code: func (d *Driver) DiffSize(id, parent string) (size int64, err error) {
  	logrus.Debugf("lcowdriver: diffsize: id %s", id)
  	// TODO @gupta-ak. Needs implementation with assistance from service VM
  	return 0, nil
  }
docs: // DiffSize calculates the changes between the specified layer
  // and its parent and returns the size in bytes of the changes
  // relative to its base filesystem directory.