotic thing to do.
    >
    > Only idiots like Joerg Schilling thinks that any such thing even _exists_.
    > It does not, never has, and never will.
    >
    > The way you reach a SCSI device is through the device filename, and trying
    > to use controller/channel/id/lun naming IS INSANE!
    >
    > Stop it now. We should kill that ioctl, not try to make it look like it is
    > sensible. It's not a sensible way to look up SCSI devices, and the fact
    > that some SCSI people think it is is doesn't make it so.
    >
    > The fact is, you CANNOT ID a SCSI device that way. Look at how /sys does
    > it, and realize that there's a damn good reason we do it that way. We ID
    > the same device in many different ways, because different people want to
    > ID it differently.
    >
    > You can ask "what's the first device we enumerated", you can ask "what's
    > the physical path to the device" or you can ask "what's the intrisic UUID
    > of the device". But the controller/channel/id/lun thing is just stupid.
    > You can look it up that way if you want to, but I refuse to have idiotic
    > interfaces that somehow try to make that the "official" name, when it
    > clearly is NOT.

    Linus,
    There are two things that really count:
    1) the identifier (preferably a world wide unique name)
    of the logical unit that is being addressed
    2) a topological description of how that logical unit
    is connected

    For the last 25 years various OS SCSI subsystems have used
    variants of 2) as a proxy for 1). Modern SCSI disks (and
    soon SATA disks) have a world wide unique name associated
    with their logical unit.

    So why are modern SCSI standards full of terms like
    I_T_L ** and I_T_L_Q nexus? Probably because the topology,
    especially when there are multiple paths to the same
    logical unit, is significant.

    Linux's <hctl> may be a ham fisted way of describing
    a path through a topology, but it easily beats /dev/sdabc
    and /dev/sg4711 .
    With a device node name like /dev/sdabc, a SCSI INQUIRY or
    an ATA IDENTIFY DEVICE command can be sent to ascertain 1)
    but I am unaware of any command sent to a logical unit that
    will yield 2).


    ** that is: the nexus of an Initiator port, a Target port
    and a Logical unit number.

    Doug Gilbert

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