Comment # 17 on bug 1223167 from James Carlson

Some other notes about the system that may (or may not) help in identifying the
problem.

The root file system is on NVME and uses ext4. /etc is (of course) on the root
file system, as is /bin, /usr, /lib, /lib64, /root, /opt.

/var is on ZFS as are my /home directories, /srv, /usr/local, and the bulk of
the storage is on ZFS in /export.

I would not be surprised at all to find that (like static network
configuration) the installer knows nothing of ZFS and that means (in
particular) that /var can't be mounted and appears to be empty to the
installer.

No idea if it needs something from there. If there were some way to add drivers
and import the ZFS mounts during the upgrade process, then I would do that.
Alternatively, I could try doing an experiment where I move the ZFS /var data
over to the NVME and then somehow move it back. It'll be a bit fraught, though.