Re: [chrony-users] Best way to synchronize dispersed VMʼs

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Mon, 7 May 2018, Nicolas Embriz wrote:

> I have some legacy VM’s/ instances dispersed around the world currently using npt and pool.ntp.org.

Note tha VMs are pretty horrible timekeepers at the best of times. The best
thing is to run ntp on the real hardware-- the host for the VMs, and have the VMs get their time from
that .
> I have been updating them and using chrony now, so far all good.
>
> Now I created a server, 94.16.116.82 (Germany) and start to use it on all my instances around the globe from Asia to USA, it
> works but here is were I have some doubts.
>
> - when using pool.ntp.org the server use the closest one based on its location, so a server in Singapore will have less
> latency pulling from a server in Asia than o one from Germany, same for a server in USA, they all will be let’s say “on
> time” but between them the offsets will differ and what I would like to have is rather than being on time to be as
> synchronized as possible, therefore wondering if by using a own server for all the infrastructure despite the network
> latency is the way to go?  (All my instances using my server now have stratum 3)
>
> - currently I have only one server but in the case I of having 2 just for example purposes, let’s say Germany and in USA,
> they have a stratum 2 and using  pool.ntp.org, the VM’s using the USA server will differ from the once using the server in
> Germany, so question here is how to synchronize the “master” servers , I was thinking on using in both the same source, for
> example a own stratum 1 server.
>
> At the end my goal is to have all instances well synchronize hopefully will the smallest possible offset, being on time is
> important but not as much as being almost identically in sync.

The problem is that your various clients will have different routes to the
server, and in particular will have different assymetric routes to the server.
Since ntp cannot determine whether or not the delay is assymetric, this will
mean that that assymetry will mean that your different clocks will not be
synchronized to the extent of that assymetry. That delay will be of order
100ms, so it will not take much of an assymetry to mean that your clocks are
all out by 10's of ms.

If you really want them to be synchronized ( and you do not give a time
scatter that you are willing to live with) the best bet is to use the time
standard that is most synchronized and available everywhere-- namely GPS pps
time. That way each clock will be withing microseconds of UTC and thus will
also be synchronized to within microseconds. Set up a gps pps stratum 1 in
each of the countries, or at each location, and use that.
> Thanks in advance 
>
> Regards