code: func isOld(rootDir string) (bool, error) {
  	d, err := os.Open(rootDir)
  	if err != nil {
  		return false, trace.ConvertSystemError(err)
  	}
  	defer d.Close()
  
  	files, err := d.Readdir(0)
  	if err != nil {
  		return false, trace.ConvertSystemError(err)
  	}
  
  	for _, fi := range files {
  		if fi.IsDir() {
  			return true, nil
  		}
  	}
  
  	return false, nil
  }
docs: // DELETE IN: 2.8.0
  // isOld checks if the directory backend is in the old format or not.