code: static long readTransactionIdFile(StorageDirectory sd) throws IOException {
      File txidFile = getStorageFile(sd, NameNodeFile.SEEN_TXID);
      long txid = 0L;
      if (txidFile.exists() && txidFile.canRead()) {
        BufferedReader br = new BufferedReader(new FileReader(txidFile));
        try {
          txid = Long.valueOf(br.readLine());
          br.close();
          br = null;
        } finally {
          IOUtils.cleanup(LOG, br);
        }
      }
      return txid;
    }
docs: Determine the last transaction ID noted in this storage directory.
  This txid is stored in a special seen_txid file since it might not
  correspond to the latest image or edit log. For example, an image-only
  directory will have this txid incremented when edits logs roll, even
  though the edits logs are in a different directory.
  
  @param sd StorageDirectory to check
  @return If file exists and can be read, last recorded txid. If not, 0L.
  @throws IOException On errors processing file pointed to by sd