Its usually controlled by the device name you use to access the tape.
If you look in /dev/rmt, you'll see all the tape devices.
The ones with c in their name are the compressed devices.
So if you save to /dev/rmt/0 your writing to the uncompressed device.
If you save to /dev/rmt/0c you get compression.
Thanks. I just done another test before I read your reply. I have another HP DAT72 tape drive attached to the other Sun server(Netra240, same the 1st server), and I executed the command 'tar cvf /dev/rmt/2n /data', however, the /data is about 49 GB, and the backup speed is about 4.23 MB/s...
Since there's 'media write error' in the 1st Netra240 server, and the tape drive [Clean] LED is lighted, so I put the clean tape before I did backup. And I found this situation, it can't do hardware compress!!
And may I ask if using '/dev/rmt/0c', does it mean software compress or hardware compress?
TKS!!