def sizes(self):
        """Provides block_size and transfer_size through a list
        of dict, for instance: [{'transfer': '1M 4M', 'block': '8M'}]
        """
        if self.attributes.get('sizes'):
            for settings in self.attributes.get('sizes'):
                for pair in itertools.product(
                    shlex.split(settings['block']), shlex.split(settings['transfer'])
                ):
                    yield pair
        else:
            for pair in itertools.product(self.block_size, self.transfer_size):
                yield pair