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[Xen-users] Re: Time synchronization issue

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Re: Time synchronization issue
From: Diego Molina <diegoaugustomolina@xxxxxxxxx>
Date: Tue, 19 Jul 2011 11:46:43 -0300
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2011/7/7, Diego Molina <diegoaugustomolina@xxxxxxxxx>:
> Hi, I have Gentoo Linux dom0 and paravirtual domUs with Xen 3.4.2 all
> of them with `/proc/sys/xen/independent_wallclock' set to 0 (dom0 and
> domUs) but domUs aren't in sync with dom0.
> I've noticed this after a while, because I saw a recent line in a log
> file with the time in the future. Then I went to the dom0, set time
> quickly with ntpdate and started ntpd. After this I noticed that all
> domUs still had the same time so I waited... and waited to see the
> changes take effect. Nothing happened. It's been a day since then, and
> I can see dom0 and domUs are all acting as if they were configured to
> use the independent wall clock. I was able to change the time of one
> of the domUs and still see it differs both from dom0 and the other
> domUs. I googled and searched the archives but couldn't find a way
> out. It would really mean a lot to me not to have to maintain ntp in
> all domUs...

Hi again, it's been about six days of uptime and I started a test from
the early bootup of all the machines. The test is very simple and
consists of:

  - Provision: all gentoo systems, 4 domUs, pv. At bootup, I run
ntpdate in all machines and start ntpd in dom0 and in three domUs. The
forth one is the control. Dom0 syncs against a trusty server and domUs
against dom0. Periodically (each 7 days) the date in the 5 machines
are checked simultaneously with `date +%Y%m%d-%H%M%S-%N'. After this I
stop ntpd in one domU, wait another 7 days and process starts over. I
hope this ends up showing the degradation of time in each machine.
Sincerely, I hope there's no degradation at all :)
  When I say I run something in more than one server (somthing like
ntpdate or date) I do it "at the same time" thanks to my terminal
manager, which allows me to run commands on multiple terms at the same

  - First result: we have passed the first 7 days and this is the
result of the "synchronous" date command:

dom0: 20110719-114359-087087066
1st domU/ntp: 20110719-114352-634035597
2nd domU/ntp: 20110719-114352-591847797
3rd domU/ntp: 20110719-114352-551410325
domU: 20110719-114353-232691277

This test may be dummy and may not pass most statistics test proofs.
If you can improve it, if you think it's loss of time, pointless,
please help me build a worthwhile test. Do you think domUs should sync
against the foreign server? I didn't do this because I wanted domUs to
depend on dom0. If they would be independant, then it *should* be like
any other machine syncing against a time server.

Another notice: time doesn't jump anymore, I've been seeing the logs
and time keeps advancing in a clear way though not consistent (else
there would be no drifts).

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