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[Nikola Đipanov <ndipanov@xxxxxxxxxx>]

** Also affects: nova
   Importance: Undecided
       Status: New

** Changed in: nova
   Importance: Undecided => Medium

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https://bugs.launchpad.net/bugs/1304695

Title:
  glusterfs: Instance is not using the correct volume snapshot file
  after reboot

Status in Cinder:
  New
Status in OpenStack Compute (Nova):
  New

Bug description:
  Instance is not using the correct volume snapshot file after reboot.

  Steps to recreate bug:
  1. Create a volume

  2. Attach volume to a running instance.

  3. Take an online snapshot of the volume.
  Note that the active volume used by the instance is now switched to volume-<uuid>.<snapshot-uuid>.

  4. Shutdown the instance.

  5. Start the instance.
  If you invoke virsh dumpxml <instance>, you will see that it is re-attaching the base volume ( volume-<uuid>) to the instance and not the snapshot volume (volume-<uuid>.<snapshot-uuid>).  The expected behavior is to have the snapshot volume re-attach to the instance.

  This bug will cause data corruption in the snapshot and volume.


  
  It looks like the nova volume manager is using a stale copy of the block_device_mapping. The block_device_mapping needs to be refreshed in order for the updated volume snapshot to be used.

  On power on, the nova manager (nova/compute/manager.py ) does:
  1. start_instance
  2. _power_on
  3. _get_instance_volume_block_device_info

  The structure for this method is:
  def _get_instance_volume_block_device_info(self, context, instance, refresh_conn_info=False, bdms=None):
      if not bdms:
          bdms = (block_device_obj.BlockDeviceMappingList.
                      get_by_instance_uuid(context, instance['uuid']))
          block_device_mapping = (
              driver_block_device.convert_volumes(bdms) +
              driver_block_device.convert_snapshots(bdms) +
              driver_block_device.convert_images(bdms))
      ....
  block_device_obj.BlockDeviceMappingList.get_by_instance_uuid() goes and queries the database to construct the bdms, which will contain stale data.

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