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



On Sat, Sep 20, 2014 at 10:09:40AM +0200, Michal Privoznik wrote:
On 20.09.2014 01:36, Jim Fehlig wrote:
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 suse com>
---
   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.

And so is mine. The only concern I have is that syncing time on cold
boot of the host may take ages. But on the other hand, it's better to
start domains later and with correct time than start asap with
inaccurate time. ACK then,


Maybe we could use After=ntpdate.service instead of ntp-wait.service;
although starting an ntpdate.service should result in the
ntp-wait.service to be automatically active, but this is just my
guess, I have no idea how /usr/sbin/ntp-wait works.

Wither way, ACK from me too,

Martin

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