Sessions still written to filesystem with database driver

Alright, there seem to be a lot of things mixed up on our side that made the constations of the problem nebulous. It looks like we had other applications than PMA creating these empty session files and, as Narf told us, we got confused with the error displaying something about /var/lib/php5/sessions while in the end it was accesses to the database that were problematic.

I would have another question though that i can't find an answer for concerning the database driver. I understand the requirement for a non-persistent connection but why the need to use the 'default' $this->db one ? Would there be a problem using the 'default' connection as persistent and create another specific connection just for session handling that would be non-persistent ?

In any case, thanks for you help !