According to 'man journalctl', option --boot (-b) "show messages from a
specific boot". However, on my system:
$ journalctl -b -1
Specifying boot ID has no effect, no persistent journal was found
According to 'man journald.conf':
Storage=
Controls where to store journal data. One of "volatile",
"persistent", "auto" and "none". If "volatile", journal log data will be
stored only in memory, i.e. below the /run/log/journal hierarchy (which is
created if needed). If "persistent", data will be stored preferably on disk,
i.e. below the /var/log/journal hierarchy (which is created if needed), with
a fallback to /run/log/journal (which is created if needed), during early
boot and if the disk is not writable. "auto" is similar to "persistent" but
the directory /var/log/journal is not created if needed, so that its
existence controls where log data goes. "none" turns off all storage, all
log data received will be dropped. Forwarding to other targets, such as the
console, the kernel log buffer, or a syslog socket will still work however.
Defaults to "auto".
In my /etc/systemd/journald.conf, Storage is left to its default value. If
it is the same on yours, you may want "Storage=persistent" in it.
