Comment 3 for bug 1304695

I debugged this problem a bit more. 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.

In order for this to work properly, refresh_conn_info must be set to True. However, since _get_instance_volume_block_device_info is used by everyone, I am not sure if setting refresh_conn_info = True is such a good solution, since it will affect other cinder drivers also.