@Override
  public boolean seekToPreviousRow(ExtendedCell key) throws IOException {
    if (checkFlushed()) {
      reopenAfterFlush();
    }
    return this.heap.seekToPreviousRow(key);
  }
Do a backwardSeek in a reversed StoreScanner(scan backward)
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ReversedStoreScanner.java