[libvirt] latency between LIFECYCLE event and notification generation

[Daniel P. Berrange]

On Mon, May 13, 2013 at 09:44:35AM -0600, Eric Blake wrote:
> On 05/11/2013 07:41 AM, nishant burte wrote:
> > Hi,
> > 
> > 
> > I want to know following about LIFECYCLE events of libvirt.
> > 
> > 1. about the the latency of these events happening and notification
> > generation.
> > e.g. suppose a VM goes down. How much time it takes to realize that the
> > particular VM has gone down(going to say, DEFINED state) and then
> > notification is generated?
> 
> Libvirt is not a real-time scheduler.  We make no guarantees about when
> events will be delivered, and while it is likely that events are
> delivered in order, I'm not even brave enough to state that libvirt even
> guarantees in-order delivery to remote hosts.  All I know is that
> libvirt tries to deliver events as soon as it knows about them, but that
> events are always best-effort, and you have to be prepared for guest
> state to have changed yet again in between when libvirt detected that an
> event should be delivered and when your code receives the event.

FYI, we *do* guarantee to deliver events to clients in exactly the
same order that libvirtd detects the events, even to remote hosts,
and we do not drop events. The RPC protocol is strictly serialized
in its dispatch of events.

We make no guarantees about latency though. There can be an arbitrary
delay between libvirtd detecting the event & the client receiving it,
though of course we aim to keep this latency as small as we can.

Daniel
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