@Override
  protected void checkScanOrder(Cell prevKV, Cell kv, CellComparator comparator)
    throws IOException {
    // Check that the heap gives us KVs in an increasing order for same row and
    // decreasing order for different rows.
    assert prevKV == null || comparator == null || comparator.compareRows(kv, prevKV) < 0
      || (CellUtil.matchingRows(kv, prevKV) && comparator.compare(kv, prevKV) >= 0)
      : "Key " + prevKV + " followed by a " + "error order key " + kv + " in cf " + store
        + " in reversed scan";
  }
Do a backwardSeek in a reversed StoreScanner(scan backward)
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ReversedStoreScanner.java