Re: Synchronized time with kvm_clock

Athanasius
2010-04-26T16:43:28Z

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On Mon, Apr 26, 2010 at 10:32:51AM -0400, John Buswell wrote:
> You don't need to run ntp on each guest. You can enable rtc support in
> the guest kernel and on the hypervisor. Run ntp client on the hypervisor
> via cron, and use hwclock -w on the hypervisor after you run ntp, to
> sync the hardware clock to the system clock (which is now updated by
> ntpdate). On the guests, periodically run hwclock -s to set the system
> clock from the hw clock.

What a *horribly* hacky way to do it, meaning you'll get time warps
all over the place, admittedly of short intervals if you run those cron
jobs often enough. It seems much simpler to me to simply run ntpd in
all the guests. It's not like the extra CPU or bandwidth is going to be
a problem. At the very least you want to run ntpd, not ntpdate out of
cron, in the hypervisor, and only use cron for those hwclock -w's.

> This seems to work extremely well, the clocksource on the guests as
> kvm_clock, and as long as you have the clocksource as hpet or acpi_pm on
> the hypervisor, there doesn't seem to be any problems with keeping time.
>
> The only thing I've noticed is that when you reboot, the very first
> guest will have the wrong time on boot, so the uptime is messed up.

And I think many people would find this unacceptable.

Really, I appreciate that "keep the time sync'd via ntpd on the
hypervisor and have it passed accurately to the guests" has a certain
elegant simplicity about it. But if you achieve the latter by
periodically resyncing against what the guest sees as its hardware clock
you've lost that elegance again. It really needs to 'just work' via KVM
code in the guest kernel using the exact same time as the hypervisor
kernel is supplying.

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