@Override
  protected void initializeKVHeap(List<KeyValueScanner> scanners,
    List<KeyValueScanner> joinedScanners, HRegion region) throws IOException {
    this.storeHeap = new ReversedKeyValueHeap(scanners, comparator);
    if (!joinedScanners.isEmpty()) {
      throw new DoNotRetryIOException("Reverse scan with loading CFs on demand is not supported");
    }
  }
ReversibleRegionScannerImpl extends from RegionScannerImpl, and is used to support reversed
scanning.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ReversedRegionScannerImpl.java