Yes you are right. I forgot that 'datadirectory' is in owncloud / config / config.php
But unfortunately this is also not the best solution. Firstly, the directory and files with the rights "root", that is, "read only". Secondly, even if you run lighttpd & php-fpm with "nobody" rights, owncloud keeps the list of files / directories in SQL, and the files copied to the 'datadirectory' will not be visible through the web interface.