A few days ago I noticed "/" reached 95% without doing anything (by myself). I noticed this in my Home Assistant UI (see screen capture below)
Home Assistant Supervisor Panel
Home Assistant is running on Docker.
root@zeus:~# df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.8G 0 1.8G 0% /dev
tmpfs 364M 2.6M 361M 1% /run
/dev/sda2 458G 436G 0 100% /
tmpfs 1.8G 0 1.8G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 1.8G 0 1.8G 0% /sys/fs/cgroup
/dev/loop2 56M 56M 0 100% /snap/core18/1885
/dev/loop1 98M 98M 0 100% /snap/core/10126
/dev/loop3 71M 71M 0 100% /snap/lxd/16922
/dev/loop0 98M 98M 0 100% /snap/core/10185
tmpfs 364M 0 364M 0% /run/user/1000
As you can see, /dev/sda2 (notebook internal HD) is full.
Before upgrading to 20.04.1 LTS I did not notice the /dev/loop[n] filesystems.
How can I determine the cause of this increment?
TIA
Your storage device is full due to a "runaway" log file taking up 7.6GB.
This is one of the most common causes of a full storage device.
The log file itself is not broken. Your system is encountering some problem very frequently (sometimes one or more times each second), and logging about it every time.
The temporary workaround is to delete the log file. It will promptly regenerate (empty) on it's own, and will promptly start growing again.
The real solution is to investigate the log file, identify whatever problem(s) are causing the tremendous log activity, and fix it(them).