@Override
  public boolean isDirty() {
    //RC: as always, with mapped storage it is tricky to say when it is 'dirty' -- 'cos almost all the
    // writes go directly to the mapped buffer, and mapped buffer dirty/flush management is up to OS.
    // We can manage .dirty flag ourself, but it seems an overhead for (almost) nothing -- there are
    // very few real usages for .isDirty() in mmapped storages. I just define: storage is not dirty
    // by default:
    return false;
  }
@Override
  public boolean isDirty()