func syncDisk() {
	time.Sleep(time.Second * g.CACHE_DELAY)   //CACHE_DELAY     = 1800    //s
	ticker := time.NewTicker(time.Millisecond * g.FLUSH_DISK_STEP)
	defer ticker.Stop()
	var idx int = 0

	for {
		select {
		case <-ticker.C:
			idx = idx % store.GraphItems.Size
			FlushRRD(idx, false)
			idx += 1
		case <-Out_done_chan:
			log.Println("cron recv sigout and exit...")
			return
		}
	}
}