def properties_root_device_name(properties):
    """Get root device name from image meta data.
    If it isn't specified, return None.
    """
    root_device_name = None
    for bdm in properties.get('mappings', []):
        if bdm['virtual'] == 'root':
            root_device_name = bdm['device']
    if 'root_device_name' in properties:
        root_device_name = properties['root_device_name']
    return root_device_name
