Device Identifiers in SMC Read Element Status
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
>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
element status page whenever the size changes. As an alternative,
vendor-specific area would also work.
A host can avoid this issue by only requesting one element descriptor
a READ ELEMENT STATUS command with the DVCID bit set.
... Erich Oetting (SMC Technical editor)
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