Re: [libvirt] [PATCH] libvirt-guests: wait for ntp service

Martin Kletzander wrote: > On Fri, Sep 19, 2014 at 02:37:12PM -0600, Jim Fehlig wrote: >> Michal Privoznik wrote: >>> On 08.09.2014 18:30, Jim Fehlig wrote: >>>> If an NTP server is configured on the host, it is possible for >>>> libvirt-guests to start before the NTP service, in which case >>>> guest clocks won't be synchronized to the host clock. >>>> >>>> Add ntp-wait.service to "After" in libvirt-guests systemd service >>>> file, ensuring NTP has synchronized the host clock before starting >>>> any guests. >>>> >>>> Signed-off-by: Jim Fehlig <jfehlig(a)suse.com&gt; >>>> --- >>>> tools/libvirt-guests.service.in | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/tools/libvirt-guests.service.in >>>> b/tools/libvirt-guests.service.in >>>> index d8d7adf..226b3bd 100644 >>>> --- a/tools/libvirt-guests.service.in >>>> +++ b/tools/libvirt-guests.service.in >>>> @@ -1,6 +1,6 @@ >>>> [Unit] >>>> Description=Suspend Active Libvirt Guests >>>> -After=network.target libvirtd.service >>>> +After=network.target libvirtd.service ntp-wait.service >>>> Documentation=man:libvirtd(8) >>>> Documentation=http://libvirt.org >>>> >>>> >>> >>> Well, guest can have their own ntp-client (and in most cases they do, >>> right?). >> >> I think most do, but know of at least two users who want to use kvmclock >> with no ntp in the guests :). >> > > I'm sure there are way more users without ntp clients inside their > guests. I'm just wondering what's the difference when libvirt-guests > starts before or after ntp has synchronized their clocks. Is it that > they have the time reset to a little bit inaccurate time? Or are they > off way too much? They are off by the ntp adjustment. As I understand it, the guests start and read the host clock, which is later adjusted by ntp. > >>> I mean, since guests can be paused, saved & restored back, their time >>> is often off. So the best is to have an ntp-client running inside the >>> guest. >> > > Yes, but if it's way off, ntp will refuse to update the time; that's > why we are resetting the time, isn't it? > >> Yep. I mentioned this, but seems they don't use save, restore, migrate, >> et. al., since it wasn't a concern. But I'm fine handling this >> downstream. Thanks! >> > > Well, if they use libvirt-guests, they use at least save/restore :) They have ON_SHUTDOWN=shutdown. > > Unfortunately I'm not very familiar with systemd files, but my guess > is that After=ntp-wait.service means it should be started after the > time is synchronized if and only if the ntp-wait.service unit is > enabled, otherwise it doesn't require it. Yes, this is my understanding too.