Xen users - Aug 2013 - keeping correct time in guest

Hello,

I know, often asked, often discussed .. No recommendation from Citrix.
What is the better solution?

1) use ntp in the XEN host and synchronize the guests with the clock
running on the control domain (independent_wallclock=0, what the
default is)
2) or use ntp in host and guest (and independent_wallclock=1)

I ask, because the XEN host (bare metal) has a more stable clock and
if the ntp servers are fail, the guest VMs can all rely on host clock,
all should guest VMs then are running in sync.
But I heard some bad things about the synchronization with the host
clock, users in internet recommend to use ntp in guest VM. My mind
tells me, to use ntp (2), but my brain in stomach is telling me, to
use syncing via XEN method (1).

Vmware recommend to use ntp in guest
(http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427),
but Citrix only tells, how can I do that..

One good configuration manual for using ntp in guest I''ve found here:
http://www.brookstevens.org/2010/06/xen-time-drift-and-ntp.html


What do you use, or what experiences do you have ?

Thanks in advance
Meike

On Wed, 2013-08-21 at 22:38 +0200, Meike Stone wrote:
> Hello,
> 
> I know, often asked, often discussed .. No recommendation from Citrix.
Citrix? Are you running on XenServer? If you want XenServer specific
advice then I suggest you contact your Support rep or use the XenServer
forums http://forums.citrix.com/category.jspa?categoryID=101 .

I''ve given some generic advice below.
> What is the better solution?
> 
> 1) use ntp in the XEN host and synchronize the guests with the clock
> running on the control domain (independent_wallclock=0, what the
> default is)
> 2) or use ntp in host and guest (and independent_wallclock=1)
> 
> I ask, because the XEN host (bare metal) has a more stable clock and
> if the ntp servers are fail, the guest VMs can all rely on host clock,
> all should guest VMs then are running in sync.
> But I heard some bad things about the synchronization with the host
> clock, users in internet recommend to use ntp in guest VM. My mind
> tells me, to use ntp (2), but my brain in stomach is telling me, to
> use syncing via XEN method (1).
PV ops (AKA upstream) Linux kernels only support option (2), so that is
what you need to use for them.

Only the classic Linux kernel port (not upstream) supported
independent_wallclock=0, I don''t know if other PV OSes (e.g. *BSD)
supported this option or not. I''m pretty certain no OS running in HVM
(including Linux as HVM or PVHVM) supported dependent wallclock.

Ian.