public function popStorage()
    {
        if ( count( $this->storageStack ) === 0 )
        {
            throw new ezcCacheStackUnderflowException();
        }
        $storageConf = array_shift( $this->storageStack );
        unset( $this->storageIdMap[$storageConf->id] );
        return $storageConf;
    }