Comment 4 for bug 1528655

So apparently you already identified some processes that hang on shutdown. /usr/share/doc/systemd/README.Debian.gz describes how to find and debug those.

If you want to keep this open against systemd, please attach /var/log/syslog right after a reboot, which should still contain most of the shutdown log from the previous boot (if that isn't sufficient, we'll need a complete journal for shutdown, after enabling persistent journal -- but let's get to that when we need it).