Following up on myself...
Paul Bijnens wrote:
The tape really bumped into -- what it diagnosed as -- end of tape
at this point.
The problem is that "bumping into end of tape" is implemented as a
"hard write error" on the same spot.
Learning more every day...
There are two main technologies for tape writing: those that use
helical scan (DDS, Exabyte 8mm, AIT) without servo-tracks, and those
that use linear, serpentine writing with servotracks (QIC, DLT, LTO).
The tapes with servo-tracks ARE able to detect EOT: that information
is found in their servo-tracks. (I'm not sure how/if a damaged servo
track can result in premature End Of Tape.)
The helical scan type drives detect End Of Tape as a hard write error
on the same spot. They cannot distinguish the real end of tape from
a hard write error. Some of the drives seem to guess the end of tape
by measuring the spindle speed of one or both the reels, but can only
approximately indicate "near" end of tape.
At least that how I understand it...
For this you have to understand that most tapedrives (disclaimer: I do
not know a VXA drive, this could be different here) have a read head
that verifies what they write. During writing, error correcting and
error detecting bits are added, and many of the write errors can be
corrected by those additional error correcting bits. When writing
a block, and the read head diagnoses this as a bad block, the tape does
not rewind, but the firmware rewrites the same block again, in the hope
it does write correctly this time. If after a certain number of these
"soft" errors, the error persists, this becomes a "hard" error.
Bumping into end of tape is the same: the tape drive tries to write
a block, but it does not succeed, resulting in a "hard error".
The result of all this guessing is that most drives report a "hard
write error" as "end of tape".
Only in some circumstances, a drive can really know that it did not
bump into the "real" end of tape, but had a real I/O error.
Cleaning the tape drive helps sometimes.
--
Paul Bijnens, Xplanation Tel +32 16 397.511
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM Fax +32 16 397.512
http://www.xplanation.com/ email: Paul.Bijnens AT xplanation DOT com
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* stop, end, F3, ~., ^]c, +++ ATH, disconnect, halt, abort, hangup, *
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