[CentOS-virt] ntp on kvm
linuxweb at gmail.com
Tue Jul 28 16:46:30 UTC 2009
Nigel Metheringham wrote:
> On 21 Jul 2009, at 00:41, Johnny Tan wrote:
>> * Setting independent_wallclock=1 (or is that a xen-only thing)?
> [Can't comment - I'm not a Xen user]
Ok, I've done a bit more research, it seems the above is a
xen-only thing, so it's a no-go for KVM.
>> * Passing kernel parameters? If so, which? The vmware KB
>> mentions "notsc divider=10" -- is that vmware-specific?
This is NOT vmware-specific. In fact, it seems like
divider=10 is the only thing that works. "notsc" (at least
by itself) does not help.
>> * Just don't run ntpd on virtual guests? And let it get its
>> time from the host.
This does not work. On Xen, domU's apparently get their time
from the physical host, so it would work for Xen. But for
KVM, this does not seem to be the case.
> My suggestions, from my position of complete ignorance on Xen (but I
> do quite a lot under VMware) is to tweak your kernel params
> (definitely divider, maybe some other tweaks depending on how the
> virtual hardware appears, but on our systems we use clocksource=acpi_pm)
Yes, divider works. I tried "clocksource=acpi_pm" -- it does
not work for our KVM guests, at least not by itself. (We're
In summary: For KVM guests, pass "divider=10" as kernel
option in grub.conf for your KVM guest, then use ntp to keep
time (also on your KVM guest).
(p.s. However, I believe these issues are fixed with RHEL 5.4.)
> Hope that this helps but you have opened a very messy can of worms here!
I figured it'd be a can of worms, but a useful one. I'm
surprised it's not asked more often. I'm guessing most
people are using Xen still, where this is less of an issue.
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