/dev/root almost full, preventing backups and soon to be a problem


I’m running on Centos 7 and Webmin 1.900, Usermin 1.751, Virtualmin 6.05.

I’m facing a problem. My daily backup failed and I got the error “cat: write error: No space left on device virtualmin”.

After searching, I changed the tmp folder to /var/tmp/.webmin but nothing changes.

df -h shows :

Filesystem Size Used Avail Use% Mounted on
/dev/root 20G 15G 3.5G 81% /
devtmpfs 16G 0 16G 0% /dev
tmpfs 16G 0 16G 0% /dev/shm
tmpfs 16G 1.8G 14G 11% /run
tmpfs 16G 0 16G 0% /sys/fs/cgroup
/dev/sda1 510M 3.6M 507M 1% /boot/efi
/dev/md3 1.8T 23G 1.7T 2% /home
tmpfs 3.2G 0 3.2G 0% /run/user/0

I see that /dev/root is almost full. It needs more size.

I actually need more space because these 15G used are a great amount of pictures uploaded by users.

Here are my SATA partitions :
https://i.imgur.com/C5LqxNo.png - https://i.imgur.com/W5138pS.png

Could you please help me figure this out ? In a few days this will be a problem.

Thank you,
Best regards.

EDIT : For now, I found a temporary solution. Revealing /var/log/virtualmin showed me several large log files. I removed them and backup is up ! But in a few days it will the limit again due to uploads.