code: func (d *Driver) Create(id, parent string, opts *graphdriver.CreateOpts) (retErr error) {
  	if opts != nil && len(opts.StorageOpt) != 0 {
  		if _, ok := opts.StorageOpt["size"]; ok {
  			return fmt.Errorf("--storage-opt size is only supported for ReadWrite Layers")
  		}
  	}
  	return d.create(id, parent, opts)
  }
docs: // Create is used to create the upper, lower, and merge directories required for overlay fs for a given id.
  // The parent filesystem is used to configure these directories for the overlay.