root partition full but cannot find big files
 

in my system the root partition is full
but there are no big files
may be system writes to opened file
please some ideas how to proceeed

What is the output from
distribution?

from "/" directory
Maybe? That will tell you what directories use up the most disk space.

The outpt of fdisk -l (lowercase L) might help as well to judge if this will be a once-off or that you will encounter this problem on a regular basis.

I've noticed a couple of posts that seem to indicate that a default Ubuntu install creates a relatively small root partition.