Where did the disk space go?

Question

In Ubuntu 18.04 as root user executing "df -h" and getting:

Filesystem                           Size  Used Avail Use% Mounted on
...
/dev/vda1                             40G   36G  2,4G  94% /
...

, that is 36 Gb is used.

Next, doing "cd /" + "du -sm *| sort -nr" and getting:

22892   home
3329    var
1735    lib
1004    usr
243 boot
176 backup
21  run
15  tmp
9   bin
8   etc
6   sbin
1   srv
1   root
1   opt
1   mnt
1   media
1   lost+found
1   lib64
0   vmlinuz.old
...

That is used: 22892+3329+1735+1004+243+176+21+15+9+8+6+1+1+1+1+1+1+1=29445 Mb

So where did 36-29 = 7 Gb go?

I am puzzled by the question due to every day several tens of megabytes disappear somewhere. How I can find the folder with those disappeared 7 Gb?

Answer 1

Info from https://askubuntu.com/questions/280342/why-do-df-and-du-commands-show-different-disk-usage solves my problem. Command:

lsof | grep '(deleted)'

shows me those processes which hold unlinked but held open files. After killing them all unlinked files was released and free up disk space. Thanks, @Mime.