Why cinder-volume-usage-audit fails on RHEL OSP 8 ?

Issue

• cinder-volume-usage-audit command is getting failed with following message on RHEL OSP 8 setup ?

# cinder-volume-usage-audit
No handlers could be found for logger "oslo_config.cfg"
/usr/lib/python2.7/site-packages/oslo_db/sqlalchemy/enginefacade.py:241: NotSupportedWarning: Configuration option(s) ['use_tpool'] not supported
  exception.NotSupportedWarning
2016-05-26 08:15:56.744 9892 ERROR cinder [req-a56f0b30-6070-4f28-8053-5b9a046b4d77 - - - - -] Exists volume notification failed: Parent instance  is not bound to a Session; lazy load operation of attribute 'volume_metadata' cannot proceed
2016-05-26 08:15:56.744 9892 ERROR cinder Traceback (most recent call last):
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib/python2.7/site-packages/cinder/cmd/volume_usage_audit.py", line 119, in main
2016-05-26 08:15:56.744 9892 ERROR cinder     'exists', extra_usage_info=extra_info)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py", line 125, in notify_about_volume_usage
2016-05-26 08:15:56.744 9892 ERROR cinder     usage_info = _usage_from_volume(context, volume, **extra_usage_info)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py", line 75, in _usage_from_volume
2016-05-26 08:15:56.744 9892 ERROR cinder     metadata=volume_ref.get('volume_metadata'),)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib/python2.7/site-packages/oslo_db/sqlalchemy/models.py", line 68, in get
2016-05-26 08:15:56.744 9892 ERROR cinder     return getattr(self, key, default)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 237, in __get__
2016-05-26 08:15:56.744 9892 ERROR cinder     return self.impl.get(instance_state(instance), dict_)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 578, in get
2016-05-26 08:15:56.744 9892 ERROR cinder     value = self.callable_(state, passive)
2016-05-26 08:15:56.744 9892 ERROR cinder   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/strategies.py", line 502, in _load_for_state
2016-05-26 08:15:56.744 9892 ERROR cinder     (orm_util.state_str(state), self.key)
2016-05-26 08:15:56.744 9892 ERROR cinder DetachedInstanceError: Parent instance  is not bound to a Session; lazy load operation of attribute 'volume_metadata' cannot proceed
2016-05-26 08:15:56.744 9892 ERROR cinder

• Why cinder-volume-usage-audit output is different in RHEL OSP 7 and OSP 8 ?