karim@uncecs.edu (Omar A. Karim) (03/20/91)
Misaligned heads can cause tape I/O to be 'unique' to a particular tape
drive. I discovered this due to the following chain of events:
(1) Machine crashed, trashing root partition.
(2) Attempts to boot of tape FAILED due to I/O error
(3) SUN came down and found tape drive was faulty, replaced drive.
(4) I attempted to restore the root partition from a level 0 dump...
(5) ... and the NEW tape drive wouldnt read the tape!!
(6) Fix: we booted off the tape using the NEW drive, switched drives after
boot. Now the OLD tape drive could read the dump tape. Restored.
I am wondering if anyone has a fix to this rather dangerous scenario:
1) Tape head gets misaligned. No symptoms, backups continue as usual, errorfree.
2) Tape drive crashes IRRETRIEVABLY, system crashes......
3) Replacement drive cannot read your DUMP tapes.
Is all lost? Is there a way to check the head alignment? I can think of
periodically reading the OS distribution tapes as a 'check'.
BOTTOM LINE: Misaligned tape heads can make ALL your dumps USELESS!
Worriedly yours
Omar A. Karim
karim@ecsvax.uncecs.edukludge@grissom.larc.nasa.gov ( Scott Dorsey) (03/24/91)
In article <1991@brchh104.bnr.ca> karim@uncecs.edu (Omar A. Karim) writes: >I am wondering if anyone has a fix to this rather dangerous scenario: >1 Tape head gets misaligned. No symptoms, backups continue as usual, errorfree. >2 Tape drive crashes IRRETRIEVABLY, system crashes...... >3 Replacement drive cannot read your DUMP tapes. On a nine-track drive, this is easy to do. I assume that your problem is not with horizontal alignment but solely with azimuth, though if you have a horizontal problem it's not difficult to do with the proper alignment tape (or even with Magna-See and a micrometer). First, put a dual trace scope on the output of the preamps for the inner and outer tracks (being careful to put them before any demod stuff), and start running a known-good tape on it. Adjust the head so that it's perpendicular to the tape by matching up the phase of the two square waves. You'll notice the corners squaring off as well, as the high frequency response improves. There, your azimuth is correct. Now, if you have a bad tape that was recorded at a slant azimuth, you can readjust the head so that the azimuth of the head matches that at which the tape was recorded (although you'll note that the wave will never square off as well, due to the length of the gap). Read the tape, and then be sure to put the machine back into correct alignment so that you don't write any tapes incorrectly. I know nothing about the 1/4" tape cartridges, but if they have multiple tracks this method should work. You'll probably find that the azimuth is less critical as the track width decreases and the tape speed increases, and more critical as the density increases. --scott Of course, the real fix is to A. Align your heads regularly B. Clean heads every time you mount a tape