Re: [chrony-users] Best way to synchronize dispersed VM’s

On Mon, May 7, 2018 at 11:00 PM, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:

On Mon, 7 May 2018, Nicolas Embriz wrote:

Hi, many thanks for the answers,
Regarding how low I want to go, currently I am only testing and understanding more about this, I don’t really know the
real limits but if I could get at a nanosecond-level would make me happy.

You cannot unless you hook up a timing gps up to each of your computers,
replace all their crystals with muchmuch higher quality and definitely get of
VMs. So if that is what you need, you are engaged in a hopeless task.

My plan is to start collecting metrics (collectd + influxdb + grafana) and try to measure the offset across all the
instances, and over the time see how I could gradually start improving the offset, I would like to give a try building a
GPS/PPS (raspberry pi + FreeBSD) and see how far I could get.

While raspberry pi's are really useful for many things, ultra high preceision
timing is not one of them.

Any ideas, suggestions more than welcome.

regards.

On Mon, May 7, 2018 at 6:51 PM, Stephen Satchell <list@xxxxxxxxxxxx> wrote:
On 05/07/2018 08:58 AM, Bill Unruh wrote:

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 .

"The right tool for the right job" -- Scotty, _Star Trek_

I've worked with a number of VM objects for the past few years, and have tried to use the "trick" of running
NTP in the host and having the virtual objects sync to them. Still have time differences.

The problem isn't the NTP daemon running on the host; it's the slipperiness of clock accuracy in the virtual
objects. NTP (and CHRONY) make the assumption that the clock in the "computer" is driven by a crystal
oscillator or AC power-line frequency.

Crystal oscillators in modern computing devices have a tracking accuracy of under 1 ppm, closer to 0.010 ppm.
Very few modern devices track power-line frequency anymore.

Virtual clocks have considerably worse tracking accuracy, so the clock in a VM object can't be regulated
nearly as well as one in bare iron. Short story: I had an automation server running on bare iron for years.
Time tracking was excellent, even without NTP running. When $DAYJOB moved that server to a virtual
environment, the clock started running slow at times. At one point, the clock was running four hours behind
wall time, which pissed the customers off no end. The "fix" was to run NTPdate every hour against an external
time standard, a fix that continues to this day. No more unpredictable clock drift.

At satchell.net, I've added an GPS-based NTP appliance. One server slaves to that (and other Stratum 1
servers). Everything else slaves to that. Last time I checked time accuracy against WWV, all the computers
were well within one second of the radio clock time.

>From some things you said, you don't own the VM hosts. That can be a...problem.

Also, latency is not as important as jitter. If you have a constant latency, the chronyd algorithms can take
that into account. Then there is the false-ticker problem, but that's another issue.

(N.B.: if you plan to use a commercial NTP server for your reference time base, make sure it can handle all
the traffic on your network. Embedded IP stacks tend to be fragile, particularly in consumer-grade products.
When I got my GPS-based NTP appliance, I found that I had to use a VLAN to isolate the poor thing from the
traffic on my network. Otherwise the TCP/IP stack would choke. That's also why ONE server synced with the
appliance, and served time to the rest of my network.)

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