code: public void writeTransactionIdFileToStorage(long txid, FSImage image)
        throws IOException {
      // Write txid marker in all storage directories
      List<StorageDirectory> badSDs = new ArrayList<StorageDirectory>();
      for (StorageDirectory sd : storageDirs) {
        try {
          writeTransactionIdFile(sd, txid);
        } catch(IOException e) {
          // Close any edits stream associated with this dir and remove directory
          LOG.warn("writeTransactionIdToStorage failed on " + sd,
              e);
          badSDs.add(sd);
        }
      }
      reportErrorsOnDirectories(badSDs, image);
      if (image != null) {
        
      }
    }
docs: Write a small file in all available storage directories that
  indicates that the namespace has reached some given transaction ID.
  
  This is used when the image is loaded to avoid accidental rollbacks
  in the case where an edit log is fully deleted but there is no
  checkpoint. See TestNameEditsConfigs.testNameEditsConfigsFailure()
  @param txid the txid that has been reached