Just curious, but on Xen if I "disable/break" the hwclock, Linux guests apparently have no problem using system time set by host (so far) and may be preventing "Time goes backwards" errors too. e.g. In a CentOS pv guest as root: cp /sbin/hwclock /sbin/hwclock.original echo exit 0 > /sbin/hwclock You may have to do that again on updates. I'm also not sure of the source of this way for Xen pv guests and have not tried it , but is the same principle: Prevent "hwclock" from being used ================================= mv /sbin/hwclock /sbin/hwclock.dist touch /sbin/hwclock chmod +x /sbin/hwclock Johnny Tan wrote: > 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 > using KVM-85.) > > > 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. > > Thanks Nigel. > > johnny > _______________________________________________ > CentOS-virt mailing list > CentOS-virt at centos.org > http://lists.centos.org/mailman/listinfo/centos-virt >