有一个codeigniter会话替代方式，行为像会话和如何实现？

Currently i am facing problem in codeigniter session size. some data doesn't save in session variable but i need this. i don't find any solution over googled. Here's my some session code. first i create session for login:

$this->session->set_userdata('member_loged', TRUE);
$this->session->set_userdata('uid', $uloged->id);
$this->session->set_userdata('user_name', $uloged->user_name);
$this->session->set_userdata('email', $uloged->email);

second session code for exam :

$onlineexam = array();

$ukey = time();
$uid = $this->session->userdata('uid');
$onlineexam['uid'] = $uid;
$onlineexam['examid'] = $eid;
$onlineexam['ukey'] = $ukey;
$onlineexam['onlineexamdate'] = date('Y-m-d H:i:s');

foreach($allque as $que):

$quid = $que['id'];
$qa['queid'] = $quid;  
$onlineexam['allq'][] = $qa;
$this->session->set_userdata('onlineexam', $onlineexam);

endforeach;

Third sesion add in online exam after click by ajax:

$onlineexam = $this->session->userdata('onlineexam');
$ans['queid'] = $queid; 
$ans['ansid'] = $ansid;
$ans['flag'] = $checkans;
$onlineexam['allanswer'][] = $ans;
$this->session->set_userdata('onlineexam', $onlineexam);

in allanswer variable fill with some data and miss maximum data