I recently had occasion to test my disaster recovery routine for my server, which is Ubuntu 18.04 LTS rooted on zfs.

The cause was that I did a command-line upgrade to 19.10. The resulting system did not boot. I am not exactly sure why. I’d hoped to enjoy a possible disc perforance improvement in 19.10.

Anyhow, after messing around for a while, I decided to revert to before the upgrade. I have a bootable flash drive with Ubuntu 18.04, not running from zfs, but with zfs tools installed.

I booted from the flash drive. Then I zfs rolled-back the /boot and root datasets to before the upgrade. Then I mounted the root dataset to /mnt/root, the boot to /mnt/root/boot, mount –rbind /dev /mnt/dev, and the same for sys, proc and run. Then chroot /mnt/root. Then update-grub (probably unnecessary) and grub-install /dev/sd{abcde}.

That’s it, although it did take 2 hours with inumerable boots and struggling to understand the Supermicro BIOS settings. I think the next time I have to do this, it will take about 30 minutes. Without knowing I could do this reasonably easily, I would never have tried the OS upgrade. The upgrade almost worked, but I’ll wait for the next LTS before trying again.