I try to debug those error.

ceph.py at 859 line:
do_full_backup = False
        if self._file_is_rbd(volume_file):
            # If volume an RBD, attempt incremental backup.
            try:
                self._backup_rbd(backup_id, volume_id, volume_file,
                                 volume_name, length)
            except exception.BackupRBDOperationFailed:
                LOG.debug("Forcing full backup of volume %s.", volume_id)
                do_full_backup = True
        else:
            do_full_backup = True

        if do_full_backup:
            self._full_backup(backup_id, volume_id, volume_file,
                              volume_name, length)

but something goes wrong and function _file_is_rbd did return FALSE.

let's see to '_file_is_rbd' function in ceph.py at 683 line:
def _file_is_rbd(self, volume_file):
        """Returns True if the volume_file is actually an RBD image."""
        return hasattr(volume_file, 'rbd_image')

It mean that attribute 'rbd_image' did not assigned.

'rbd_image' attribute belong to class 'RBDImageIOWrapper' at cinder/volume/drivers/rbd.py.

I try print out contents 'volume_file' array:
['__abs
tractmethods__', '__class__', '__delattr__', '__doc__', '__enter__', '__exit__', '__format__', '__getattribute__', '__hash__', '__init__', '__iter__', '__metaclass__', '__module__', '__new__', '__reduce__', '__re
duce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_abc_cache', '_abc_negative_cache', '_abc_negative_cache_version', '_abc_registry', '_checkClosed', '_checkReadable', '_checkSe
ekable', '_checkWritable', '_inc_offset', 'close', 'closed', 'fileno', 'flush', 'isatty', 'next', 'read', 'readable', 'readall', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write
', 'writelines']' 

those cells of array is in class RBDVolumeIOWrapper at os_brick/initiator/linuxrbd.py

In sum cinder-backup didn't identify image as rbd.

my cinder.conf at block node:
[DEFAULT]
debug = true
rootwrap_config = /etc/cinder/rootwrap.conf
api_paste_confg = /etc/cinder/api-paste.ini
iscsi_helper = tgtadm
volume_name_template = volume-%s
volume_group = cinder-volumes
verbose = True
auth_strategy = keystone
state_path = /var/lib/cinder
lock_path = /var/lock/cinder
volumes_dir = /var/lib/cinder/volumes
rpc_backend = rabbit
auth_strategy = keystone
my_ip = 10.30.17.21
enabled_backends = lvm,rbd
glance_host = controller
control_exchange = cinder
notification_driver = messagingv2
backup_driver = cinder.backup.drivers.ceph
backup_ceph_conf = /etc/ceph/bak.conf
backup_ceph_user = cinder-backup
backup_ceph_chunk_size = 134217728
backup_ceph_pool = backups
backup_ceph_stripe_unit = 0
backup_ceph_stripe_count = 0
restore_discard_excess_bytes = true
[database]
connection = mysql://cinder:XXXXXXX@controller/cinder
[oslo_messaging_rabbit]
rabbit_host = controller
rabbit_userid = openstack
rabbit_password = XXXXXXXXX
[keystone_authtoken]
auth_uri = http://controller:5000/v2.0
identity_uri = http://controller:35357
admin_tenant_name = service
admin_user = cinder
admin_password = XXXXXXX
[lvm]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_group = cinder-volumes
iscsi_protocol = iscsi
iscsi_helper = tgtadm
[rbd]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
rbd_pool = os-volumes
rbd_ceph_conf = /etc/ceph/ceph.conf
rbd_flatten_volume_from_snapshot = false
rbd_max_clone_depth = 5
rbd_store_chunk_size = 4
rados_connect_timeout = -1
glance_api_version = 2
rbd_user = cinder
rbd_secret_uuid = XXXXXXX
volume_backend_name = volume_ceph
[oslo_concurrency]
lock_path = /var/lock/cinder

and my volumes:
root@controller:/var/log# cinder list
+--------------------------------------+-----------+-------+------+-------------+----------+-------------+
|                  ID                  |   Status  |  Name | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+-------+------+-------------+----------+-------------+
| 8a554a85-a49b-45c4-9f83-a40a45f0c544 | available | test2 |  1   |     rbd     |  false   |             |
| dea5342c-4625-40fc-ab8f-d147b2d224b2 | available | test3 |  1   |     rbd     |  false   |             |
+--------------------------------------+-----------+-------+------+-------------+----------+-------------+
root@controller:/var/log# cinder type-list
+--------------------------------------+------+-------------+-----------+
|                  ID                  | Name | Description | Is_Public |
+--------------------------------------+------+-------------+-----------+
| 903bc236-f612-4253-9763-d7154987fe66 | rbd  |             |    True   |
| cae9eee2-fbad-4082-9635-e48788a2f4e1 | lvm  |      -      |    True   |
+--------------------------------------+------+-------------+-----------+
root@controller:/var/log# cinder type-show 903bc236-f612-4253-9763-d7154987fe66
+---------------------------------+--------------------------------------+
|             Property            |                Value                 |
+---------------------------------+--------------------------------------+
|           description           |                                      |
|           extra_specs           |                  {}                  |
|                id               | 903bc236-f612-4253-9763-d7154987fe66 |
|            is_public            |                 True                 |
|               name              |                 rbd                  |
| os-volume-type-access:is_public |                 True                 |
|           qos_specs_id          |                 None                 |
+---------------------------------+--------------------------------------+
cinder service-list
+------------------+-------------+------+---------+-------+----------------------------+-----------------+
|      Binary      |     Host    | Zone |  Status | State |         Updated_at         | Disabled Reason |
+------------------+-------------+------+---------+-------+----------------------------+-----------------+
|  cinder-backup   |    block1   | nova | enabled |   up  | 2016-05-19T06:52:27.000000 |        -        |
| cinder-scheduler |  controller | nova | enabled |   up  | 2016-05-19T06:52:44.000000 |        -        |
|  cinder-volume   | block1@ceph | nova | enabled |  down | 2016-05-19T05:36:40.000000 |        -        |
|  cinder-volume   |  block1@lvm | nova | enabled |   up  | 2016-05-19T06:52:24.000000 |        -        |
|  cinder-volume   |  block1@rbd | nova | enabled |   up  | 2016-05-19T06:52:23.000000 |        -        |
+------------------+-------------+------+---------+-------+----------------------------+-----------------+