stenn at nwtime.org
Wed May 1 23:20:56 UTC 2019
On 5/1/2019 4:17 PM, Brandon Martin wrote:
> On 5/1/19 7:03 PM, Harald Koch wrote:
>> Properly deployed NTP should calibrate the local hardware clocks to
>> prevent drift even during connectivity outages. (I'm talking both the
>> low resolution hardware clocks used for timing across power cycles and
>> reboots, and the oscillators used while the OS is running). While most
>> computer hardware is temperature sensitive, if your datacenter is
>> suddenly changing temperature enough to cause clock drift, well, you
>> have bigger problems.:)
> For sure, sudden loss of time "shouldn't" happen, but having a local
> refclock is comparatively cheap insurance against it in many deployments.
BCP these days is "orphan mode", not "local refclock".
> I've seen things like this when there's a sudden power loss across a
> small site e.g. a remote PoP. Think a loss of utility power and UPS
> fails to transfer for some unanticipated reason. Everything will come
> back up when either the utility power comes back or generator spins up,
> but it will all be hard reset. Depending on your NTP implementation,
> the local hardware clock may not be particularly accurate. Even good
> implementations often lack the necessary hardware capabilities to trim
> the low-resolution hardware reference and have to resort to simply
> flushing the time to hardware every so often.
> Relative inaccuracies of a few seconds are pretty normal in that kind of
> situation in my experience. Putting everything together from logs where
> there's an unknown time offset of a few seconds after the fact can be
> tough. Then again, maybe you don't care in this example case since the
> cause of the problem is proximate - the frigging UPS didn't do its job.
> More complex scenarios might be easily envisioned, though.
> Now, obviously you've still got an issue of the fact that the GPS refclk
> will take a while to lock and start serving time, but at least you've
> potentially got known-good time info before you start bringing
> higher-level network protocols up (and can purposely delay until you do,
> if desired) which is potentially impossible if your only source of time
> is the network itself.
Ah, this is the dance with "have enough sources of time"...
Harlan Stenn, Network Time Foundation
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