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

[Nicolas Embriz <nbari@xxxxxxxxxx>]

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.

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.

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.