How to investigate sessions

To list all current sessions:

loginctl list-sessions

To investigate details of particular session run:

loginctl show-session [session_id]

The sessions files are located in /run/systemd/sessions on Fedora-based systems and in /run/systemd/sessions on Debian-based systems.

If sessions fill up the /run directory for 100% then per recommendation on
https://www.centos.org/forums/viewtopic.php?f=48&t=65472:

It's because you've updated systemd and not rebooted and it's lost its dbus connection. It's a known bug and is fixed in newer systemd versions but I am unsure if the fix will get backported to RHEL 7's systemd. You'll need to search bugzilla.redhat.com and see if there is an existing bz for it and if not, raise one.

bug report is on https://bugs.centos.org/view.php?id=14278