Source: https://unix.stackexchange.com/a/194058

# Retain only the past two days:
journalctl --vacuum-time=2d

# Retain only the past 500 MB:
journalctl --vacuum-size=500M

# or edit /etc/systemd/journald.conf to set SystemMaxUse=100M

# show all errors since last boot
journalctl -p 3 -xb