Create device file for tape drive specifying no compression

[EEESEC]

I have a SCSI DAT 72 tape drive connected to a HP-UX 11iv2 server that has hardware compression enabled by default.  For various reasons, I'd like to disable this compression.  According to docs for the drive I can override the default compression with "software settings."  I'd like to  specify a non-compression device file to test to see if that will actually disable it.

The man page for mt(7) is confusing me. 

Currently I have these device files in /dev/rmt:
c0t0d0BEST, c0t0d0BESTb, c0t0d0BESTn,
c0t0d0BESTnb, c0t0d0DDS, c0t0d0DDSb, c0t0d0DDSn, c0t0d0DDSnb, c0t1d0BEST,
c0t1d0BESTb, c0t1d0BESTn, c0t1d0BESTnb, c0t1d0DDS, c0t1d0DDSb, c0t1d0DDSn,
c0t1d0DDSnb,

I know that "BEST" will use compression. Are the "DDS" dfs set to NOT use compression?  What would be the mksf command to create the appropriate device file without compression?  Any assistance would be appreciated.

Thanks.

[Dennis Handly]

>What would be the mksf command to create the appropriate device file without compression?

Have you looked at the lssf(1M) output for each of your device files?

[EEESEC]

The BEST device files show best density of course, the DDS device files don't really specify.  I'm not sure how much sense it would make to try and use the DDS ones either since it looks like they are expecting a DDS1 cartridge.

#lssf /dev/rmt/c0t0d0DDS
stape card instance 0 SCSI target 0 SCSI LUN 0 at&t DDS1 DAT tape cartridge at a
ddress 0/0/1/0.0.0 /dev/rmt/c0t0d0DDS

#lssf /dev/rmt/c0t0d0BEST
stape card instance 0 SCSI target 0 SCSI LUN 0 at&t best density available at ad
dress 0/0/1/0.0.0 /dev/rmt/c0t0d0BEST

mt status isn't much help either:

mt -f /dev/rmt/c0t0d0DDS status
Drive:  HP C7438A
Format:
Status: [41014700] BOT online immediate-report-mode
File:   0

I've found an option in HP's Library and Tape Tools utility (ltt)  that says it disables compression, but I'm not conviced it works.  (The LTT utility that test's compression automatically turns it back on when it runs regardless so that is no help.)

Would prefer to create my own appropriate device file for it if I can figure it out.

(Updated  first post for mksf typo)

Thanks.

[Pete Randall]

Take a look at "man 7 mt", particularly the Options section referring to density and the Warnings section in the third paragraph where it discusses data interchange between devices supporting and not supporting compression.  It says that you need to specify a "density option with an argument of DDS1, and must not include a compression option".

[EEESEC]

Saw that as well and I did make a couple of device files with the DDS options makeing sure I didn't specify compression:

/sbin/mksf  -v -H 0/0/1/0.0.0 -b DDS4 -u

/sbin/mksf  -v -H 0/0/1/0.0.0 -b DDS1 -u

Even thought it dosen't mention it in the man page there is a DDS3 and 4 option which you can see in the mtio.h header.  The problem here is that I'm specifying a wrong density.  The DDS4 is the closest option they have to DAT72.

Looks like I can write to the drive fine using either of the new device files but I haven't had a chance yet to see how much data it will actually allow me to write to it.  Are there known issues with choosing a different density than the drive is supposed to use?

Thanks for the replies.