[PLUG] Looking forward and backward at the same time, updating to Ubuntu 20.04

[Russell Senior]

On one of my desktop machines, I decided it was time to update to 20.04
from 16.04. The installation is on a RAID1 with LVM2 on top. The initial
update to 18.04 went smoothly using do-release-upgrade. The second upgrade
to 20.04 choked though. A couple additional complications are that I have
my /home mounted from NFS, but that won't enter in to the problematic bits
here.

When I encountered the hitch in the update to 20.04 (some package conflict
that was never very clear), I thought "okay, fine, my homedir is safely on
NFS, the only other non-package files are on another logical volume, I'll
just update the rootfs from the installer.

Pretty quick, I was refamiliarized with the fact that Ubuntu's installer
doesn't think RAID on the desktop is a thing.  And, since it's been most of
4 years since I did the 16,04 install, I'd forgotten how I'd worked around
this before.

Turns out, it isn't terribly difficult. The basic thumbnail sketch is as
follows:

  install mdadm in the live-boot environment;
  sudo mdadm --assemble --scan (to detect the existing RAID array);
  the installer will now happily detect the LVM volume groups and logical
volumes;
  don't reboot yet!
  the installed system doesn't have mdadm yet and its initramfs needs
rebuilding; so
  chroot to the installed rootfs with the various virtual filesystems
mounted;
  apt install mdadm;
  put at least raid1 (I threw in lvm2 as well, but not sure that's
required) in /etc/initramfs-tools/modules;
  update-initramfs;
  update-grub;
  exit chroot;
  okay, now reboot.

If you forgot something, you can return to the live-boot'd thumbdrive and
retry the chroot.

Hope this helps someone, even if the someone is me in another 4 years.

-- 
Russell Senior
russell at personaltelco.net