code: func (a *Driver) DiffSize(id, parent string) (size int64, err error) {
  	if !a.isParent(id, parent) {
  		return a.naiveDiff.DiffSize(id, parent)
  	}
  	// AUFS doesn't need the parent layer to calculate the diff size.
  	return directory.Size(context.TODO(), path.Join(a.rootPath(), "diff", id))
  }
docs: // DiffSize calculates the changes between the specified id
  // and its parent and returns the size in bytes of the changes
  // relative to its base filesystem directory.