There are mysql errors in the c-bak logs when the volume goes into 'error_restoring' status:

http://logs.openstack.org/82/202782/15/gate/gate-tempest-dsvm-neutron-full/372a95f/logs/screen-c-bak.txt.gz?level=TRACE

2015-08-10 21:36:37.420 ERROR oslo_messaging.rpc.dispatcher [req-1abbfe4f-97f2-4184-b598-522a1e0e2c19 None] Exception during message handling: (pymysql.err.OperationalError) (2013, 'Lost connection to MySQL server during query')
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher Traceback (most recent call last):
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 142, in _dispatch_and_reply
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     executor_callback))
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 186, in _dispatch
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     executor_callback)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 129, in _do_dispatch
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     result = func(ctxt, **new_args)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/osprofiler/profiler.py", line 105, in wrapper
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return f(*args, **kwargs)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/manager.py", line 437, in restore_backup
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     backup.save()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 119, in __exit__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     six.reraise(self.type_, self.value, self.tb)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/manager.py", line 431, in restore_backup
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     backup_service)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/osprofiler/profiler.py", line 105, in wrapper
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return f(*args, **kwargs)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/volume/drivers/lvm.py", line 500, in restore_backup
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     backup_service.restore(backup, volume['id'], volume_file)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/chunkeddriver.py", line 684, in restore
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self.put_metadata(volume_id, volume_meta)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/driver.py", line 336, in put_metadata
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self.backup_meta_api.put(volume_id, json_metadata)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/driver.py", line 315, in put
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     func(meta_container[type], volume_id, fields)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/backup/driver.py", line 177, in _restore_vol_base_meta
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self.db.volume_update(self.context, volume_id, metadata)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/db/api.py", line 245, in volume_update
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return IMPL.volume_update(context, volume_id, values)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/db/sqlalchemy/api.py", line 182, in wrapper
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return f(*args, **kwargs)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/db/sqlalchemy/api.py", line 1696, in volume_update
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     volume_ref = _volume_get(context, volume_id, session=session)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/db/sqlalchemy/api.py", line 182, in wrapper
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return f(*args, **kwargs)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/opt/stack/new/cinder/cinder/db/sqlalchemy/api.py", line 1278, in _volume_get
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     filter_by(id=volume_id).\
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2445, in first
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     ret = list(self[0:1])
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2281, in __getitem__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return list(res)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2516, in __iter__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return self._execute_and_instances(context)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2529, in _execute_and_instances
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     close_with_result=True)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2520, in _connection_from_session
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     **kw)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 882, in connection
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     execution_options=execution_options)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 887, in _connection_for_bind
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     engine, execution_options)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 334, in _connection_for_bind
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     conn = bind.contextual_connect()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2039, in contextual_connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self._wrap_pool_connect(self.pool.connect, None),
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2078, in _wrap_pool_connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     e, dialect, self)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1401, in _handle_dbapi_exception_noconnection
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     util.raise_from_cause(newraise, exc_info)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     reraise(type(exception), exception, tb=exc_tb)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2074, in _wrap_pool_connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return fn()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 376, in connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return _ConnectionFairy._checkout(self)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 713, in _checkout
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     fairy = _ConnectionRecord.checkout(pool)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 480, in checkout
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     rec = pool._do_get()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 1060, in _do_get
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self._dec_overflow()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     compat.reraise(exc_type, exc_value, exc_tb)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 1057, in _do_get
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return self._create_connection()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 323, in _create_connection
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return _ConnectionRecord(self)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 449, in __init__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self.connection = self.__connect()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 607, in __connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     connection = self.__pool._invoke_creator(self)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/strategies.py", line 97, in connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return dialect.connect(*cargs, **cparams)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/default.py", line 385, in connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return self.dbapi.connect(*cargs, **cparams)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/__init__.py", line 88, in Connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     return Connection(*args, **kwargs)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/connections.py", line 644, in __init__
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self._connect()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/connections.py", line 838, in _connect
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     self._request_authentication()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/connections.py", line 1021, in _request_authentication
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     auth_packet = self._read_packet()
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/connections.py", line 882, in _read_packet
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     packet_header = self._read_bytes(4)
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/pymysql/connections.py", line 909, in _read_bytes
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher     2013, "Lost connection to MySQL server during query")
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher DBConnectionError: (pymysql.err.OperationalError) (2013, 'Lost connection to MySQL server during query')
2015-08-10 21:36:37.420 9136 ERROR oslo_messaging.rpc.dispatcher 

So that's probably the root issue here.