Exabyte Tape Errors

[George Goble]

In article <3421 at brchh104.bnr.ca> adw at isg.cs.reading.ac.uk (Anthony Worrall) writes:
>We have beening doing dumps onto an Exabyte connected to a 4/380 under 
>SUNOS 4.1.1 rev B using Sony QG-112M tapes. Recently we have started to 
>get write errors with the error message 
>
>	st1:  write failed
>	st1 error:  sense key(0x5): illegal request
>
>
>This error message seems to be tape dependent and has occurred on the Exabyte
>tape supplied with the drive as well as the Sony tapes. I had the Sun engineer
>in and we changed the drive the cables and finally the CPU board which seemed
>to cure the problem, but now it is back again. 

This is probably "cockpit error", not anything wrong with the tape drive.
Exabytes (8200) can only write at the following places:
1) starting at the BOT
2) after the end of the last data written on the tape (EOM)
3) at the "front" side of a filemark.

If a write is attempted anyplace else (e.g. in the middle of a file)
you get an illegal req error.

A tar "append" (tar r or u) will cause this also, when an attempt is
made to overwrite data near the end of file.  Say one has a tape with
3 files on it (dump images?), and only wants to save the first two, and
write over the third..

On normal magtape:
mt fsf 2
write on it.

on Exabyte
mt fsf 2
mt bsf 1  (or "mt bsfm 1" if sunos4.0 or later)
mt weof 1  (this writes a filemark over itself, but erases area after EOF)
write on it

This restriction is due to the helical scan heads cannot erase tape.
The 8500 has slightly less restrictions, it may be able to start a 
write at the EOT side of a filemark as well.
--ghg