All the dump reports say amanda dumped 264.7 MB irrespectively of the real size. I don't use compression.
Trying with a whole directory tree (about 1.5GB) or only one file (about 330MB) results the same report:
Original Size (meg) 264.7 Avg Compressed Size (%) -- Filesystems Dumped 1 Avg Dump Rate (k/s) 2765.6 Tape Time (hrs:min) 0:01 Tape Size (meg) 264.7 Tape Used (%) 0.8
Thanks for the answers!
