[CentOS] How to configure time on virtual clients using KVM?

Theo Band

theo.band at greenpeak.com
Thu Jun 28 16:49:49 UTC 2012

I use a CentOS5.8 server with KVM. I have several virtual machines
running on it. When I reboot the server (takes 10 minutes) all VMs are
saved and correctly restored. The time on the clients is however of by
10 minutes.
nptd is running on the clients and that is able to correct this big
mismatch. But what I don't understand is that the host does not seem to
help the client with it's timekeeping.

On a fedora16 machine I can tell that kvm-clock is being used:
cat /sys/devices/system/clocksource/clocksource0/available_clocksource
kvm-clock tsc acpi_pm
cat /sys/devices/system/clocksource/clocksource0/current_clocksource

One the centos5 client I can think I see the same:
dmesg|grep time.c
time.c: Using 1.193182 MHz WALL KVM GTOD KVM timer.

If I suspend a client, I see the time is correct just after resume. If I
save and restore the time is left at the saved time:

ssh valk5 date;virsh save valk5 /var/lib/libvirt/images/save ;sleep
60;virsh restore /var/lib/libvirt/images/save;ssh valk5 date;date
Thu Jun 28 18:37:59 CEST 2012
Domain valk5 saved to /var/lib/libvirt/images/save

Domain restored from /var/lib/libvirt/images/save

Thu Jun 28 18:40:00 CEST 2012
Thu Jun 28 18:41:07 CEST 2012

So my question is, is this intended behaviour? Is there something I can
change to kick the time during a restore?

A second related question is what happens after a live migrate. I can
migrate the VMs to a different machine, but that machine has slightly
different specs. I notice that the clock speed is really off (minutes
per hour need to be corrected after a live migrate). I guess the client
gets a differt CPU clock on the other host. Is there a way to update the
client without a reboot? A reboot helps, so I guess some timer
calibration takes place then. But that defeats the purpose of a live


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