we have a similar bug downstream against queens in the other direction alos
https://bugzilla.redhat.com/show_bug.cgi?id=1838786

in the downstream bug the failure is on attach
libvirtError: Requested operation is not valid: target vdf already exists

where as upstream we have  "nova.exception.DeviceDetachFailed: Device detach failed for vdb: Unable to detach the device from the live config."

in both caes its as if the live domain view and nova's/cinder'd view or the domain are getting out of sync with each other


the call trace upstream traceback for detach looks like this

 Traceback (most recent call last):
   File "/opt/stack/nova/nova/virt/block_device.py", line 328, in driver_detach
     virt_driver.detach_volume(context, connection_info, instance, mp,
   File "/opt/stack/nova/nova/virt/libvirt/driver.py", line 2004, in detach_volume
     wait_for_detach = guest.detach_device_with_retry(guest.get_disk,
   File "/opt/stack/nova/nova/virt/libvirt/guest.py", line 425, in detach_device_with_retry
     _try_detach_device(conf, persistent, live)
   File "/opt/stack/nova/nova/virt/libvirt/guest.py", line 414, in _try_detach_device
     ctx.reraise = True
   File "/usr/local/lib/python3.8/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
     self.force_reraise()
   File "/usr/local/lib/python3.8/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
     six.reraise(self.type_, self.value, self.tb)
   File "/usr/local/lib/python3.8/dist-packages/six.py", line 703, in reraise
     raise value
   File "/opt/stack/nova/nova/virt/libvirt/guest.py", line 387, in _try_detach_device
     self.detach_device(conf, persistent=persistent, live=live)
   File "/opt/stack/nova/nova/virt/libvirt/guest.py", line 475, in detach_device
     self._domain.detachDeviceFlags(device_xml, flags=flags)
   File "/usr/local/lib/python3.8/dist-packages/eventlet/tpool.py", line 190, in doit
     result = proxy_call(self._autowrap, f, *args, **kwargs)
   File "/usr/local/lib/python3.8/dist-packages/eventlet/tpool.py", line 148, in proxy_call
     rv = execute(f, *args, **kwargs)
   File "/usr/local/lib/python3.8/dist-packages/eventlet/tpool.py", line 129, in execute
     six.reraise(c, e, tb)
   File "/usr/local/lib/python3.8/dist-packages/six.py", line 703, in reraise
     raise value
   File "/usr/local/lib/python3.8/dist-packages/eventlet/tpool.py", line 83, in tworker
     rv = meth(*args, **kwargs)
   File "/usr/local/lib/python3.8/dist-packages/libvirt.py", line 1408, in detachDeviceFlags
     if ret == -1: raise libvirtError ('virDomainDetachDeviceFlags() failed', dom=self)
 libvirt.libvirtError: device not found: no target device vdb


and the down stream traceback for attach looks like

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 5433, in _attach_volume
    do_driver_attach=True)
  File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 46, in wrapped
    ret_val = method(obj, context, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 624, in attach
    virt_driver, do_driver_attach)
  File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 602, in _do_attach
    do_driver_attach)
  File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 549, in _volume_attach
    attachment_id)
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
    self.force_reraise()
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
    six.reraise(self.type_, self.value, self.tb)
  File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 540, in _volume_attach
    device_type=self['device_type'], encryption=encryption)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 1520, in attach_volume
    encryption=encryption)
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
    self.force_reraise()
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
    six.reraise(self.type_, self.value, self.tb)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 1493, in attach_volume
    guest.attach_device(conf, persistent=True, live=live)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/guest.py", line 306, in attach_device
    self._domain.attachDeviceFlags(device_xml, flags=flags)
  File "/usr/lib/python2.7/site-packages/eventlet/tpool.py", line 186, in doit
    result = proxy_call(self._autowrap, f, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/eventlet/tpool.py", line 144, in proxy_call
    rv = execute(f, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/eventlet/tpool.py", line 125, in execute
    six.reraise(c, e, tb)
  File "/usr/lib/python2.7/site-packages/eventlet/tpool.py", line 83, in tworker
    rv = meth(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 605, in attachDeviceFlags
    if ret == -1: raise libvirtError ('virDomainAttachDeviceFlags() failed', dom=self)
libvirtError: Requested operation is not valid: target vdf already exists


these bug may be unrelated but i find it interesting that we are seeint issue with both attach and detach at the same time even if they are with two master vs queens and both seam to be realted to nova's view of the live domain mismatching with livbvirts.