Enabling compression for tape drive?

[Andrew Mickish <mickish cmu edu>]

I have an Exabyte Eliant 820E SCSI tape drive, and I would like to
enable data compression under the RedHat 5.2 distribution.
I've been following the steps at

        http://www.exabyte.com/suppserv/techsupp/8mm/misc/in0181.html

Writing uncompressed to /dev/nst0 has been working for a long time,
and my understanding is that I need to set the density for this
device in order to write compressed data.

However, after setting the density using the 'mt' command as in
the instructions, the amount of data that can be written to one tape
does not change.  Additionally, the status keyword for 'mt' does not
show that my commands are having any effect:

        [root wolf /]# mt -f /dev/nst0 status
        SCSI 2 tape drive:
        File number=-1, block number=-1, partition=0.
        Tape block size 1024 bytes. Density code 0x0 (default).
        Soft error count since last status=0
        General status bits on (1010000):
         ONLINE IM_REP_EN
        
        [root wolf /]# mt -f /dev/nst0 setdensity 140
        
        [root wolf /]# mt -f /dev/nst0 status
        SCSI 2 tape drive:
        File number=-1, block number=-1, partition=0.
        Tape block size 1024 bytes. Density code 0x0 (default).
        Soft error count since last status=0
        General status bits on (1010000):
         ONLINE IM_REP_EN


I have been using commands like this to dump to tape:

        [root wolf /]# dump 0dsbfu 54000 6000 126 /backups/wolf/root.0 /dev/hda11


Is there something else I should be doing to enable compression
for a tape device?

Thanks,

--Andrew Mickish