Journald

Persistent logging across reboots on debian

First, make sure that persistent storage is enabled in /etc/systemd/journald.conf

[Journal]
Storage=persistent

Second, make sure your /etc/machine-id is persistent across reboots. One way to diagnose this is running

journalctl --merge

If that shows you the logging of previous boots while plain journalctl does not, your machine-id is volatile. Another way to check this is to see if each boot creates a new subdirectory in /var/log/journal

The following commands make a volatile machine-id persistent:

mount --bind / /mnt
cp /etc/machine-id /mnt/etc/machine-id
shutdown -r now