code: private FSDataOutputStream createNewPartStream(Path dst, int partId) throws IOException {
      String partName = PART_PREFIX + partId;
      Path output = new Path(dst, partName);
      FileSystem destFs = output.getFileSystem(conf);
      FSDataOutputStream partStream = destFs.create(output, false,
          conf.getInt("io.file.buffer.size", 4096), destFs.getDefaultReplication(),
          conf.getLong(HAR_BLOCKSIZE_LABEL, HAR_BLOCKSIZE_DEFAULT));
      return partStream;
    }
docs: Creates new stream to write actual file data
  @param dst parent of the part-id file
  @param partId id of the part
  @return the open stream
  @throws IOException