Well, since I got no answers, I risked proceeding with the installation anyway.
I'm reporting what I did, and the result, so it may help others and myself in the future.
What I did
- Before starting, make an external backup of the important stuff in
/home, of course.
- Boot the live Ubuntu 20.04 from a USB flash drive,
and chose "Try Ubuntu without installing".
/dev/sda2 btrfs partition on
- Delete everything inside the
/mnt/@/ root subvolume.
(I previously made a btrfs snapshot of the subvolume,
just in case something went wrong and I would have to restore the old system back.
Fortunately this was not needed, but I'll keep the snapshot around for a while
so I can compare configuration files in
/etc, for instance.)
- Start the Ubuntu installer.
- On the "Installation type" window (see it on the tutorial),
choose "Something else".
- On the partitioning window (a screenshot would be nice here),
/dev/sda2 partition, specify mount point =
/ and no formatting.
(There is no option for mount point
/, /home, which would be clearer.)
- On the "Who are you" window (see it on the tutorial),
enter a username equal to my previous main (
(This should be optional, but this way the main user will immediately "inherit" the previous homedir from
The installation proceeded fine from then on.
Upon reboot, the system started fine, both
/home were mounted from the
@home subvolumes, as expected, and I logged in to my account with no issues.
Apparently the Ubuntu installer will keep the
@home subvolumes if it finds them on the brtfs filesystem, and will mount them as
If you want to overwrite the
/ volume, as I did, you should clear it first.
I tried once without clearing, and the installation eventually failed.