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Re: [Xen-users] one domU out of 4 looses time sync

Dimitris Zilaskos wrote:

To follow up a bit on this, I have installed another SL4 domU. The new one not only also gets of sync, but it also has the same time as the other domU that gets out of sync. So out of the 5 domUs, 2 are in sync together but not with the dom0, and the other 3 are in sync with the dom0.

I found out that it seems drift occurs when for some reason the dom0 looses contact with the ntp server:

Jun 15 20:38:26 xen03 ntpd[4633]: no servers reachable

When that happens the time in the domUs starts to drift. Now why there are two groups of domUs, those that keep in sync with the dom0 and those that do not keep up with dom0 but keep up with each other, I guess it has something to do with xen/kernel internals.

Weird thing is that the ntp server is in the same lan (but different vlan), and other systems in the same location network wise do not exhibit the same problem.


Dimitris Zilaskos wrote:
Hi all,

I am running Scientific Linux 5.1 (rhel 5.1 compatible) 64bit on a 2x4core system with 16 gb ram with 3 Scientific Linux 4.6 (rhel 4.6 compatible) guests an one Scientific Linux 5.1 guest.

I do not run ntp on the guests but only on dom0.

One specific Scientific Linux 4 domU always looses time synchronization with dom0. All the others SL4 and the SL5 domUs remain synchronized. There is no difference in the xen configuration from the others apart from different memory size.

If I restart ntpd on the dom0 the time is synchronized across all domUs, including the problematic one. After about a week of uptime the time different can be up to 2 minutes 30 seconds backwards compared to the domUs that work properly.

All domUs and dom0 have xen.independent_wallclock = 0

Dom0 is running Linux 2.6.18-128.1.6.el5xen #1 SMP Wed Apr 1 07:21:08 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

the SL4 domUs run Linux 2.6.9-78.0.17.ELxenU #1 SMP Thu Mar 12 14:49:52 CDT 2009 x86_64 x86_64 x86_64 GNU/Linux

and on dom0


This is very puzzling. I did quiet some googling but I was unable to find something that resolved the issue, which seems to be unheard of (only one domU getting out of sync).

I would appreciate any ideas on how to resolve this.


Dimitris Zilaskos
GridAUTH Operations Centre @ Aristotle University of Thessaloniki , Greece
Tel: +302310998988 Fax: +302310994309

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