'Get the balance of the ring. The balance value is the highest
percentage of the desired amount of partitions a given device
wants. For instance, if the "worst" device wants (based on its
weight relative to the sum of all the devices\' weights) 123
partitions and it has 124 partitions, the balance value would
be 0.83 (1 extra / 123 wanted * 100 for percentage).
:returns: balance of the ring'

def get_balance(self):

    balance_per_dev = self._build_balance_per_dev()
    return max((abs(b) for b in balance_per_dev.values()))
