Sessions still written to filesystem with database driver

Nothing's written to the file-system when you use the database driver.

"/var/lib/php5/sessions" is only present in the error message because CI doesn't need to set the session.save_path INI when using a database, leaving the PHP engine "in the dark" when it comes to reporting these errors.

The error means that you don't have a default database configured or that an attempt to connect to it has failed.