I am following https://github.com/openzfs/zfs/wiki/Ubuntu-18.04-Root-on-ZFS but doing it for Ubuntu 20.04.

When I get to:

update-initramfs -u -k all

nothing happens: It returns after 0.1 second and normally update-initramfs takes several seconds on my machine.

update-grub also complains about a missing initrd:

# update-grub
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: vmlinuz-5.4.0-29-generic in rpool/ROOT/ubuntu
Warning: Couldn't find any valid initrd for dataset rpool/ROOT/ubuntu.
Warning: didn't find any valid initrd or kernel.
Found Ubuntu 20.04 LTS (20.04) on /dev/sda5

And when booting I get a grub prompt (no menu).

It seems there is some crucial step missing. Something that tells update-initramfs which initrd to build.

I have tested that the machine can boot on UEFI (the normal, unencrypted ext4 Ubuntu can install just fine with UEFI).

  • This is, as I'm sure you know, absolutely on topic and welcome here. However, as it sounds like a very Ubuntu-specific issue, you might have better chances of getting an answer if you delete this and ask over at Ask Ubuntu. Completely up to you, I just thought I'd point it out. – terdon May 8 '20 at 16:13

There is no initial initramfs, so updating none does nothing.

The solution was to create a new one:

update-initramfs -c -k all

-c being the magic change.

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