code: func execRbdMap(b rbdMounter, rbdCmd string, mon string) ([]byte, error) {
  	// Commandline: rbdCmd map imgPath ...
  	// do not change this format - some tools like rbd-nbd are strict about it.
  	imgPath := fmt.Sprintf("%s/%s", b.Pool, b.Image)
  	if b.Secret != "" {
  		return b.exec.Run(rbdCmd,
  			"map", imgPath, "--id", b.Id, "-m", mon, "--key="+b.Secret)
  	} else {
  		return b.exec.Run(rbdCmd,
  			"map", imgPath, "--id", b.Id, "-m", mon, "-k", b.Keyring)
  	}
  }
docs: // Execute command to map a rbd device for mounter.
  // rbdCmd is driver dependent and either "rbd" or "rbd-nbd".