/var/log not mounted when journalctl looks for it

[nefus]

according to Step 3: System Installation /var/log is on his own volume.
At my pretty standard system it's not mounted when journalctl looks for /var/log/journal at boot time, so, as journalctl is configured "# Storage=auto", it uses RAM and does not persist logs between boots.

My workaround:

zfs set mountpoint=legacy rpool/var/log
echo "rpool/var/log /var/log zfs defaults 0 0" >> /etc/fstab