On Wednesday 07 August 2002 17:46, Paul G. Allen wrote:
>My changer has an HP35480A DAT drive. Should I enable drive
> compression for use with Amanda and NOT use software compression
> for backups, or should I disable the drive compression and use
> software compression for backups? The compression on these drives
> is enabled/disabled with DIP switches. It can be controlled
> through software, IF the SCSI driver supports it and it looks
> like scsi-changer does not (I have no indication that I am able
> to turn the drive compression on/off, even though the DIP
> switches are currently set to allow it).
>
>PGA

1. Turn the compression on the drives off as using it hides the true 
capacity of the drive from amanda.  Most of the software 
compressors can handily beat that dumb 2-7 RLL in the drives by 
rather large amounts under the right conditions.  I have a couple 
of directories that often compress to 15% of their original size.

2.  If the tapes have been labeled with the compression on, then 
this setting is recorded in the tape header which is not accessable 
to you, and it becomes very difficult to turn it back off as the 
header will tell the drive to turn it back on the instant it gets a 
chance to re-read that header.

The only way I've found to get around this is to dd the tape label 
out to a scratch file, rewind, turn all the compression options off 
with mt, then feed the drive, using dd, about 10 or more megs worth 
of /dev/zero.   This will force the drive to flush its buffers to 
tape, and at that point the compression flags in the headers are 
reset to the current 'off' settings.  At that point you can rewind 
the tape and rewrite the label using dd with that scratch file as 
src.  The data on the tape will be gone, but at least you'll have 
an amanda usable tape when you are done.

-- 
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz  512M
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