Amanda:Should I use software or hardware compression?
From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
When you enable software compression, you drastically reduce the compression that might be achieved by hardware. In fact, tape drives will usually use *more* tape if you tell them to try to further compress already compressed data.
Thus, you must choose whether you're going to use software or hardware compression; don't ever enable both unless you want to waste tape space.
Since Amanda prefers to have complete information about tape sizes and compression rates, it can do a better job if you use software compression. However, if you can't afford the extra CPU usage, Amanda can live with the unpredictability of hardware compression, but you'll have to be very conservative about the specified tape size, specially if there are filesystems that contain mostly uncompressible data.
You might want to run amtapetype to determine if you have hardware-compression enabled for your tape-drive.