Re: Installing grub2-bios into a raw partition instead of MBR gap

[Pascal-liste]

Le 17/02/2021 à 09:20, Christian Eggers a écrit :
> I would like to install grub2 to a non-EFI machine with Windows+Linux (MBR 
> partition
> table). As Linux uses LVM and the first partition starts at sector 63,

That must be a very old partition table.

> there is not enough
> space in the MBR gap for grub. But I have a spare partition before the LVM 
> starts (/dev/
> sda5, 1 MB) where I would like
> to install grub.
>
> # grub2-install /dev/sda5
> grub2-install error embedding is not possible but this is required for raid and 
> lvm install
>
> Why is "embedding" required for a LVM install?

Because block lists (the opposite of embedding), beside being 
unreliable, can be used only on disks or plain partitions and some 
filesystems, not on complex devices such as LVM or RAID.

Anyway this command would not do what you want. GRUB-install installs 
two parts :
- the boot image in the specified device boot sector
- a core image (rescue shell + modules required to read /boot/grub), 
either embedded in a dedicated location (MBR gap, BIOS boot partition, 
btrfs or LVM reserved boot area...) or as a regular file 
/boot/grub/i386-pc/core.img.

Your command would write the boot image in /dev/sda5 boot sector, not in 
the disk MBR. This requires another boot loader to be installed in the 
MBR and chainload GRUB. If you want to go that way you need a bigger 
partition so that you can format it as btrfs which has a reserved boot 
area. AFAIK, the minimum size of a btrfs partition is 16 MiB.

> why can't I use a bare partition (like on an EFI system)?
Regretfully GRUB developpers did not implement a option to explicitly 
select an arbitrary area for the core image. With a GPT partition table, 
you could create a "BIOS boot" partition where grub-install would write 
the core image, but there is no such thing with a DOS partition table, 
and Windows stands in the way of converting the disk to GPT. A 
workaround would be to create a hybrid DOS/GPT MBR, but this is quite 
hackish and unstable.