Hopefully this will help somebody else out, its taken me a long time to get the bottom of this.
I have EmonPi with a hardwired Ethernet connection and WiFi Ethernet connection, everything EmonPi/EmonHub related works perfectly but (except occasionally I cant connect to it (SSH), but it is operational, I normally connect to EmonCMS so did not link this issue to the one described below, but it does correspond to the 15-20 minute down periods)
EmonCMS has portions of frozen data (varies but about 15-20 mins in every 3 or 4 hours) where info has not been received form EmonHub, I trawled the logs, the config files, etc, etc but found nothing that was really helpful, I stumbled across an unrelated article on ARP errors causing similar gaps in other systems, investigated with Wireshark, but no this was not my problem, as I could not WireShark the WiFi connection I removed the WiFi dongle to force all comms to use wired Ethernet and Voila, all fixed !
It appears when the WiFi signal is marginal (as mine is, the EmonPi is under the stairs), whatever negotiation goes on between the wired and WiFi Ethernet, appears to take approx 15 mins, this may because the WiFi signal occasionally is strong enough to connect during the negotiation period (i'm guessing)
The Bottom Line
If you have wired AND WiFi Ethernet to your EmonPI/EmonHub and see frozen data/data gaps/drop outs to EmonCMS, try pulling out the WiFi dongle and rebooting (actually..shutdown the EmonPi correctly, using the pin in the reset hole, de-power the EmonPi to preserve your data, remove the WiFi dongle, then re-power the EmonPi, but you know what I mean)
Keith
Re: Solved -- EmonHub working fine - EmonCMS data freezes/gaps
I think this issue may have revealed itself in a couple of ways in the past, IMO there is a long standing issue with the emonPi sdcard image switching interfaces ie ifupdown, When I brought it up after losing a day to it, the general opinion was "you don't need both" which is fair enough I guess, but when you do not know leaving "the other" connected will cause problems with your primary interface, problems will ensue, at least one other user has had the same issue since I posted.
Paul