libvirt users - Sep 2015 - Re: Time syncing after VM suspend/resume

Hi all.

When resuming guest after suspend, the time is wrong (because the guest
doesn't know it has been suspended).

Apparently, ntp running on the guest doesn't solve this. At least on
Linux system. I read it works on Windows guests. I don't understand why,
so I'd appreciate a link to an explanation, but I understand it is out
of scope for this list.

The HW clock is correct and the guest agent is setup to use kvm-clock.

Using the guest-set-time command does set the clock on the guest
according to the host. But this is not triggered automatically on
resume.

I found discussions about this, even a patch proposal to call
guest-set-time on resume, but all of this is one year old and I can't
find any newer information.

http://serverfault.com/questions/334698/how-to-keep-time-on-resumed-kvm-guest-with-libvirt
https://www.redhat.com/archives/libvirt-users/2014-October/msg00009.html
https://www.redhat.com/archives/libvir-list/2014-February/msg00520.html

Is there any fresher information I have missed ?

Thanks.

-- 
Jérôme

> -----Oorspronkelijk bericht-----
> Van: libvirt-users-bounces@redhat.com [mailto:libvirt-users-
> bounces@redhat.com] Namens Jérôme
> Verzonden: woensdag 23 september 2015 15:03
> Aan: libvirt-users@redhat.com
> Onderwerp: [libvirt-users] Time syncing after VM suspend/resume
> 
> Hi all.
> 
> When resuming guest after suspend, the time is wrong (because the guest
> doesn't know it has been suspended).
> 
> Apparently, ntp running on the guest doesn't solve this. At least on
Linux
> system. I read it works on Windows guests. I don't understand why, so
I'd
> appreciate a link to an explanation, but I understand it is out of scope
for this
> list.
Linux has two methods to use ntp:

ntpdate:
It will run once at boot time to sync time. (This is probably installed on your
system)
It will not run after suspend and resume... => no correction

ntpd:
Continuously adjusts time. The deamon also calculates the drift to anticipate
differences.
I use this one and works perfectly.
> 
> The HW clock is correct and the guest agent is setup to use kvm-clock.
> 
> Using the guest-set-time command does set the clock on the guest according
to
> the host. But this is not triggered automatically on resume.
> 
> I found discussions about this, even a patch proposal to call
guest-set-time on
> resume, but all of this is one year old and I can't find any newer
information.
> 
> http://serverfault.com/questions/334698/how-to-keep-time-on-resumed-kvm-
> guest-with-libvirt
> https://www.redhat.com/archives/libvirt-users/2014-October/msg00009.html
> https://www.redhat.com/archives/libvir-list/2014-February/msg00520.html
> 
> Is there any fresher information I have missed ?
> 
> Thanks.
> 
> --
> Jérôme
> 
> _______________________________________________
> libvirt-users mailing list
> libvirt-users@redhat.com
> https://www.redhat.com/mailman/listinfo/libvirt-users

Hi.

Thanks for answering.

Le 2015-09-23 17:34, Dominique Ramaekers a écrit :
> Linux has two methods to use ntp:
> 
> ntpdate:
> It will run once at boot time to sync time. (This is probably
> installed on your system)
> It will not run after suspend and resume... => no correction
Nope. This is not installed on my system.
 
> ntpd:
> Continuously adjusts time. The deamon also calculates the drift to
> anticipate differences.
> I use this one and works perfectly.
This is installed (package ntp, Debian Jessie) and runs (I see
/usr/sbin/ntpd in `ps aux`). But after more than 24h, the 2'17 gap
between hwclock and date has not reduced a bit.

I guess this is not the place for me to debug my ntp issues, apart maybe
from what could be related to the virtualization itself.

I understood from my readings (can't remember where precisely) that the
fact that ntp wouldn't work was "normal", but if it is not, maybe
trying
to have it working is the way to go, rather than searching for a way to
automatize guest-set-time.

-- 
Jérôme

On 23.09.2015 15:03, Jérôme wrote:
> Hi all.
> 
> When resuming guest after suspend, the time is wrong (because the guest
> doesn't know it has been suspended).
> 
> Apparently, ntp running on the guest doesn't solve this. At least on
> Linux system. I read it works on Windows guests. I don't understand
why,
> so I'd appreciate a link to an explanation, but I understand it is out
> of scope for this list.
> 
> The HW clock is correct and the guest agent is setup to use kvm-clock.
> 
> Using the guest-set-time command does set the clock on the guest
> according to the host. But this is not triggered automatically on
> resume.
> 
> I found discussions about this, even a patch proposal to call
> guest-set-time on resume, but all of this is one year old and I can't
> find any newer information.
> 
>
http://serverfault.com/questions/334698/how-to-keep-time-on-resumed-kvm-guest-with-libvirt
> https://www.redhat.com/archives/libvirt-users/2014-October/msg00009.html
> https://www.redhat.com/archives/libvir-list/2014-February/msg00520.html
> 
> Is there any fresher information I have missed ?
You can use "virsh resume $dom && virsh domtime --sync $dom"
which will
resume and resync the domain's clock from domain RTC. There were some
attempts to wrap these into a single API, however, I was persuaded that
it's a wrong idea. Wrapping two distinct operations into a single API is
always wrong.

Michal