dans /var/log/journal/journal.conf :

SystemMaxUse=100M

Stockage (par défaut) dans : /var/log/journal

sudo journalctl --disk-usage
sudo journalctl --vacuum-size=100M

see also : http://unix.stackexchange.com/questions/139513/how-to-clear-journalctl

Explorer le journal :

Show all messages from date (and optional time):

# journalctl --since="2012-10-30 18:17:16"

Show all messages since 20 minutes ago:

# journalctl --since "20 min ago"

Follow new messages:

# journalctl -f

Show all messages by a specific executable:

# journalctl /usr/lib/systemd/systemd