persistent journal

A journal written to /var/log/journal, the only place that carries it across a reboot. The journal is not a log file that survives by nature: Storage=auto, the commented default in /etc/systemd/journald.conf, writes there only if the directory already exists. An Ubuntu Server 24.04 install ships it, so an ordinary machine is persistent; a container image, a trimmed cloud image or a Storage=volatile line is not, and journald then falls back to /run/log/journal, a tmpfs. Everything in it is gone at the next boot. journalctl --list-boots printing a single line says so, and journalctl -b -1 answering Data from the specified boot (-1) is not available says it another way: the machine restarted after a crash, and the evidence left with the memory. sudo mkdir -p /var/log/journal then sudo systemctl restart systemd-journald is something you do before you need it.