Recently upgraded my laptop from Ubuntu 16.04 to 18.04.  Relatively few issues with the upgrade.  However, after several days my laptop ran out of space.  The “/” root partition (assigned 40GB) filled up completely.

I suspected some sort of a logging issue.  The “du” command is excellent for checking disk usage, and I used the following to determine the offending file(s):

$ sudo du -hs *

Although expecting the offending file to be under /var/log/, I quickly zoned in on /var/lib/.

The offending file was:

/var/lib/gdm3/.local/share/xorg/Xorg.0.log

Quick google yielded this:

Ubuntu Linux 18.04 Run Out of Disk Space

The fix was pretty simple,

$ sudo apt-get remove xserver-xorg-input-synaptics

From the link:

I got the root cause of the issue now. It seems that Xorg replaces input-synaptics with libinput. The Ubuntu upgrader did not remove xserver-xorg-input-synaptics package after upgrade and confuses xorg package, so that the error message keeps added to the log.

A reboot then cleared the problem!