Re: Low-level documentation

[Fredrik Tolf]

On Sat, 12 Apr 2014, Andrey Borzenkov wrote:
> > Grub 2, on the other hand, seems to try to go the "user-friendly" way,
> > with `grub-install' wanting to do all the work for me. This is fine, I
> > guess, so long as the circumstances aren't too strange for it to work.
>
> Could you describe your configuration where grub-install fails?

I can't say I have any case readily handy, or in fresh memory, but it has 
usually been in cases where I'm repairing a broken system. Things like, 
you know, having only one leg of a broken 0.9-metadata md mirror mounted 
raw, or moving a disk from a broken system to another system with the 
intention of moving it back, or having the device order out-of-whack due 
to bootable USB sticks being mounted, or using unusual filesystems or 
device remapping layers, or the like. Sometimes when the system is broken, 
I just don't want grub-install to go scanning devices.

More importantly, however, I'd just generally like to know what is going 
on. It is not very seldom that I find myself doing unusual things, like 
experimenting with the boot structure, or setting up systems in unusual 
ways due to various restrictions, or repartition disks later on, moving 
partitions around; and I'd like to be able to know and control just what 
happens.

For instance, as you write about grub-bios-setup:

> It tries to embed core.img into either post-MBR gap or reserved area on
> partition if filesystem is known to have one. It the worst case it
> gives you a rope to use file on filesystem directly. In all cases it
> installs boot block (MBR or PBR) which points to the installed
> core.img. That's all.

To me, this raises such questions as:

 * How do I know whether and where it embeds core.img?
 * How can I control the same?
 * How can I tell it which device to address?
 * The Texinfo manual tells me that boot.img merely loads the first sector
   of core.img. Does this mean that boot.img and core.img are both patched
   with their own sets of block lists and device numbers?
 * What does the "--allow-floppy" option do, more precisely?
 * Maybe I'm just blind, but I still haven't found how core.img actually
   finds the configuration file once it boots.
 * Is there any more detailed information on exactly what the various
   choices for the --format option to grub-mkimage actually do?

If you're up for simply answering those questions, that's great. The thing 
is that I never really needed to ask them with grub-legacy, however. Is 
there no documentation available other than reading through the source 
code?

Thanks for answering!

--
Fredrik Tolf