journalctl — Query the systemd journal. journalctl may be used to query the contents of the systemd journal as written by systemd-journald.service.
root@edison:~# journalctl --disk-usage
root@edison:~# nano /etc/systemd/journald.conf
SystemMaxUse=20M
root@edison:~# nano /etc/systemd/journald.conf
#Storage=persistent
Storage=None
root@edison:~# cd /var/log/journal/
root@edison:/var/log/journal# rm -rf *
root@edison:~# journalctl --disk-usage
No journal files were found.
Journals take up 0B on disk.