[ovirt-devel] [vdsm] vdsm hosts clock sync

[Alon Bar-Lev]

----- Original Message -----
> From: "Shahar Havivi" <shaharh at redhat.com>
> To: "Alon Bar-Lev" <alonbl at redhat.com>
> Cc: vdsm-devel at lists.fedorahosted.org
> Sent: Monday, April 6, 2015 3:05:20 PM
> Subject: Re: [vdsm] vdsm hosts clock sync
> 
> On 06.04.15 08:00, Alon Bar-Lev wrote:
> > 
> > 
> > ----- Original Message -----
> > > From: "Shahar Havivi" <shaharh at redhat.com>
> > > To: "Alon Bar-Lev" <alonbl at redhat.com>
> > > Cc: vdsm-devel at lists.fedorahosted.org
> > > Sent: Monday, April 6, 2015 2:54:07 PM
> > > Subject: Re: [vdsm] vdsm hosts clock sync
> > > 
> > > On 06.04.15 07:50, Alon Bar-Lev wrote:
> > > > 
> > > > 
> > > > ----- Original Message -----
> > > > > From: "Shahar Havivi" <shaharh at redhat.com>
> > > > > To: vdsm-devel at lists.fedorahosted.org
> > > > > Sent: Monday, April 6, 2015 2:44:06 PM
> > > > > Subject: [vdsm] vdsm hosts clock sync
> > > > > 
> > > > > Hi,
> > > > > 
> > > > > I want to add a new feature that reports migration actual downtime
> > > > > (the
> > > > > time
> > > > > that the VM was inaccessible to the user).
> > > > > 
> > > > > Libvirt reports that information but the vdsm hosts need to be in
> > > > > sync by
> > > > > clock
> > > > > time.
> > > > > I can measure the ping for NTP server an report back to the user if
> > > > > the
> > > > > ping
> > > > > is too long (more then ~100ms or so) - a way to do that is via
> > > > > ntpstat
> > > > > shell
> > > > > command.
> > > > > The NTP delay can be report back via vdsStats and can be performed
> > > > > every
> > > > > few hours or so.
> > > > > 
> > > > > Anyone knows of a better way that we can sync between hosts?
> > > > 
> > > > I am unsure how a ping to clock source is helping, can you please
> > > > explain
> > > > more?
> > > In this case I can only report back to the user that its hosts clock is
> > > delayed and need to be set...
> > > > 
> > > > If you assume clocks are synced why anything more is needed?
> > > Why do I assume that?
> > > > 
> > > > Or would you like to have a solution in which you do not require clock
> > > > sync?
> > >
> > > I do need the clock to be in sync - if not libvirts "actual downtime
> > > migration" will be not accurate.
> > 
> > you do not need clock to sync, you need to know the delta between hosts.
> > 
> > but if you assume clock are in sync so what is the actual question?
>
> As I understand from your answer is by having configured ntp the hosts clock
> are in sync.

I do  not follow.

1. if you are using ntpd on hosts and hosts are in sync you can use distributed timestamps.

detecting if host is synced can be done using either (based on what actually installed):

ntpq -c rv | grep -q clock_sync && echo ok
chronyc waitsync 1 && echo ok

2. you can also measure the time diff between hosts by clockdiff or similar process and avoid the need of ntpd deployed.

3. you may be able to calculate the time within in the handover is given to remote and eliminate the network time.

Not sure what your implementation is.

Alon