def create(self, volume_id, backup_id=None, timestamp=None):
        """
        create a backup of a volume
        """
        backup_id = backup_id or str(uuid.uuid4())
        timestamp = timestamp or int(time())
        return self.http_put('/volumes/%s/backups/%s' % (volume_id, backup_id),
                             params={'timestamp': timestamp})
create a backup of a volume