On Fri, Sep 30, 2005 at 02:10:51PM +0200, uwe.kaufmann AT infoconsult DOT nu 
wrote:
> Hi Listers,
> 
...
> 
> BTW: Is it true that a tape formerly written with hw-compression will
> "override" the 'drive compression off setting' because the compression
> on/off status has been written on the header of the tape? If yes, I should
> "reformat" a lot of tapes as described by Gene :-/

First, any tape written with HW compression will cause the drive to
switch modes when the data is read.

The problem comes when that switch persists during writes even though
you might have tried to set the device to no-compression.

Some OS, eg solaris, have different device names for compressed writes
and uncompressed writes.  No quibbles there, whatever the current mode
of the device, it is switched to the mode of the device.

The standard st and nst devices on Linux do no behave this way.  They
use whatever is the current mode of the tape.

At the start of tape operations amanda checks the identity of the tape
by reading the header and therein lies the problem.

- you have a tape with hw compress data
- you want to use is without hw compress so you do some mt magic
  hand wave and sure enough the tape is in no-hw-compress mode
- you start an amanda operation
- amanda reads the hw compressed data -- the tape drive switches mode
- now amanda writes but hw compression is on because of the header read

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)