[time-nuts] Noob question, NTP stratum 1.

[Achim Gratz]

Tim Shoppa writes:
> Real ntpd uses a drift file to track the local processor's frequency offset
> and has a good estimate of processor clock drift after a day of tracking.

That assumes stable median temperature and predictable temperature
swings.

> The Raspberry Pi processor clock, like any motherboards', will often be off
> by off anywhere from +/-200ppm but the good news is that it usually varies
> by less than +/-10ppm over a day and ntpd does a good job tracking this and
> using the drift correction during no-signal periods.

All rasPi I've bought so far (5 of them, all different models from three
different sources) were ~10ppm slow at RT and within -6ppm and 0ppm at
the apex temperature point of the crystal (around 60°C).  It is possible
to keep the temperature within about 0.2K of that point by using the CPU
itself as a heater and thermometer and the resulting frequency within
+-20ppb over a day when disciplined via pps-gpio.  The residual drift is
initially positive and gets smaller over time, mine are down to about
20~30ppb per week.

> +/-10ppm over 24 hours is about +/-1 second.

I've never tried to hold over since it's really hard to figure out when
to start holding and when to switch back.  A 24 hour holdover is pretty
severe, but I guess you could keep it within double-digit ms territory
without getting too fancy if you've got that right.

Instead, by having enough independent NTP servers (all with their own
antenna) the clients figure out by themselves when one is off by more
than about a millisecond and switch to another one.  Each server
monitors the other ones and if it finds itself off from that bunch too
much will drop down from stratum-1 to stratum-2.  When exactly that
happens depends a bit on how fast the drift rate is, but usually between
3ms and 5ms.

> I would strongly discourage using a Raspberry Pi in any not-just-for-fun
> application.

Indeed.  I'd opt for something like PCEngines' APU line since they have
accessible GPIO and PTP capable ethernet.

> I have had very poor luck keeping even brand-name hi-grade
> microSD cards working for a year in 24x7 Raspberry Pi hardware. No-name or
> store brand microSD cards seem to barely work at all when new.

While I support that sentiment, after initial problems with off-brand
cards the ones I have now are all getting into their third year with no
signs of problems (knocks on wood).  In a datacenter you'd almost
certainly net boot the rasPi instead of giving them a microSD card.

> Unless your server farms are completely isolated from the internet at
> large, you should consider using a curated list of public NTP sources.

The key to NTP reliability is copious redundancy, so you'll certainly
want local servers in addition to that, even if you have redundant
outside connectivity.


Regards,
Achim.
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