code: func (jc *JobConfig) Validate() error {
  	switch {
  	case jc.ShardCount < 1:
  		return errors.Reason("ShardCount should be >= 1, try 8").Err()
  	case jc.PageSize <= 0:
  		return errors.Reason("PageSize should be > 0, try 256").Err()
  	case jc.PagesPerTask < 0:
  		return errors.Reason("PagesPerTask should be >= 0, keep 0 for default").Err()
  	case jc.TaskDuration < 0:
  		return errors.Reason("TaskDuration should be >= 0, keep 0 for default").Err()
  	}
  	return nil
  }
docs: // Validate returns an error of the config is invalid.
  //
  // Mapper existence is not checked.