[GLLUG] Booting biosboot

[Tim Woodall]

On Mon, 7 Mar 2022, Henrik Morsing via GLLUG wrote:

>
> Good morning,
>
> I have installed CentOS on the latter half of a disk in an old lpatop, where 
> the first half conatins Ubuntu.
>
> The CentOS install, after booting, did not appear to have altered or 
> re-installed a boot loader. 
> I've read the CentOS manual, which just says you need a biosboot partition on 
> BIOS/GPT disks (same does the installer, if you try to remove it), the GRUB 
> documentation, which describes you need it, and various online blog that 
> generously describe how to create it. Not one bit of information described 
> what to do to GRUB to boot it.
>
> I've tried adding a manual grub.cfg entry to point to gpt#, # being both the 
> biosboot partition and the root partition, but nether worked.
>
> What is the solution here? And is the CentOS installer broken?
>

Do you mean a partition table that looks something like this?
root at xen17:~# fdisk -l /dev/sda
Disk /dev/sda: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: Samsung SSD 870
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: DAD8D524-D8F7-D04D-A21E-C8E14F39285B

Device      Start        End    Sectors   Size Type
/dev/sda1    2048       4095       2048     1M BIOS boot
/dev/sda2    4096     264191     260096   127M EFI System
/dev/sda3  264192 1953525134 1953260943 931.4G Linux filesystem

Then you don't always need the bios boot partition.

root at dirac:~# fdisk -l /dev/nvme0n1
Disk /dev/nvme0n1: 119.2 GiB, 128035676160 bytes, 250069680 sectors
Disk model: LITEON CA1-8D128
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: A9C70A11-3BDB-4338-ACB2-DAB386027BAC

Device          Start       End   Sectors   Size Type
/dev/nvme0n1p1     34    264191    264158   129M EFI System
/dev/nvme0n1p3 264192 250069646 249805455 119.1G Linux filesystem

IIRC you only need a bios boot to use legacy boot. If you're exclusively
EFI (dirac doesn't support anything else) then the bios boot partition
won't be used.

IIUC the problem is that grub needs some disk space for legacy boot and
the GPT format doesn't have enough unused space for grub to sneek in so
it needs an explicit partition reserved for it.

Tim.