Athanasius wrote:
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.
Not really. You don't get time warps at all, the only place you get a
time warp is on the initial guest, and thats not a problem with the
workaround I suggested. It seems to be an issue with the clock on the
initial guest. There is no point wasting resources by running ntpd on
each guest when you don't have to.
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.
This particular problem has nothing to do with what I suggested above.
This is some kind of issue with kvm_clock on the first guest starting up.
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.
I agree. Unfortunately, kvm_clock doesn't seem to be quite there yet. So
using rtc0 as a comparison, and keeping the hypervisor clock in sync
with reality, is a good way to avoid having to run N+1 copies of ntpd on
the guests :)
--
John Buswell
CEO, Carbon Mountain LLC
http://www.carbonmountain.com
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