Toralf Lund wrote:
Paul Bijnens wrote:
amtapetype will tell you too if hardware compression is on.
OK.
Does amanda have any built-in support for switching it off? I mean, can
any of the changer scripts or whatever do this? Or even amdump itself?
No. Controlling hardware compression is a very OS and hardware
specific issue.
It can even be manipulated by dipswitches on the tapedrive itself.
It can even be more complicated: when reading a tape, the drive
adjusts itself automatically to the correct setting.
On some(*) linux versions this results in automatically writing
with whatever setting the tape was previously written.
I.e. if you have a tape that was written with hardware compression,
and you first read some bytes of the tape (as amanda does to verify
the label), and subsequently write something to the tape, the
tape is written with hardware compression, whatever you switched
the drive before. To get out of this situation, you have to
insert the tape, disable hardware compression, and write some data
to the tape WITHOUT reading something first.
(*) I used to say "on all linux versions", but it seems there
are different implementations in different versions.
Some systems can control the tapesettings with the file
/etc/stinit.def (see "man stinit" if that exists).
There is also a method to use different device names (**), just
as in Solaris, where the letters c, h, m, l are indications
of compression and density to use when writing. This avoids
the above problems, because when you open() a drive for writing,
the device name (minor device number actually) contains the settings
for the drive implicitly.
The whole problem is still not completely clear to me.
And the lack of documentation on the st-driver in Linux does not help
either. I mean: any pointers are welcome.
(**) at least the comments in the kernel source code in
drivers/scsi/st.c seem to imply this, but I cannot find any other
decent documentation of this.
--
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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