code: public void flushIndexesBlocking()
      {
          // despatch flushes for all CFS backed indexes
          List<Future<?>> wait = new ArrayList<>();
          synchronized (baseCfs.getDataTracker())
          {
              for (SecondaryIndex index : allIndexes)
                  if (index.getIndexCfs() != null)
                      wait.add(index.getIndexCfs().forceFlush());
          }
  
          // blockingFlush any non-CFS-backed indexes
          for (SecondaryIndex index : allIndexes)
              if (index.getIndexCfs() == null)
                  index.forceBlockingFlush();
  
          // wait for the CFS-backed index flushes to complete
          FBUtilities.waitOnFutures(wait);
      }
docs: Flush all indexes to disk