NAS upgrade: the honeymoon is over
Getting the Synology DS213+ NAS to sleep is trickier than anticipated. Judging from the folklore on the interwebs I am not alone.
The people's guess is, that it's an ARP request that wakes up the Synology. That sounds sensible and somewhat unavoidable. By my own observations, I'd say that ANY ARP request wakes up the Synology from deep sleep, not just the ones looking for the NAS ethernet address. ARPs are broadcast, but other broadcast traffic I observed didn't affect it.
Case in point, when my "workstation" turns off or goes to sleep, my NAS magically awakes, presumably because my gateway is looking for the workstation again. (Speculation). Also when my "gateway" periodically awakens at night because of Wake on Demand, then it will apparently also send out ARP packets.
The main and fixable problem seems to be, that, if the NAS has "accidentally" risen, it won't find sleep again soon. Instead it waits again for the configured period. (30 minutes in my case).
Another speculation is, that periodic awakenings from sleep mode have something to do with the ntp daemon, who's trying to contact a time server about every hour. I have now disabled ntp for testing, let's see how that works. On a hunch I would say, that this isn't the problem, as the ntp daemon shouldn't be running in sleep mode in the first place.
22.02: Last night my NAS did get a good nights sleep. So experimentally I am turning ntp back on.
27.02:Predictably ntp had no averse side effect on sleep.