Information
Data from journald may be stored in volatile memory or persisted locally on the server. Logs in memory will be lost upon a system reboot. By persisting logs to local disk on the server they are protected from loss due to a reboot.
Writing log data to disk will provide the ability to forensically reconstruct events which may have impacted the operations or security of a system even after a system crash or reboot.
Solution
Edit the /etc/systemd/journald.conf file and add the following line:
Storage=persistent
Restart the service:
# systemctl restart systemd-journald.service