private List<String> getFilePaths() {
    List<String> filePaths = new ArrayList<>();
    for (int i = 0; i < fileIndex; i++) {
      filePaths.add(filePrefix + String.format("%05d", i) + FILE_SUFFIX);
    }
    return filePaths;
  }
todo: directly serialize the sorted line instead of copy into a new tsBlock.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/sort/DiskSpiller.java