Dor Laor wrote: > On 03/21/2010 01:29 PM, Thomas Løcke wrote: >> Hey, >> >> What is considered "best practice" when running a KVM host with a >> mixture of Linux and Windows guests? >> >> Currently I have ntpd running on the host, and I start my guests using >> "-rtc base=localhost,clock=host", with an extra "-tdf" added for >> Windows guests, just to keep their clock from drifting madly during >> load. >> >> But with this setup, all my guests are constantly 1-2 seconds behind >> the host. I can live with that for the Windows guests, as they are not > > Is it just during boot time? If you run ntpdate after the boot inside > the guest, does the time is 100% in sync with the host from that moment on? > > Glauber once analyzed it and blames hwclock call in rc.sysinit > >> running anything that depends heavily on the time being set perfect, >> but for some of the Linux guests it's an issue. >> >> Would I be better of using ntpd and "-rtc base=localhost,clock=vm" for >> all the Linux guests, or is there some other magic way of ensuring >> that the clock is perfectly in sync with the host? Perhaps there are >> some kernel configuration I can do to optimize the host for KVM? > > Jan is the expert here, but last I checked clock=vm is not appropriate > since this is virtual time and not host time - if qemu is > stopped/migrated you won't notice it with virtual time withing the guest > but the drift will grow. Don't know what Windows does with the RTC, but the idea behind -rtc clock=host is to provide an accurate time source to guest without paravirtualized guest kernel drivers or an ntp installation in the guest. Last time I checked, hwclock run in a Linux guest was in sync with the host system time. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
