After a series of mishaps with Secure Boot (trying to sign VMWare Player kernel modules), GRUB stopped showing the entries corresponding to Ubuntu 19.10, installed as ZFS root (default 19.10 installer options), booting in the System Setup by default.
I was able to boot into my system by doing the following (from memory; some details below may be off).
In the GRUB shell, I entered
set root=(hd0,gptXXX) linux /BOOT/ubuntu_YYY/@/vmlinuz root=ZFS=rpool/ROOT/ubuntu_YYY boot=zfs initrd /BOOT/ubuntu_YYY/@/initrd.img boot
To find which
gptXXX partition to use, I ran
ls and then
ls (hd0,gptXXX)/ until I found the boot files.
ubuntu_YYY seems a unique identifier for the Ubuntu system. I found it by
ls-ing around from GRUB, I got a few errors concerning the
compression algorithm 115 not supported.
I don't have any other operating system installed, and I disabled Secure Boot for now.
I wonder now what steps to take to restore the GRUB bootloader correct operation. If I can't fix it, I may simply end up installing a fresh 20.04 LTS copy. But at least the above can be of help for anybody ending up in the same situation.
Edit: I have
compression=lz4 set on
rpool (but not on
bpool), it may be related.