about the cache

[eluser]FightingMan[/eluser]

Code:

$this->cache->save('foo', 'bar', 10);
$a = $this->cache->get('foo');
print_r($a);

i haven't load the driver,but i can cached foo=>bar successful,anyone can tell me what's the reason?
it return me 'bar' and doesn't write into application/cache directory