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.
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.
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.
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.
GRUB_CMDLINE_LINUX="rd.luks.uuid=24682f88-9115-4a8d-81fb-a03ec61d870b rd.luks.uuid=1815e7a4-532f-4a6d-a5c6-370797ef2450 rootfs=btrfs root=/dev/dm-0 rd.luks=1"
After the changes, I did run dracut --force --kver 5.15.85-gentoo-dist and grub-mkconfig -o /boot/grub/grub.cfg
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.
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