Adding data to a session

[eluser]elmne[/eluser]
If i have different sets of data that i want to add to a session, is this the correct way to do it?

Code:

$option_data = array(
'item_id' => '',
'quantity' => ''
);
$this->session->set_userdata($option_data);


$person_data = array(
'firstname' => '',
'middlename' => '',
'surname' => ''
);
$this->session->set_userdata($person_data);

As this line

Code:

$this->session->set_userdata($person_data);

Seems different from the Hybrid session's format of

Code:

$this->session->set_userdata($newdata = array(), $newval = '');

And what's the limit to the data amount that can be added to a session?