code: private synchronized void initDataBlockScanner(Configuration conf) {
      if (blockScanner != null) {
        return;
      }
      //initialize periodic block scanner
      String reason = null;
      if (conf.getInt("dfs.datanode.scan.period.hours", 0) < 0) {
        reason = "verification is turned off by configuration";
      } else if ( !(data instanceof FSDataset) ) {
        reason = "verifcation is supported only with FSDataset";
      } 
  
      if ( reason == null ) {
        blockScanner = new DataBlockScannerSet(this, (FSDataset)data, conf);      
      } else {
        LOG.info("Periodic Block Verification is disabled because " +
                 reason + ".");
      }
    }
docs: See {@link DataBlockScanner}