Re: Booting from Devices > 2TB with BIOS and GPT

[Pascal Hambourg]

Le 08/06/2021 à 21:45, Michael Hierweck a écrit :
> today I experienced GRUB 2.02 was not able to boot successfully a QEMU 
> based virtual machine in BIOS mode from a GPT partitioned virtual disk 
> when the root partition (containing /boot) exceeds 2 TB even when a 
> there is a tiny bios_grub partition (ef02) exists at the beginning of 
> the disk.
>
> I was dropped to the GRUB rescue shell but was not able to load any 
> modules: "attempt to read or write outside of disk 'hd0'".

You have a broken BIOS. Nothing in the BIOS interface limits the usable 
size of a disk to 2 TiB.

> After that I did some experiments. Reinstalled the system but assigned 
> only 1 TB to the root/swap partition. The system booted successfully. 
> After that I increased the partition to 4 TB and increased the 
> filesystem. GRUB was still able to boot.

Until you reinstall GRUB, the kernel or the initramfs in a location 
beyond the BIOS limit.

> I suppose the problem occurs when (after stage 1 and 1.5 have been 
> successfully loaded) parts of stage 2 (modules from /boot/grub...) have 
> to been loaded from disk sectors beyond the 2 TB limit.

GRUB 2 has no stage 1/1.5/2. It has a boot image which loads a core 
image which loads files.

> Can you confirm that not only the bios_grub partition but also 
> /boot/grub has to be located in the first 2 TB of the disk?

No. If the BIOS is broken, the whole /boot (including kernel and 
initramfs, not only /boot/grub) must be located within the BIOS limit. 
If the BIOS is not broken, /boot can be located anywhere.