code: func (d *DataDirectory) Initialize(target *EtcdVersionPair) error {
  	isEmpty, err := d.IsEmpty()
  	if err != nil {
  		return err
  	}
  	if isEmpty {
  		klog.Infof("data directory '%s' is empty, writing target version '%s' to version.txt", d.path, target)
  		err = d.versionFile.Write(target)
  		if err != nil {
  			return fmt.Errorf("failed to write version.txt to '%s': %v", d.path, err)
  		}
  		return nil
  	}
  	return nil
  }
docs: // Initialize set the version.txt to the target version if the data
  // directory is empty. If the data directory is non-empty, no
  // version.txt file will be written since the actual version of etcd
  // used to create the data is unknown.