Codeigniter session doesn't store session data with using database

my settings in config file

PHP Code:

$config['sess_driver'] = 'database';//'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'sessdata';//this is my table name
$config['sess_match_ip'] = TRUE;//FALSE;
$config['sess_time_to_update'] = 300;

$config['cookie_prefix'] = '';
$config['cookie_domain'] = '.';
$config['cookie_path'] = '/';
$config['cookie_secure'] = FALSE;
$config['cookie_httponly'] = FALSE; 


My Controller

PHP Code:

public function index() {

        echo '<pre>';
        print_r($this->session->userdata());
        //print_r($_SESSION);
        echo '</pre>';
        $newdata = array(
            'username' => 'johndoe',
            'email' => '[email protected]',
            'logged_in' => TRUE
        );

        $this->session->set_userdata($newdata);
        $this->load->view('welcome_message');
    }