Re: Time sync in KVM Guest

[Dor Laor <dlaor@redhat.com>]

On 06/23/2011 09:31 AM, Chaitra Gorantla wrote:
> Thanks for the information.
>
> Is there any way to calculate the delta between host and the guest?

Use ntddate server
The ntp server can either be on the host or anywhere else.

>
> 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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