SMC - Do any media changers need unique ID's for storage/export/transport elements?

Ballard, Curtis C (StorageWorks) curtis.ballard at hp.com
Wed Jul 7 20:25:01 PDT 2010


* From the T10 Reflector (t10 at t10.org), posted by:
* "Ballard, Curtis C (StorageWorks)" <curtis.ballard at hp.com>
*
The SMC-3 working group has been working on a new command set to provide a
more flexible alternative to the complex READ ELEMENT STATUS command and has
reviewed that command set to make certain that all of the functionality in
RES is duplicated in the new command set.
One thing we have identified as not completely duplicated is that READ
ELEMENT STATUS has the ability to return a VPD page 83h descriptor (SPC-3
Identification Descriptor, SPC-4 Designation Descriptor) for all of the
elements.  Primarily that is used to report either the WWN or the Serial
Number of the data transfer device located at that element.
I am not aware of any devices that report an identification descriptor for
anything except the drive but reporting it for other element types is
allowed.
In researching the command history there were some hints that there may be a
model where a media changer uses an Import/Export element as a way for
transferring media between two different media changers and that in that
model the identifier can be used by an application client to determine that
the I/E port it has available in one changer is the same as the I/E port that
is available in another changer.
All of the modern media changers I am aware of use an invisible
"pass-through" device to pass media between changers and when changers are
connected one of the changers becomes the master and reports the total size
of all connected changers.  If the total slot count exceeded 64K the two
media changers would have to reported independently when using READ ELEMENT
STATUS since that is the largest count that RES can report however in the new
command set we extended the element count to 4 bytes so a media changer would
have to contain more than 4 billion pieces of media before it ran out of
space.
Is anybody aware of a need to provide a method for reporting a unique
identifier for the I/E elements, storage elements, or transport elements?
If there is a need to provide a unique identifier for non-DTD elements, is
the Identification Descriptor the right method or would a media changer
assigned "unique ID" be sufficient?  
If a unique ID would be sufficient it would be easier to just assign a couple
of bytes in a current REPORT ELEMENT INFORMATION page that can be used for an
ID.  If we want the full identification descriptor I think I would probably
create a new page of information that could be returned in REPORT ELEMENT
INFORMATION. 
Curtis Ballard
Hewlett Packard
StorageWorks Platforms Tape
Fort Collins, CO
(970) 898-3013
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