Looking to read past EOT on DDS/DLT/LTO

Nico de Jong nico at farumdata.dk
Wed Jan 25 16:20:12 CST 2017


I've had the same problem some years back, with a DDS-3, IIRC

I did some research, and the most reasonable outcome was that it was not 
possible by normal means, because some algorithm reading synchronisation 
data couldnt find out what was happening, so, the backup was ruined.....

  ----- Original Message ----- 
  From: Stan Sieler
  To: cctalk at classiccmp.org
  Sent: Wednesday, January 25, 2017 8:24 PM
  Subject: Looking to read past EOT on DDS/DLT/LTO


  We have a friend with a "tape" (DDS, DLT, or LTO ... don't know which yet)
  to which was written a system backup.  Thousands of files, with an EOF
  between each file, and a double EOF + EOT at the end.

  The problem:
  They then accidentally overwrote the start of the tape yesterday
  with about 1 KB of data, plus EOT.

  (By EOT I mean the "logical end-of-tape" indicator placed there
  by the drive firmware when a tape is rewound / unloaded after
  it's been written to.)

  So, we want to try to recover the files after that 1 KB + EOT.

  On reel tape, no problem.  I'd read until I hit the double EOF, and then
  skip the partial file (by looking for the next EOF), and then...gravy.

  But, DDS/DLT/LTO drives won't let me get past that darn logical 

  Are there solutions anyone can suggest?

  I've only heard of two approaches:

  1. find a drive with modified firmware, one that treats an EOT like an EOF

  While I heard some existed for DDS-1 drives, that lead was 20 years old :)

  2. insert tape, start writing (at the start), write until I'm sure I've
  written over the area where the EOT was,
  power cycle the drive
  read from the start, skipping over the data I just wrote, handle the
  possible corrupted record, and the I've got good data from past the EOT
  (note that this method loses a small amount of old data past the EOT
  that we overwrote).
  On DDS drives, I used this technique once or twice successfully,
  although I seem to recall I couldn't do the "read until I get to
  the good stuff" because of the probability of a bad record on the
  tape (where the power fail was) ... but the solution was to load the tape
  and then say "skip forward 3 EOFs", and the skip-to-EOF would
  quietly ignore (usually) any bad record before the EOF.

  BTW, if this is DDS, it's unlikely to be newer than DDS-4 (but possible),
  if it's DLT, it's likely one of the first two or three generations,
  and if it's LTO, it's extremely unlikely to be anything other than LTO-1.



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