RE: iSCSI: problem with LUN discovery

[Robert Snively <rsnively@Brocade.COM>]

The question posed below is answered perfectly by the SCSI
logical unit identifier specified in SPC-2.  A logical
unit is identified using the INQUIRY VPD page 83h.
The identifier has several possible formats, but the
format appropriate for RAIDs and other devices with
virtual logical units is the Fibre Channel 
"IEEE Registered Extended" format.  The first 64 bits
of that are typically the WWN of the node (typically
one of the attached controllers) that created the virtual
logical unit.  The subsequent 64 bits of the 128 bit
identifier are created by the controller in such a manner
that they are never duplicated by the controller, typically
using a high resolution time stamp and/or sequence number.

Replacement of the controller and access to the LUN by
another controller do not change the ID, which is part
of the meta-data of the logical unit.  The value is
never used again and dies with the formal de-creation
(destruction, removal) of the logical unit.

>  The question is if a storage controller keeps a different 
>  GUID for every LUN
>  created. If not we are in the same problem as WWN, LUN naming.
>  The question is how to identify a LUN in a storage 
>  controller that can
>  create and delete thousands of LUNs, and that the same WWN, 
>  LUN combination
>  can address different volumes depend on the intiator.
>  Also to keep this identifier in the EEPROM of the controller 
>  is not a good
>  idea, as cause problems when the controller is replaced.
>  
>  I think that the best way is to identify a LUN by a string 
>  composed by a
>  <GUID> (or S/N) <date> and <time> of the LUN creation. This 
>  method allows to
>  create infinite LUNs (over the time) in a given controller.
>  Keeping this string in the media and presented it in a 
>  Inquiry page avoids
>  the identification problems in controllers with multiple 
>  channels, that
>  supports different mapping for multiple initiators.