Tape Backup

Anyone familiar with Travan tape drives?

My back-ups have started to exceed the capacity of one tape. I know that the -M option will prompt for further tapes if needed (ought to be the default surely), but at my command line :

tar -czvMf /dev/nst0 /home/Duke_Nukem/Data/

… bash immediately responds :

tar: Cannot use multi-volume compressed archives.

Yet advice I have read elsewhere advises just such a command line, and that files can be carried across media boundaries - the tarball is like a single file is it not? The back-up did work when I did not ask for compression (ie omitting the z).

Is this a special shortcoming of Travan or is this general with tar? It seems to me that when multi-volumes are required, this is the time when you most need compression.

Another question. Does recording include erasing previous data? At the moment I do :

mt -f /dev/nst0 retension
mt -f /dev/nst0 erase
mt -f /dev/nst0 rewind
tar [etc, as above]

Do I need the erase and rewind steps ?

Worryingly, when I do the erase command the tape winds only briefly and the command prompt soon returns, when I would expect it to take an hour or more, as streaming the data does. Does “erase” merely set a flag at the beginning of the tape (like a hard drive file) rather than physically erasing it all?

Grateful for any help.