Device Identifiers in SMC Read Element Status

Erich Oetting erich_oetting at stortek.com
Mon Mar 2 10:19:32 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Erich Oetting <erich_oetting at stortek.com>
*
>In SMC revision 10, the paragraph describing the IDENTIFIER field in the
>definitions of every element descriptor (sections 6.5.3, 6.5.4, 6.5.5,
>and 6.5.6) contains the following sentence:
>"If no device identifier is available for this element, or the DVCID bit
>in the CDB is zero, this field shall be omitted."
>
>In addition, the first paragraph following table 16 in section 6.5.7
>contains the following two sentences:
>"Device identifiers may be available for some or all elements in a
>medium changer. If no device identifier is available or the DVCID bit in
>the CDB is zero, the IDENTIFIER LENGTH shall be zero, the IDENTIFIER
>field is omitted, and the CODE SET and IDENTIFIER TYPE fields shall be
>zero."
>
>This seems to imply that not all the element descriptors for an element
>type must be the same size (e.g. if one tape drive in an autoloader
>supports device identifiers but a second doesn't, or their device
>identifiers are of different sizes, the descriptors will have different
>sizes).  If this is the case, what values should be used for the BYTE
>COUNT OF REPORT AVAILABLE field of the element status data header and
>the BYTE COUNT OF DESCRIPTOR DATA AVAILABLE and (most importantly)
>ELEMENT DESCRIPTOR LENGTH fields of the element status page header?
>Also, this would apparently require an application to parse every field
>in every preceding element descriptor of the same type in order to find
>the descriptor for a specific element address.
> 
>As a possible solution, the end of the vendor-specific area of each
>element descriptor can be "padded" with zeroes so that all element
>descriptors of the same type have the same size.
>
>Please let me know if I am misinterpreting SMC in this regard, or else
>if there is a "proper" solution to this problem.

Adding support for device identifiers does make targets more complicated.

If element descriptors are of different sizes, the solution is to return 
a new
element status page whenever the size changes.  As an alternative, 
padding the
vendor-specific area would also work.  

A host can avoid this issue by only requesting one element descriptor 
when issuing
a READ ELEMENT STATUS command with the DVCID bit set.

  ... Erich Oetting (SMC Technical editor)
*
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