Yesterday I upgraded my Ubuntu Linux from 17.10 to 18.04. I found a strange phenomena. My free disk space (about 60GB) runs out quickly after several hours, but I did not download anything that has such a big size.
I thought that maybe someone had the same problem as me, so I tried googling to see if anyone had the same issue. But I found nothing. Maybe my search keywords is not correct. So I tried searches again to find some ways to check where goes wrong. I use sudo du -hs * starting from /. And I found that a single log file /var/lib/gdm3/.local/share/xorg/Xorg.0.log occupy all the free disk space and are still growing until the system told me I have no disk space.
I used the keyword Xorg.0.log to search again to see if there is any noticable issue with it. But I found nothing meaningful again. Then I tried to use head -n 1000 to print first 1000 lines of the log, and found that an error message Touchpad: Read error 9 keeps added to the log and finally used up all my disk space.
I again use the error message to search again  and found that someone had the same problem as me . Bingo! 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.
The final solution is simple. Run
$ sudo apt-get remove xserver-xorg-input-synaptics
After removing the package, reboot the system. And everything works fine now.
|||[Resolved] Synaptics TouchPad: Read error 9 / Newbie Corner / Arch Linux Forums|
|||Ubuntu to remove xserver-xorg-input-synaptics on upgrade - Desktop - Ubuntu Community Hub|