Clock skew is a known problem in virtualized systems.
VMWare wrote a whitepaper a while back which goes into some of the
problems that they faced while dealing with this same issue...
http://www.vmware.com/pdf/vmware_timekeeping.pdf
The Xen project is not immune to these problems - however, some efforts
have been made recently on the list to address some of these issues
See these threads:
http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01010.html
http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00554.html
What it basically comes down to is that it is necessary to keep the
clock skew under 0.05% for NTP to work - so that was the design goal.
The code introduced by some recent patches claims to be accurate to
0.02% (for at least HVM guests)
Hope this helps.
Ben
Tõnu Raitviir wrote:
Hello!
I have understood that when /proc/sys/xen/independent_wallclock is 0,
then domU clock stays in sync with dom0 clock. Now I have a case with
Xen 3.1.2 (also occured with 3.1.1, but I haven't documented it) where
domU clock runs significantly faster - after 65 hours domU clock is 20
seconds ahead of dom0 clock.
In this case dom0 is not running ntpd. When ntpd is running on dom0,
domU clock also stays correct. Is this normal behaviour?
Some examples to show clock differences:
domU# uptime
14:54:09 up 2 days, 17:05, 1 user, load average: 0.75, 0.49, 0.43
domU# ntpdate -q ntp.eenet.ee
server 193.40.133.142, stratum 1, offset -20.856732, delay 0.02579
19 Nov 14:54:13 ntpdate[8467]: step time server 193.40.133.142 offset
-20.856732 sec
dom0# uptime
14:55:02 up 2 days, 17:06, 1 user, load average: 0.07, 0.02, 0.00
dom0# ntpdate -q ntp.eenet.ee
server 193.40.133.142, stratum 1, offset -0.438055, delay 0.02589
19 Nov 14:55:17 ntpdate[32284]: adjust time server 193.40.133.142
offset -0.438055 sec
Xen version used:
Xen version 3.1.2 (jussuf@localdomain) (gcc version 4.1.2 20061115
(prerelease) (Debian 4.1.1-21)) Fri Nov 16 17:52:59 EET 2007
Latest ChangeSet: Wed Nov 14 23:35:43 2007 +0000 15502:c6776b6da8ee
Linux xen-2 2.6.18.8-xen0 #1 SMP Fri Nov 16 17:39:31 EET 2007 x86_64
GNU/Linux
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