code: public Long retrieveChecksum(Block lostBlock, Path p,
        long lostBlockOffset, FileSystem fs, Context context) 
            throws IOException {
      Long oldCRC = null;  
      if (checksumStore != null) { 
        IOException caughtException = null;
        try {
          oldCRC = checksumStore.getChecksum(lostBlock); 
        } catch (IOException ioe) {
          LOG.error(" Fail to get checksum for block " + lostBlock, ioe);
          caughtException = ioe;
        }
        // Checksum is not record, we should report
        if (oldCRC == null) {
          LogUtils.logRaidReconstructionMetrics(LOGRESULTS.FAILURE, 0,
              codec, p, lostBlockOffset,
              LOGTYPES.OFFLINE_RECONSTRUCTION_GET_CHECKSUM,
              fs, caughtException, context);
        }
      }
      return oldCRC;
    }
docs: Retrieve checksums from checksum store and record checksum lost
  if possible