def get_next_device_name(instance, device_name_list,
                         root_device_name=None, device=None):
    """Validates (or generates) a device name for instance.

    If device is not set, it will generate a unique device appropriate
    for the instance. It uses the root_device_name (if provided) and
    the list of used devices to find valid device names. If the device
    name is valid but applicable to a different backend (for example
    /dev/vdc is specified but the backend uses /dev/xvdc), the device
    name will be converted to the appropriate format.
    """

    req_prefix = None
    req_letter = None

    if device:
        try:
            req_prefix, req_letter = block_device.match_device(device)
        except (TypeError, AttributeError, ValueError):
            raise exception.InvalidDevicePath(path=device)

    if not root_device_name:
        root_device_name = block_device.DEFAULT_ROOT_DEV_NAME

    try:
        prefix = block_device.match_device(
                block_device.prepend_dev(root_device_name))[0]
    except (TypeError, AttributeError, ValueError):
        raise exception.InvalidDevicePath(path=root_device_name)

    # NOTE(vish): remove this when xenapi is setting default_root_device
    if driver.is_xenapi():
        prefix = '/dev/xvd'

    if req_prefix != prefix:
        LOG.debug("Using %(prefix)s instead of %(req_prefix)s",
                  {'prefix': prefix, 'req_prefix': req_prefix})

    used_letters = set()
    for device_path in device_name_list:
        letter = block_device.get_device_letter(device_path)
        used_letters.add(letter)

    # NOTE(vish): remove this when xenapi is properly setting
    #             default_ephemeral_device and default_swap_device
    if driver.is_xenapi():
        flavor = instance.get_flavor()
        if flavor.ephemeral_gb:
            used_letters.add('b')

        if flavor.swap:
            used_letters.add('c')

    if not req_letter:
        req_letter = _get_unused_letter(used_letters)

    if req_letter in used_letters:
        raise exception.DevicePathInUse(path=device)

    return prefix + req_letter

def update_bdm(bdm, info):
    device_name_field = ('device_name'
                         if 'device_name' in bdm
                         else 'mount_device')
    # Do not update the device name if it was already present
    bdm.update(dict(zip((device_name_field,
                         'disk_bus', 'device_type'),
                        ((bdm.get(device_name_field) or
                          block_device.prepend_dev(info['dev'])),
                         info['bus'], info['type']))))

    def __init__(self, instance_id, mountpoint, dev):
        """
        Setup critical information for attaching/detaching volumes from
        instance_id.

        :param instance_id:
        :param mountpoint: connection_info['data']['mount_device']
        :param dev:
        """
        #TODO(imsplitbit): Need to make this happen while supporting block
        # devices and externally mounted filesystems at the same time.
        # Currently it does not.
        self.instance_id = instance_id
        self.mountpoint = block_device.prepend_dev(mountpoint)
        self.device = dev

    def _get_guest_storage_config(self, context, block_device_info,
                                  reboot=False):
        devices = []
        block_device_mapping = driver.block_device_info_get_mapping(
            block_device_info)

        LOG.info(_LI('block_device_mapping %s'), block_device_mapping)
        for vol in block_device.get_bdms_to_connect(block_device_mapping):
            connection_info = vol['connection_info']
            vol_dev = block_device.prepend_dev(vol['mount_device'])
            info = {'dev': vol['mount_device']}
            self._connect_volume(connection_info, info)
            device_path = connection_info['data'].get('device_path')
            if device_path:
                real_path = os.path.realpath(device_path)
                devices.append(real_path + ':' + vol['mount_device'])
            bdm = objects.BlockDeviceMapping.get_by_volume_id(
                context, connection_info['data']['volume_id'])
            driver_bdm = driver_block_device.DriverVolumeBlockDevice(bdm)
            driver_bdm['connection_info'] = connection_info
            driver_bdm.save()
        return devices

def get_disk_mapping(virt_type, instance,
                     disk_bus, cdrom_bus,
                     block_device_info=None,
                     image_meta=None, rescue=False):
    """Determine how to map default disks to the virtual machine.

       This is about figuring out whether the default 'disk',
       'disk.local', 'disk.swap' and 'disk.config' images have
       been overridden by the block device mapping.

       Returns the guest disk mapping for the devices.
    """

    inst_type = flavors.extract_flavor(instance)

    mapping = {}

    pre_assigned_device_names = \
    [block_device.strip_dev(get_device_name(bdm)) for bdm in itertools.chain(
        driver.block_device_info_get_ephemerals(block_device_info),
        [driver.block_device_info_get_swap(block_device_info)],
        driver.block_device_info_get_mapping(block_device_info))
     if get_device_name(bdm)]

    if rescue:
        rescue_info = get_next_disk_info(mapping,
                                         disk_bus, boot_index=1)
        mapping['disk.rescue'] = rescue_info
        mapping['root'] = rescue_info

        os_info = get_next_disk_info(mapping,
                                     disk_bus)
        mapping['disk'] = os_info

        return mapping

    # NOTE (ndipanov): root_bdm can be None when we boot from image
    # as there is no driver represenation of local targeted images
    # and they will not be in block_device_info list.
    root_bdm = block_device.get_root_bdm(
        driver.block_device_info_get_mapping(block_device_info))

    root_device_name = block_device.strip_dev(
        driver.block_device_info_get_root(block_device_info))
    root_info = get_root_info(virt_type, image_meta, root_bdm,
                              disk_bus, cdrom_bus, root_device_name)

    mapping['root'] = root_info
    # NOTE (ndipanov): This implicitly relies on image->local BDMs not
    #                  being considered in the driver layer - so missing
    #                  bdm with boot_index 0 means - use image, unless it was
    #                  overridden. This can happen when using legacy syntax and
    #                  no root_device_name is set on the instance.
    if not root_bdm and not block_device.volume_in_mapping(root_info['dev'],
                                                           block_device_info):
        mapping['disk'] = root_info

    default_eph = has_default_ephemeral(instance, disk_bus, block_device_info,
                                        mapping)
    if default_eph:
        mapping['disk.local'] = default_eph

    for idx, eph in enumerate(driver.block_device_info_get_ephemerals(
            block_device_info)):
        eph_info = get_info_from_bdm(
            virt_type, eph, mapping, disk_bus,
            assigned_devices=pre_assigned_device_names)
        mapping[get_eph_disk(idx)] = eph_info
        update_bdm(eph, eph_info)

    swap = driver.block_device_info_get_swap(block_device_info)
    if swap and swap.get('swap_size', 0) > 0:
        swap_info = get_info_from_bdm(virt_type, swap, mapping, disk_bus)
        mapping['disk.swap'] = swap_info
        update_bdm(swap, swap_info)
    elif inst_type['swap'] > 0:
        swap_info = get_next_disk_info(mapping,
                                       disk_bus)
        if not block_device.volume_in_mapping(swap_info['dev'],
                                              block_device_info):
            mapping['disk.swap'] = swap_info

    block_device_mapping = driver.block_device_info_get_mapping(
        block_device_info)

    for vol in block_device_mapping:
        vol_info = get_info_from_bdm(
            virt_type, vol, mapping,
            assigned_devices=pre_assigned_device_names)
        mapping[block_device.prepend_dev(vol_info['dev'])] = vol_info
        update_bdm(vol, vol_info)

    if configdrive.required_by(instance):
        device_type = get_config_drive_type()
        disk_bus = get_disk_bus_for_device_type(virt_type,
                                                image_meta,
                                                device_type)
        config_info = get_next_disk_info(mapping,
                                         disk_bus,
                                         device_type,
#.........这里部分代码省略.........