On Thursday 30 January 2003 18:09, Jason L Tibbitts III wrote:
>>>>>> "lw" == lone wolf <lone_wolf_20 AT yahoo DOT com> writes:
>
>lw> 2) Has anyone used the SDLT 320 drives w/ Amanda?  I searched
> the lw> group archives and didn't get much from them on these
> drives, same lw> with the FAQ (tape-info would be nice for these
> drives).
>
>I have an SDLT220 drive; it works just fine.  The only issue I
> have is with compression: I do software compression but I can't
> seem to get the drive to turn off its internal compression
> properly.  This seems to result in erratic tape capacity.

This sounds as if that drive type uses a scheme similar to the DDS 
drives use, wherein the compression state in effect when that tape 
was first written, is recorded in a hidden header only the drive 
can access. So when a tape that been compressed before is 
re-inserted and recognized, even if the dip switch setting is off, 
it will turn the compression back on in the process of recognizing 
the tape.

For a DDS drive, one must use mt to turn it off, and then force 
enough data to the drive to cause it to have to flush the buffers 
to tape.  This then will rewrite that hidden header with the 
compression flag bits turned off, and you should then have an 
uncompressed tape.

To preserve the tape label, do it something like this, commands 
adjusted for your environment of course.

mt -f /dev/device rewind
#save the tapes label
dd if=/dev/device of=scratch bs=32k
mt -f /dev/device rewind
# shut the compression off
mt -f /dev/device compression off
# now force a buffer flush to update the flags
dd if=/dev/zero of=/dev/device bs=32k count=100000
mt -f /dev/device rewind
# restore the tapes label
dd if=./scratch of=/dev/device bs=32k
mt -f /dev/device rewind

One could clean this up and make it work then put this into a script 
and run it 30 minutes before amdump is fired off.

>lw> 3) I am planning on using a Linux box (running RedHat 8.0)
> with lw> sufficient power to do software compression.
>
>Well, I back up a couple of terabytes of disk to a single SDLT220
>drive without problems; the dumps can sometimes take six hours or
> so but they're always finished by the morning.

-- 
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz  512M
99.22% setiathome rank, not too shabby for a WV hillbilly