Dualboot with 2nd Linux distro (EFI + Grub)

[jcdenton251]

Hi,

I have a laptop that came preinstalled with Ubuntu on an encrypted LVM volume. Now, I want to add a 2nd Linux distribution (Arch Linux) so I can select to boot either Ubuntu or Arch Linux during boot.

My idea is to have a new logical volume inside the existing volume group, so the same password is used both for Ubuntu and Arch Linux. But so far, I have had no success because the /boot/grub/grub.cfg file generated by grub-mkconfig is not correct. If you can suggest any tutorials, please let me know. This is what I have tried so far:

- The config of grub was changed so that the grub menu appears for 30 seconds during boot, so I can select which OS to boot.

- LVM volumes have been adapted to make room for a third logical volume, which I have named system-root--archlinux. Here's the output of lsblk:

NAME                         MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
nvme0n1                      259:0    0 931,5G  0 disk
├─nvme0n1p1                  259:1    0   512M  0 part  /boot
├─nvme0n1p2                  259:2    0   512M  0 part  /boot/efi
└─nvme0n1p3                  259:3    0 930,5G  0 part
  └─crypt_dev_nvme0n1p3      253:0    0 930,5G  0 crypt
    ├─system-root            253:1    0   300G  0 lvm   /
    ├─system-swap            253:2    0     8G  0 lvm   [SWAP]
    └─system-root--archlinux 253:3    0 180,3G  0 lvm

- ArchLinux has been installed on system-root--archlinux. During the installation of ArchLinux, I have mounted /dev/nvme0n1p1 on /boot and /dev/nvme0n1p2 on /boot/efi (just like on Ubuntu), so that I now have different kernels lying on /dev/nvme0n1p1 (both for Ubuntu and ArchLinux)

- With Ubuntu booted, I have modified /etc/default/grub by adding the line

GRUB_DISABLE_OS_PROBER=false

so that grub-mkconfig detects a 2nd operating system.

I ran

grub mkconfig -o /boot/grub/grub.cfg

and the output of that command included the following:

Found Arch Linux on /dev/mapper/system-root--archlinux

However, the command seems to have created a faulty menuentry for Ubuntu. This is the new menuentry for Ubuntu:

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-a50719c8-4978-4b75-8cae-1cfe6fc7efb2' {
    recordfail
    load_video
    gfxmode $linux_gfx_mode
    insmod gzio
    if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
    insmod part_gpt
    insmod ext2
    set root='hd0,gpt1'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint='hd0,gpt1'  13f8547c-f4ac-4553-ae1c-4dbda14b9cc0
    else
      search --no-floppy --fs-uuid --set=root 13f8547c-f4ac-4553-ae1c-4dbda14b9cc0
    fi
    linux    /vmlinuz-linux root=/dev/mapper/system-root ro  quiet splash xhci_hcd.quirks=1073741824 $vt_handoff
    initrd    /initramfs-linux.img
}

The problems are the last two lines in the menuentry section (linux and initrd): The files vmlinuz-linux and initramfs-linux.img were created by ArchLinux. Ubuntu should instead use the 5.11 kernel and initrd image. Prior to running `grub mkconfig`, the last two lines were:

    linux    /vmlinuz-5.11.0-34-generic root=/dev/mapper/system-root ro  quiet splash xhci_hcd.quirks=1073741824 $vt_handoff
    initrd    /initrd.img-5.11.0-34-generic

Unfortunately, I didn't find any documentation about grub-mkconfig, so I'm not sure how I should tell this tool to use one kernel for Arch Linux and a different kernel for Ubuntu.

Any ideas what I should be doing differently to generate correct menuentries? If possible, I would like to avoid to manually edit the file and instead generate it with grub-mkconfig.

[tea for one]

The config of grub was changed so that the grub menu appears for 30 seconds during boot, so I can select which OS to boot.

With two Linux Systems, Grub from the last installation usually takes care of identifying both systems.
What was wrong here if you can select either OS?

Do you want Ubuntu to control Grub?

[oldfred]

I know with Ubuntu you cannot share a /boot partition.
Not sure with other distributions, but issue is that kernels may not be unique or it tries to configure wrong one.

[Dennis N]

...the [grub mkconfig] command seems to have created a faulty menuentry for Ubuntu. Any ideas what I should be doing differently to generate correct menuentries?

These incorrect menuentries happen when the distros are not closely related and one probes the other to make a grub menuentry. Two Ubuntu-based distros would likely not do this.

Solution
With UEFI, you can instead have each distro boot using it's own grub version if you chainload the bootloader of the second OS from the grub menu of the first. This requires a custom menuentry that you construct yourself.

Process Outline:

START UP
Boot to the Arch Linux Grub Menu.
EITHER
Select the custom menuentry to chainload to the Ubuntu boot loader which is inside the EFI system partition and thus reach the Ubuntu Grub menu,
OR
Select the Arch Menuentry and boot Arch.

I don't use encryption, so you must solve how you incorporate encryption into this process.