code: public void setWaterMarks(long lwmScn, long hwmScn) throws IOException {
        if(lwmScn <= hwmScn) {
            writeHwmScn(hwmScn);
            _writer.flush();
            writeLwmScn(lwmScn);
            _writer.flush();
        } else {
            throw new IOException("Invalid water marks: lwmScn=" + lwmScn + " hwmScn=" + hwmScn);
        }
    }
docs: Sets the water marks of this ArrayFile.
  
  @param lwmScn - the low water mark
  @param hwmScn - the high water mark
  @throws IOException if the <code>lwmScn</code> is greater than the <code>hwmScn</code>
  or the changes to the underlying file cannot be flushed.