Is there some reason you aren't using amanda? Give it some holding disk space and it will run multiple backups at once, buffering on disk, and figure out how they should go on the tape for you.
I'm archiving, not backing up.
I looked at Amanda for a few days and it would be really clunky to do what I want.
Anyway I found this :
[root@solexa-db tmp]# export GZIP=-v [root@solexa-db tmp]# tar czf files.tar.gz file{1,2,3,4,5} 98.4%
So I'm golden :-)