On Wednesday 04 April 2007, Jon LaBadie wrote:
>On Wed, Apr 04, 2007 at 11:01:20AM +0200, Sebastian Henrich wrote:
>> > On Tuesday 03 April 2007, Sebastian Henrich wrote:
>> > >Hello,
>> > >
>> > >I'm using amanda 2.4.5 to backup my server. The tape is a HP DAT
>> > > 72. This drives stores 36 GB of uncompressed data. I switched the
>> > > hardware compression off and tried to use the software
>> > > compression. Now I have the problem that amanda tells me every
>> > > time
>> >
>> > You may be getting bit by something I ran into several years ago
>> > now.
>> >
>> > If the tape is written with the hardware compression on, then the
>> > drive will detect that and turn the compression on even if you think
>> > its turned off.
>
>...
>
>> I diabled the hardware compression before I used the tapes for the
>> first time like it's described in the docs. mt shows me that hardware
>> compression is off after inserting a tape. So I think that it's really
>> disabled.
>
>The switch may not occur unless the drive actually attempts to read the
> tape. Amanda does an initial read before any attempt to write to a tape
> to check if it is actually an amanda tape and is the expected amanda
> tape.

That has not been my experience Jon.  The drive reads that hidden header 
to see what the tape is before it acknowledges it and allows us access to 
it, and it is during that recognition read that the compression bit was 
getting turned back on.  I could turn it off, insert a tape, and if the 
tape had it set, by the time the drive had recognized the tape, the 
compression bit had been set back to on.  The amcheck read hadn't come 
into the scene yet.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Computer programmers do it byte by byte.