Long story short:

journalctl --disk-usage

Archived and active journals take up 1.8G in the file system.

journalctl --vacuum-size=500M

Vacuuming done, freed 1.3G of archived journals from /var/log/journal/6947493b350448bba40817dc19e5762a.
Vacuuming done, freed 0B of archived journals from /run/log/journal

A ajouter à mes scripts de nettoyage, je suppose