Find out what filled the disk.

df

What is on each filesystem

df -hT

Hide pseudo filesystems

df -hT -x tmpfs -x devtmpfs -x overlay

Which filesystem holds a path

df -h /var/log

Inodes as well as bytes

df -i

A filesystem can show free space while df -i reports full inodes, which is what a directory with millions of small files produces.

Scope: df