When volume flatten time is too long, the state of the cinder-volume becomes down.

When ceph does the backend of cinder, if the clone volume depth exceeds the configured size, the volume will be flattened.
If the flatten operation is too long, the cinder-volume can not report the status to the cinder-scheduler,
The cinder-scheduler considers the cinder-volume to be down.

cinder-volume log
--------------------------------------------
2017-03-21 21:06:00.949 31396 DEBUG cinder.volume.drivers.rbd [req-3659d466-af71-4774-9d06-e877d4b761f3 331aa7019d414a9cb48fde06cfbc7b64 aefa89215813437ab91a2c1acd9338d8 - default default] opening connection to ceph cluster (timeout=30). _connect_to_rados /usr/lib/python2.7/site-packages/cinder/volume/drivers/rbd.py:328
2017-03-21 21:06:01.048 31396 DEBUG cinder.volume.drivers.rbd [req-3659d466-af71-4774-9d06-e877d4b761f3 331aa7019d414a9cb48fde06cfbc7b64 aefa89215813437ab91a2c1acd9338d8 - default default] Volume volume-a53d2778-d436-484c-8fe5-67651253f727 is not a clone. _get_clone_info /usr/lib/python2.7/site-packages/cinder/volume/drivers/rbd.py:634
2017-03-21 21:06:01.050 31396 DEBUG cinder.volume.drivers.rbd [req-3659d466-af71-4774-9d06-e877d4b761f3 331aa7019d414a9cb48fde06cfbc7b64 aefa89215813437ab91a2c1acd9338d8 - default default] maximum clone depth (2) has been reached - flattening source volume create_cloned_volume /usr/lib/python2.7/site-packages/cinder/volume/drivers/rbd.py:487
2017-03-21 21:06:01.077 31396 DEBUG cinder.volume.drivers.rbd [req-3659d466-af71-4774-9d06-e877d4b761f3 331aa7019d414a9cb48fde06cfbc7b64 aefa89215813437ab91a2c1acd9338d8 - default default] flattening source volume volume-67e4b760-7213-49d9-a13d-285cdd7db955 create_cloned_volume /usr/lib/python2.7/site-packages/cinder/volume/drivers/rbd.py:497
2017-03-21 21:06:54.615 31450 DEBUG oslo_service.periodic_task [req-34c3a89d-025d-4e9e-b225-5cbfd6fcfee5 - - - - -] Running periodic task VolumeManager._publish_service_capabilities run_periodic_tasks /usr/lib/python2.7/site-packages/oslo_service/periodic_task.py:215
2017-03-21 21:06:54.620 31450 DEBUG cinder.manager [req-34c3a89d-025d-4e9e-b225-5cbfd6fcfee5 - - - - -] Notifying Schedulers of capabilities ... _publish_service_capabilities /usr/lib/python2.7/site-packages/cinder/manager.py:165
2017-03-21 21:06:54.623 31450 DEBUG oslo_messaging._drivers.amqpdriver [req-34c3a89d-025d-4e9e-b225-5cbfd6fcfee5 - - - - -] CAST unique_id: 7fc461297cf94b45ac4fd1e515ad9e1f FANOUT topic 'cinder-scheduler' _send /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/amqpdriver.py:437
2017-03-21 21:06:54.638 31450 DEBUG oslo_service.periodic_task [req-34c3a89d-025d-4e9e-b225-5cbfd6fcfee5 - - - - -] Running periodic task VolumeManager._report_driver_status run_periodic_tasks /usr/lib/python2.7/site-packages/oslo_service/periodic_task.py:215
2017-03-21 21:06:54.639 31450 DEBUG cinder.volume.drivers.lvm [req-34c3a89d-025d-4e9e-b225-5cbfd6fcfee5 - - - - -] Updating volume stats _update_volume_stats /usr/lib/python2.7/site-packages/cinder/volume/drivers/lvm.py:204
------------------------------------------------

cinder-volume service status
-------------------------------------------
# cinder service-list
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| Binary | Host | Zone | Status | State | Updated_at | Disabled Reason |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| cinder-backup | newton | nova | enabled | up | 2017-03-21T13:06:45.000000 | - |
| cinder-scheduler | newton | nova | enabled | up | 2017-03-21T13:06:53.000000 | - |
| cinder-volume | newton@lvm | nova | disabled | up | 2017-03-21T13:06:49.000000 | - |
| cinder-volume | newton@rbd | nova | enabled | up | 2017-03-21T13:05:57.000000 | - |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
# cinder service-list
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| Binary | Host | Zone | Status | State | Updated_at | Disabled Reason |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| cinder-backup | newton | nova | enabled | up | 2017-03-21T13:06:55.000000 | - |
| cinder-scheduler | newton | nova | enabled | up | 2017-03-21T13:06:53.000000 | - |
| cinder-volume | newton@lvm | nova | disabled | up | 2017-03-21T13:06:49.000000 | - |
| cinder-volume | newton@rbd | nova | enabled | down | 2017-03-21T13:05:57.000000 | - |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
# cinder service-list
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| Binary | Host | Zone | Status | State | Updated_at | Disabled Reason |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
| cinder-backup | newton | nova | enabled | up | 2017-03-21T13:06:55.000000 | - |
| cinder-scheduler | newton | nova | enabled | up | 2017-03-21T13:06:53.000000 | - |
| cinder-volume | newton@lvm | nova | disabled | up | 2017-03-21T13:06:59.000000 | - |
| cinder-volume | newton@rbd | nova | enabled | up | 2017-03-21T13:06:57.000000 | - |
+------------------+------------+------+----------+-------+----------------------------+-----------------+
#
Reproduce the steps
-------------------------------
1.modify config
#vim /etc/cinder/cinder.conf
rbd_max_clone_depth = 2

2.Create a volume vol1 ,size 100G
3.Create a volume vol2 from vol1
4.Create a volume vol3 from vol2
5.Check cinder-volume service status