Compression and TLZ07

[Ayman Altounji]

We recently moved our AlphaStation 250 from DEC Unix 4.0d to Linux (Debian linux, using the 2.4.14 kernel). This machine serves as our fileserver and has a DEC TLZ07 DDS2 DAT drive attached on the SCSI channel. I'm having trouble getting compression on the drive under Linux.

Under DEC Unix, we used the /dev/nrmt0h device, which is the no rewind compression-enable tape device. Typically, we could back the entire machine - a 2 GB internal drive and a 4 GB external drive - onto a single tape. The total data on the two drives is just over 4 GB, probably about 4.2-4.5 GB. Since switching to Linux, I cannot get any compression on the drive at all. Once 4 GB is written to the tape, it ejects.

I have attempted to modify the behavior of the drive several ways. I have tried using the standard Linux mt tools with the command "mt -f /dev/nst0 datcompress" and the enhanced mt-st tools with "mt-st -f /dev/nst0 defcompress" without success. I have also modified the stinit.def file used by stinit to initialize the drive, in which a number of modes are defined with compression on. No success with that either. What am I missing? Do I need to define a density value perhaps? Are there known problems with this drive under Linux?

Thanks for any help.