--On 3 November 2015 at 12:21:20 +0100 Roger Pau Monné <roger....@citrix.com> wrote:

Can't you tell ntpd to just resynchronize no matter what skew it detects?

Probably - I'll have to do some digging - I had hoped someone had hit this already, though I'm also a little concerned about throwing a 7 second 'back in time' jump on the system.

Are either of those fixes likely to cure this?

Hm, I don't think so. Both commits are present from 4.3 onwards, and
IIRC XenServer 6.5 is based on Xen 4.4, which should contain both fixes.

Is XenServer also synchronized using ntpd? When a guest resumes from
migration is updates it's current time based on the time provided by the
hypervisor, so if the hosts themselves are not synchronized you could
see skews like that.

All the clocks on the XenServers are NTP sync'd - and are in sync (confirmed from the consoles).

When the domU moves - it's clock moves >ahead< in time of both the server it's leaving, and the server it's moved to.

I re-ran the test adding checks for the source (Xen1) and destination (Xen2) XenServer clocks - and I get:

 Me: Tue Nov  3 18:56:29 GMT 2015
Them: Tue Nov  3 18:56:29 GMT 2015
Xen1: Tue Nov  3 18:56:29 GMT 2015
Xen2: Tue Nov  3 18:56:29 GMT 2015

(All in sync)

[moving FreeBSD from Xen1 to Xen2]

 Me: Tue Nov  3 18:56:39 GMT 2015
(Gained 4 seconds)
Them: Tue Nov  3 18:56:35 GMT 2015
Xen1: Tue Nov  3 18:56:35 GMT 2015
Xen2: Tue Nov  3 18:56:35 GMT 2015

 Me: Tue Nov  3 18:56:43 GMT 2015
(Completed - gained 7 seconds total)
Them: Tue Nov  3 18:56:36 GMT 2015
Xen1: Tue Nov  3 18:56:36 GMT 2015
Xen2: Tue Nov  3 18:56:36 GMT 2015

 Me: Tue Nov  3 18:56:44 GMT 2015
Them: Tue Nov  3 18:56:37 GMT 2015
Xen1: Tue Nov  3 18:56:37 GMT 2015
Xen2: Tue Nov  3 18:56:37 GMT 2015

So you can see it's now 7 seconds ahead of both 'Them' (external bare metal machine running ntp) and Xen1/Xen2.

I re-ran the test without ntp loaded on the domU - and the same thing happens - so it's not ntp causing it.

We've kind of 'lived with it' until now - but as the quantity of VM's increases - it becomes more of a pain :(

-Karl




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