RE: Time sync in KVM Guest

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Thanks for the information.

Is there any way to calculate the delta between host and the guest?

On 06/22/2011 06:17 AM, Chaitra Gorantla wrote:
> Hi all,
>
> We are working on Fedora 15 Host. And the KVM is used to create Fedora 14 guest.
> The clock-source details are as below.
>
> Our Host supports constant_tsc.
>
> on HOST OS
> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> tsc
>
> on GUEST OS
> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> kvm-clock
>
> We are not having Wall-clock time Sync in Both Host and guest.
> The Time in Host OS lags behind guest by atleast 2 seconds.
> However, this time difference remains constant.

That's good

> Does this mean that the Host and Guest clocks are synchronized ?
> By using kvm-clock as clock source on the guest, the time sync should happen itself ?
> Please correct me if I am wrong...
> How exactly we can find the time difference between Host and guest?

Usually Linux OS reads the rtc cmos to get the time when it boots.
Both the guest and the host do that using hwclock.

 From there own the time is tracked by the clock source as delta.

It is recommended that both the guest and the host will run ntpd to keep 
their time updated. That's the only thing that you're recommended to do.

Thanks,
dor

>
> Thanks in advance.
>
>
>
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