tape compression

[Vu Pham]

My Linux server has a tape drive SDLT320, which is supposed to be able 
to store 320Gb data with compression.

Recently when the data is more than 160Gb , I guess it is about 180Gb 
now, then the tar command ends up with the error "tar: /dev/st0: Wrote 
only 0 of 10240 bytes" which I understand as the tape is full.

The data itself is mostly document, I can estimate more than half  of 
them are .doc, .xls, .eml which are compressed pretty well.

It looks like the tape drive does not compress at all.

Below is the mt output:
[root at coke root]# mt -f /dev/st0 status
SCSI 2 tape drive:
File number=0, block number=0, partition=0.
Tape block size 0 bytes. Density code 0x49 (Quantum SDLT320).
Soft error count since last status=0
General status bits on (41010000):
  BOT ONLINE IM_REP_EN


Do I have to run any extra command to force the tape drive to use 
compression mode ? I already tried "mt -f /dev/st0 compression 1" but it 
does not help.

Any advice is greatly appreciated.

Vu