code: func BufferSize(size int) FileStoreOption {
  	return func(o *FileStoreOptions) error {
  		if size < 0 {
  			return fmt.Errorf("buffer size value must be a positive number")
  		}
  		o.BufferSize = size
  		return nil
  	}
  }
docs: // BufferSize is a FileStore option that sets the size of the buffer used
  // during store writes. This can help improve write performance.