Booting Gentoo on a BTRFS from multiple LUKS devices

Author: Solène

Date: 02 January 2023

Tags: linux gentoo btrfs

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Introduction

This is mostly a reminder for myself. I installed Gentoo on a machine, but I reused the same BTRFS filesystem where NixOS is already installed, the trick is the BTRFS filesystem is composed of two partitions (a bit like raid 0) but they are from two different LUKS partitions.

It wasn't straightforward to unlock that thing at boot.

Fix grub error

Grub was trying to autodetect the root partition to add `root=/dev/something`, but as my root filesystem requires `/dev/mapper/ssd1` and `/dev/mapper/ssd2`, it was simply adding `root=/dev/mapper/ssd1 /dev/mapper/ssd2`, which is wrong.

This required a change in the file `/etc/grub.d/10_linux` where I entirely deleted the `root=` parameter.

Compile systemd with cryptsetup

A mistake I made was to try to boot without systemd compiled with cryptsetup support, this was just failing because in the initramfs, some systemd services were used to unlock the partitions, but without proper support for cryptsetup it didn't work.

Linux command line parameters

In `/etc/default/grub`, I added this line, it contains the UUID of both LUKS partitions needed, and a `root=/dev/dm-0` which is unexpectedly the first unlocked device path, and `rd.luks=1` to enble LUKS support.

Run Dracut and grub

After the changes, I did run `dracut --force --kver 5.15.85-gentoo-dist` and `grub-mkconfig -o /boot/grub/grub.cfg`

Conclusion

It's working fine now, I thought it would require me to write a custom initrd script, but dracut is providing all I needed, but there were many quirks on the path with no really helpful message to understand what's failing.

Now, I can enjoy my dual boot Gentoo / NixOS (they are quite antagonists :D), but they share the same filesystem and I really enjoy this weird setup.