In my system the log space was getting big (more than 4Gb). Since nowadays we have these small solid state disks this can be a problem.
I checked Can I remove files in /var/log/journal and /var/cache/abrt-di/usr? and this instruction does the magic:
sudo journalctl --vacuum-size=500M
Moreover, you can activate log rotation with:
sudo systemctl kill --kill-who=main --signal=SIGUSR2 systemd-journald.service
and you'll need to restart the service:
sudo systemctl restart systemd-journald.service
Finally, if you want to check the size of your journaling logging space:
Or, in the old-fashioned (and more 'physical') way:
du -h /var/log/journal/
No more complaints about the size of the root filesystem, for now.