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

[Pascal Hambourg]

Le 15/03/2019 à 20:44, Nathan Stratton Treadway a écrit :
> On Fri, Mar 15, 2019 at 19:43:18 +0100, Pascal Hambourg wrote:
> > Le 15/03/2019 à 01:53, Nathan Stratton Treadway a écrit :
> > > Question in brief: how can you get Grub, from the "grub recovery>"
> > > prompt, to display its boot-time view of disk drive geometry, etc?
> >
> > You can run 'ls' to print all detected disks and partitions, then
> > run 'ls' with each disk or partition as argument to print its size,
> > sector size, starting position...
>
> Thanks, this sounds like part of what we're looking for... but what's
> the exact syntax?
>
> When we tried it, "ls" did give the list of detected partitions:
>
>    grub rescue> ls
>    (hd0) (hd0,msdos1)
>
> , but doing ls on a partition only showed the filesystem type:
>
>    grub rescue> ls (hd0,msdos1)
>    (hd0,msdos1): Filesystem is ext2

Sorry, the position and size information is shown only when the module 
"ls" is loaded. I thought that running normal_exit to exit normal mode 
and go back to rescue mode would be the same as booting straight into 
rescue mode, but it is not : loaded modules are still loaded.

So you must either boot with another GRUB in normal mode, or include the 
"ls" module in the core image when installing GRUB with

grub-install --modules=ls /dev/...

> Would it be "ls (hd0)" that would show the full partition table
> information for the device?

It will show the device size, not the partition table.

> > As a workaround, you can try to create a small partition for /boot
> > at the beginning of the card.
>
> Yes, this is our plan... but we're trying to figure out what size the
> partitition can safely be.  If the limit is large enough, we might
> decide to split the partitions between "root" (medium sized) and "misc"
> (the rest of the space), rather than very-tiny /boot and the rest as
> the root... or something.

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.