code: func (i *ImageService) GetContainerLayerSize(containerID string) (int64, int64) {
  	var (
  		sizeRw, sizeRootfs int64
  		err                error
  	)
  
  	// Safe to index by runtime.GOOS as Unix hosts don't support multiple
  	// container operating systems.
  	rwlayer, err := i.layerStores[runtime.GOOS].GetRWLayer(containerID)
  	if err != nil {
  		logrus.Errorf("Failed to compute size of container rootfs %v: %v", containerID, err)
  		return sizeRw, sizeRootfs
  	}
  	defer i.layerStores[runtime.GOOS].ReleaseRWLayer(rwlayer)
  
  	sizeRw, err = rwlayer.Size()
  	if err != nil {
  		logrus.Errorf("Driver %s couldn't return diff size of container %s: %s",
  			i.layerStores[runtime.GOOS].DriverName(), containerID, err)
  		// FIXME: GetSize should return an error. Not changing it now in case
  		// there is a side-effect.
  		sizeRw = -1
  	}
  
  	if parent := rwlayer.Parent(); parent != nil {
  		sizeRootfs, err = parent.Size()
  		if err != nil {
  			sizeRootfs = -1
  		} else if sizeRw != -1 {
  			sizeRootfs += sizeRw
  		}
  	}
  	return sizeRw, sizeRootfs
  }
docs: // GetContainerLayerSize returns the real size & virtual size of the container.