OT:DAT remaining capacity response

[Alan Dayley]

This is not directly Linux related but I am having a <bleep> of a time
finding an answer.  All of you on this list a experienced in other things
besides Linux and I hope to tap a knowledgable brain somewhere.  Respond
off the list if you would like.

I tested a Seagate DAT drive to determine the way it calculates the
remaining capactiy response to the Log Sense Page 0x31 command.  I have
conflicting information from other sources about the answer I should expect.

Question
--------
Do SCSI DAT drives report remaining capacity as an ABSOLUTE value or a
value RELATIVE to the current location?

Hypothetical example: Take a 2GB tape.  Write 1GB of data to the tape.
Rewind the tape.  Ask for remaining capacity (Log Sense Page 0x31).  The
ABSOLUTE value response would be 1GB (total - used = remaining).  The
RELATIVE value response would report 2GB (total - position = remaining).
Which is the industry standard?  Is the other calculation method ever used?

Test
----
Using the Linux tar command, I wrote 10, 1.2MB backups to a tape.  This
created 10, 1.2MB files on the tape.  I queried remaining capacity three
times: 1)after the last write, 2)after a back space of 5 file marks and
3)after a rewind.

Results
-------
Operation          Remaining Capacity
After last write   3,777,480 KB
After back spaces  3,784,037 KB
After rewind       3,795,909 KB

Total capacity of media: 3,795,909KB

Conclusion
----------
The Seagate DAT tape drive reports remaining capacity RELATIVE to the
current location.

The question remains: What do other manufacturer's drives report?  Should I
trust the Seagate results as representative of all DAT drives?  Since Log
Sense Page 0x31 is a vendor specific page, I am not sure I should trust a
universal remaining capacity method of calculation.

I ask this question here because there is an amazing lack of information on
the net about how DAT drives should or do operate.  Info or pointers to
info on this subject would be greatly appreciated.

Alan