LUs, GUIDs, LUNs

["Jim Hafner/Almaden/IBM" <hafner@almaden.ibm.com>]

<soapbox>

I'd like to help us all keep our terms straight.

LU = a SCSI logical unit that lives on a SCSI Target Device (see my last
posting or the discussion in T10 about multi-ported devices).   There may
be 1 or tens or hundreds or thousands of these things in a Target Device.
An LU is not a name, or an identifier or an address, it is an entity
(unit).

LUN = an *address* for a LU valid in the context of the initiator_target
connection (I_T Nexus in SAM-2).  This is NOT a globally unique identifier
or globally unique address for a LU as this number may be different within
each I_T nexus (as a result of standardized or vendor-specific LUN
Mapping).

LU WWID = (GUID) globally unique *identifier* for a logical unit.  This is
reported in EVPD page 83h of the SCSI INQUIRY command.  This is (by recent
requirements) mandatory (by T10) for a LU to have, and should be valid for
that LU only (and not be reused if that LU goes away, e.g., in a
controller).  NOT all controllers today have either of these properties
(though some do).

NOTE:
1) Having a LU WWID doesn't help one address the LU.  You still need a
target address (transport specific) and a LUN.  Where you get that from the
LU WWID is not specified anywhere.

2) Having a LUN doesn't give you the LU WWID directly.  You get that via
INQUIRY.  But, as this address is valid only in the context of an I_T
nexus, they aren't necessarily the best things to throw around for
third-party stuff.

3) Traditionally, LUN and LU have been synonymous (only because in parallel
SCSI only days, there was a one-to-one mapping of LUN to LU).  Those days
are gone.

</soapbox>
Jim Hafner