def METHOD_NAME(img, root_dir):
    """ Returns size in bytes that the given image is actually using
        :param img: QemuImg instance of the image being checked
        :param root_dir: root data dir in which images are left
        :returns: an int with the disk usage of the image
    """
    img_path = os.path.join(root_dir, img.image_filename)
    try:
        res = process.run(f"ls -l {img_path}", shell=True).stdout_text
        # Taking 5.th element from output. Similar to '| awk...' but safer
        size = int(res.split()[4])
    except (process.CmdError, ValueError):
        test.error(f"Couldn't extract {img_path} size")
    except IndexError:
        test.error(f"cmd 'ls -l {img_path}' didn't work as expected")
    return size