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Re: Low-level documentation

From: Fredrik Tolf
Subject: Re: Low-level documentation
Date: Sun, 13 Apr 2014 20:02:12 +0200 (CEST)
User-agent: Alpine 2.10 (DEB 1266 2009-07-14)

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

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