FAQ:Should I use software or hardware compression?
From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
LTO drives use an algorithm for hardware compression that would not try to compress already compressed data. [ http://en.wikipedia.org/wiki/Linear_Tape-Open#Compression] To make it short, data is tested on the fly by the drive to check if compression makes it smaller than what is natively received or not.

