code: func tar2ext4Actual(dest string, diff io.Reader) (int64, error) {
  	// maxDiskSize is not relating to the sandbox size - this is the
  	// maximum possible size a layer VHD generated can be from an EXT4
  	// layout perspective.
  	const maxDiskSize = 128 * 1024 * 1024 * 1024 // 128GB
  	out, err := os.Create(dest)
  	if err != nil {
  		return 0, err
  	}
  	defer out.Close()
  	if err := tar2ext4.Convert(
  		diff,
  		out,
  		tar2ext4.AppendVhdFooter,
  		tar2ext4.ConvertWhiteout,
  		tar2ext4.MaximumDiskSize(maxDiskSize)); err != nil {
  		return 0, err
  	}
  	fi, err := os.Stat(dest)
  	if err != nil {
  		return 0, err
  	}
  	return fi.Size(), nil
  }
docs: // tar2ext4Actual is the implementation of tar2ext to write a layer from a tar file.
  // It can be called through re-exec (default), or inline for debugging.