Time sync in KVM Guest

Time sync in KVM Guest

[Chaitra Gorantla]

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.
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?

Thanks in advance.



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______________________________________________________________________

Re: Time sync in KVM Guest

[Dor Laor]

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.
>
>
>
> The contents of this e-mail and any attachment(s) may contain confidential or privileged information for the intended recipient(s). Unintended recipients are prohibited from taking action on the basis of information in this e-mail and  using or disseminating the information,  and must notify the sender and delete it from their system. L&T Infotech will not accept responsibility or liability for the accuracy or completeness of, or the presence of any virus or disabling code in this e-mail"
>
> ______________________________________________________________________
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

RE: Time sync in KVM Guest

[Chaitra Gorantla]

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.
>
>
>
> The contents of this e-mail and any attachment(s) may contain confidential or privileged information for the intended recipient(s). Unintended recipients are prohibited from taking action on the basis of information in this e-mail and  using or disseminating the information,  and must notify the sender and delete it from their system. L&T Infotech will not accept responsibility or liability for the accuracy or completeness of, or the presence of any virus or disabling code in this e-mail"
>
> ______________________________________________________________________
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html


______________________________________________________________________

______________________________________________________________________

Re: Time sync in KVM Guest

[Dor Laor]

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.
>>
>>
>>
>> The contents of this e-mail and any attachment(s) may contain confidential or privileged information for the intended recipient(s). Unintended recipients are prohibited from taking action on the basis of information in this e-mail and  using or disseminating the information,  and must notify the sender and delete it from their system. L&T Infotech will not accept responsibility or liability for the accuracy or completeness of, or the presence of any virus or disabling code in this e-mail"
>>
>> ______________________________________________________________________
>> --
>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>
> ______________________________________________________________________
>
> ______________________________________________________________________
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html