mysterious diskspace usage by mariadb

Hi all,

Working on debian 10 (4.19.0-16-amd64 #1 SMP Debian 4.19.181-1 (2021-03-19) x86_64 GNU/Linux) with mariadb ( version 10.3.27) All the mariadb data files are on a separate disk mounted on /data. For reasons unknown to me the root (/) filesystem is filling up. This space will become available again after a restart of mariadb. Check this out:

root@lhddb03-1c:/# df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       18G   14G  3.8G  78% /

14GB is in use when I check this with du the total diskusage is around 5,2GB

root@lhddb03-1c:/# du -sh * 2>/dev/null
0       bin
85M     boot
0       dev
4.0M    etc
180K    home
0       initrd.img
0       initrd.img.old
0       lib
0       lib32
0       lib64
0       libx32
16K     lost+found
8.0K    media
4.0K    mnt
4.0K    opt
0       proc
72K     root
57M     run
0       sbin
4.0K    srv
0       sys
44K     tmp
1.3G    usr
3.9G    var
0       vmlinuz
0       vmlinuz.old

Now I do a restart of the mysqld service (systemctl restart mysql) and again a df -h

root@lhddb03-1c:/# systemctl restart mysql 
root@lhddb03-1c:/# df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       18G  5.2G   12G  31% /

Can someone explain this ? This node is part of a galera cluster. Thanks in advance, Carlo