Jon LaBadie wrote:
I don't think it is actually a 'flag', but merely the presence of HW
compressed data at the front of the tape. The compression setting
is only relevant when writing tapes. Reading switches automatically
based on the type of data (HW compressed or not) written before.
And that "auto switch" is what causes the problem. You set the
tape to compression off, read a bit of compressed data (like the
amanda label at the start of the tape previously written with
compression on) and the drive "auto switches" to compression on.
To clear this state for amanda you have to write a bunch of data
at the beginning of the tape, after setting compression off, and
be sure to not read anything between the two.
The good news is, that when I try to reproduce the problem, I cannot
(anymore).
A few years ago, I struggled with the compression settings of my
tape drive (a DDS-2 drive), and I used sequences like below
to test the hardware compression status of the drive.
It was then that I added the hwcompr test heuristics to amtapetype,
at that time still called "tapetype".
I'm not 100% sure, but I believe I used a Linux kernel 2.4.18.
I try to reproduce the problem now, but my current config does NOT
autoswitch after reading a tape:
$ mt -f /dev/st2 compression 1
$ amtapetype -oc -f /dev/st2
... yes hw compression enabled
$ mt -f /dev/st2 compression 0
$ amtapetype -oc -f /dev/st2
... no warning about hw compression!!!
Amtapetype does read the tape before writing! I even added a
"dd if=/dev/st2 of=/dev/null bs=32k" inbetween, and still got
same result.
That either means that phase of the moon is correct for handling of
tapes, or that current versions of Linux do not have the problem
anymore. It tried it both on a 2.4.20 kernel and a 2.6.9 kernel,
and both do not autoswitch for writing (they do for reading, just as
expected).
Can anyone still reproduce the problem? And under what circumstances?
--
Paul Bijnens, Xplanation Tel +32 16 397.511
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM Fax +32 16 397.512
http://www.xplanation.com/ email: Paul.Bijnens AT xplanation DOT com
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