Karsten Fuhrmann wrote:

This should mean i can backup about 40G on a single tape. I am using software compression and turned off hw compression on the drive. But today it happend that even a dump with a compressed size of 33075 MB failed to dump because of a tape error [out of tape]

*** A TAPE ERROR OCCURRED: [[writing file: short write]].

In your amanda report there is a line telling where exactly it
hit end of tape.  See in the "NOTES" section.  From my report
when writing to a AIT-1 tape (35 Gbyte native capacity):

Was it really around 33 Gbyte when EOT was encountered?
Maybe you are a tiny bit too optimistic about the tapelength.  Tapes

when the databuffer is empty the drive rewrites the complete
datablock again, instead of a much slower stop/start.  Also the error
detection and correction results in some block multiple times on a tape.
This all lowers the optimistic capacity of 40 GByte. And tape
manufacturers used to count in marketing K- G- or M- prefixes meaning
K=1000 instead of the amanda K=1024 . The program "amtapetype -e 40g"
gives you a realistic capacity of your drive.

If you really hit into EOT around 33 Gbyte instead of 38.5 Gbyte, then
maybe you DO have compression on because you once labelled the tape

Many drives adjust themselves to (un-)compressed mode when they read
the tape.  Before amanda writes to the tape, it verifies the label,
and if that was written in compressed mode, then the drive automatically
switches to compressed mode again.
Relabeling with "amlabel" is not enough (because amlabel too verifies
the current tape before writing.  You have to disable compression, and
write some data to the tape without any read inbetween.
Verify the lights on the drive (a DLT4000 drive does have LEDs that
indicate compression status, I guess a DLT8000 drive has too).

--
Paul @ Home