private float inuseRatio(final Exchange ex) throws Exception {
        float total = 0;
        float used = 0;
        //
        // forward sequential
        //
        for (long page = 2; page < 20; page++) {
            final Buffer buffer = ex.getBufferPool().get(ex.getVolume(), page, false, true);
            if (buffer.isDataPage()) {
                int available = buffer.getAvailableSize();
                System.out.println(buffer + " avail=" + available);
                total = total + buffer.getBufferSize();
                used = used + buffer.getBufferSize() - buffer.getAvailableSize();
            }
            buffer.releaseTouched();
        }
        return used / total;
    }