[brandmeyer]

That doesn't make it any less unilateral. IMO, it makes it rather worse to have been doing this for five years. The situation would be much better if they had been working to build a broader consensus over all that time. As near as I can tell, they don't even have consensus within Linux, let alone POSIX or the ITU.

[TheGrumpyBrit]

In the absense of a published and agreed standard, every approach is unilateral. My company is taking a similar approach - disconnecting from external NTP servers on 31st December, stepping the change in gradually, and reconnecting when we're "right" again.

Google have never been in the NTP business - there's no reason for them to have worked towards a concensus on this. But when a company their size makes their approach publicly available to all, it starts to pave the way for a consistent standard for everyone.

[toast0]

Google is in the NTP business. Chromebooks sync time from their servers, android devices can (but also from carrier provide time signals), and for hosts in their cloud services.

[detaro]

Honest question: what's so bad about others' systems running on slightly off time? I get why people care about internal consistency, and why deviations should be quite small, but this?

[toast0]

During the last leap second, I had servers configured against google's semi-public servers and some other good sources of time. ntpd marked the google servers as a false ticker sometime during the distortion, and when it was done, was happy with it again. However, I have more non-google servers than google servers, and high minpoll times which tends to result in time checks between servers happening far apart in time, so even if I had multiple google servers, they wouldn't look very close together.

[brandmeyer]

Slightly contrived example: Lets say that you were running a distributed database, and you had distributed instances across different cloud providers for increased reliability. if your database relies on high-resolution timestamps for distributed conflict resolution, then you're going to have a hard time.

Another example: Suppose that a portion of an industrial monitoring system processes remote sensor data in a cloud datacenter with smeared time, while the sensor nodes keep strict UTC time. Your SCADA system had better not have any hard-baked assumptions like "messages cannot come from the future", or you're going to have a hard time, too.

Lets say that a company's internal NTC servers include several sources for reliability and redundancy. Much like Google DNS, perhaps one of the sources is Google NTP, while another is derived from the NTP pool. How do you expect the NTP daemon to behave in this situation? It will certainly be able to observe a 500ms difference between its source timeservers.

[oasisbob]

Both of those examples strike me as very contrived.

I can't think of anyone who cares that much about timekeeping who isn't running their own internal NTP infrastructure.

Google's Spanner requires accurate global time, so they deployed GPS and atomic clocks. Same for CDMA. There are some applications for high-resolution time (eg finance), so protocols like PTP exist.

A smeared NTP source in an otherwise normal list of time sources doesn't seem like that big of a deal either - eventually the daemon is just going to mark it as a falseticker and life goes on.

[subway]

Everywhere Google documents the service, they clearly state you should not mix their smearing NTP servers with non-smearing NTP servers.

[oasisbob]

I'm not convinced it would do anything harmful, so long as you have enough NTP sources (which you should have anyway).

From their FAQ:

> We recommend that you do not mix smeared and non-smeared NTP servers. The results during a leap second may be unpredictable.

I read that as a soft SHOULD NOT, not MUST NOT. Would be a fun exercise to try doing it intentionally with common NTP implementations and see what happens.

[detaro]

first example: ok, yes, if they offer the DB as a service, that would be bad.

If you run it on a VM it's IMHO your responsibility to make sure your time sensitive database nodes have shared time.

Same for the second example, interesting point for SaaS scenario, although it seems like that could break through normal deviations already.

EDIT: ok, the blog post actually mentions "local clocks in sync with VM instances running on Google Compute Engine", my bad. Not sure what to think about that. In comparison, Amazon recommends running NTP on your VMs and their Linux AMIs come with pool.ntp.org configured as default. </edit>

Third: It's going to figure out some solution (if Google is only one source it's probably going to drop it as faulty), but you probably should not have added a time source that's officially documented to not strictly follow standards. It's not like Google offered a NTP service for years and now suddenly switched how it works.

I guess I underestimate the amount of trust people put into random time sources: practice is probably messier than theory.