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Re: workaround for BIOS / CHS stuff

From: Szakacsits Szabolcs
Subject: Re: workaround for BIOS / CHS stuff
Date: Sun, 27 Jun 2004 18:20:54 +0200 (MEST)

On Sun, 27 Jun 2004, Andrew Clausen wrote:

> I just had a look at the QtParted source, and it doesn't set a constraint.
> It should set a constraint telling libparted not to move the start.

Thanks for checking it out.
> > They say, parted is broken.
> Do they?

If they don't then I'll ;-) Both problems (unbootability, shifted
partition starts) are basically caused by parted trusting the untrustable
kernel providing the right logical geometry.
> > Yes, used to. But because it wasn't reliable (see the destroyed partition
> > tables by parted running on 2.4 kernels), kernel even stopped guessing. 
> Which one are you referring to here?

2.4 kernels tried to guess the "right", logical geometry but since it
didn't always work and it was unreliable thus 2.6 kernels don't guess
it anymore.
> > > > They report the disk geometry and usually this is not what parted 
> > > > expects.
> > > 
> > > You mean "usually wrong".
> > 
> > Yes. Most often the number of logical heads are 240 or 255. But 2.6
> > kernels report often 16 in the problematic cases. Parted rewrites the
> > partition table with incorrect CHS, adjust to the wrong, imaginary
> > cylinder boundary thus the partition is gone,
> ?

The above bug you pointed out above. Constraint wasn't set thus parted
adjusted the partition start based on the incorrect CHS what parted
thought was correct.
> > or in a better case it's only unbootable.
> (you can always switch the BIOS to lba)

No. There are a lot of boxes, notebooks, etc where this is not possible
to do. And for whatever reason, this also doesn't work always.
> > One of the explanations why parted damages partition tables on 2.4
> > kernels,
> > 
> >    http://linux.derkeiler.com/Mailing-Lists/Kernel/2003-10/4682.html
> Parted isn't mentioned, but I take your point about even 2.4 being
> imperfect.

Yes, 2.4 was also imperfect. That's why the logical geometry guessing was
dropped completely in 2.6 kernels.

> >    http://qtparted.sourceforge.net/forums/viewtopic.php?t=144
> This looks like the above bug in qtparted.

It is, due to parted using the wrong geometry on the latest 2.4 kernel.


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