Questions about sessions in CI 3.0

1. I see 3 method for accessing/creating sessions:

PHP Code:

$name = $_SESSION['name'];

// or:

$name = $this->session->name

// or:

$name = $this->session->userdata('name'); 


What do you recommand for using and why ?

Files Driver

2. Under what circumstances should I change 'sess_save_path' from NULL ?

3. I have changed 'sess_save_path' from NULL to "sessions", but nothing happend.
Where should a I create the "sessions" folder ?

Database Driver

4. Can I use $_SESSION and Database Driver ?
When we use $_SESSION, the writting/reading are still going through Session library ?