Re: getting grub's view of disk geometry from "grub rescue>" prompt?

[Pascal Hambourg]

Le 16/03/2019 à 00:00, Nathan Stratton Treadway a écrit :
> On Fri, Mar 15, 2019 at 21:54:27 +0100, Pascal Hambourg wrote:
>
> > You can try dichotomy. Create a partition starting at 50% of the size.
> > Mount it and install GRUB using the mount point as boot directory :
> >
> > grub-install --boot-directory=<mount-point> /dev/xxx
> >
> > If GRUB boots in normal mode, repeat at 75%. If it boots in rescue
> > mode, repeat at 25%. And so on.
>
> Unfortunately I don't think that procedure will work reliably, because
> grub could boot fine from a bigger-than-the-unknown-threshold partition
> if the /boot directory contents happened to be installed early in the
> partition... but then later fail to boot against that that same
> partition if the files got moved near the end of the filesystem.

You are misinterpreting what I wrote. It is about the starting position, 
not the size. If the partition starting at X% of the disk size works, it 
means that the threshold is at least X%.
You can use a small test partition, 100 MB in more than enough for GRUB.