public void logFile(TsFileResource tsFile, String flag) throws IOException {
    String log =
        flag
            + TsFileIdentifier.INFO_SEPARATOR
            + TsFileIdentifier.getFileIdentifierFromFilePath(tsFile.getTsFile().getAbsolutePath());
    logStream.write(log.getBytes());
    logStream.write(System.lineSeparator().getBytes());
    logStream.flush();
  }
MergeLogger records the progress of a merge in file "merge.log" as text lines.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/compaction/execute/utils/log/CompactionLogger.java