Thanks Guys,

Ideally I would like to have event driven (rather than slower polled)
inputs into pacemaker to quickly trigger the fall over. I assume adding
event driven inputs to pacemaker isn't straightforward? If it was possible
to add event inputs to pacemaker is pacemaker itself fast enough? Or is it
also going to be relatively slow to switch?

It would seem based on this discussion it may still work still work to use
pacemaker & corosync for initial setup & handle services which can handle a
slower switch over time. For our services that require a much faster switch
over time it would appear we need something propriety.

Regards
David

On 12 April 2018 at 02:56, Klaus Wenninger <kwenn...@redhat.com> wrote:

> On 04/11/2018 10:44 AM, Jan Friesse wrote:
> > David,
> >
> >> Hi,
> >>
> >> We are planning on creating a HA product in an active/standby
> >> configuration
> >> whereby the standby unit needs to take over from the active unit very
> >> fast
> >> (<50ms including all services restored).
> >>
> >> We are able to do very fast signaling (say 1000Hz) between the two
> >> units to
> >> detect failures so detecting a failure isn't really an issue.
> >>
> >> Pacemaker looks to be a very useful piece of software for managing
> >> resources so rather than roll our own it would make sense to reuse
> >> pacemaker.
> >>
> >> So my initial questions are:
> >>
> >>     1. Do people think pacemaker is the right thing to use? Everything I
> >>     read seem to be talking about multiple seconds for failure
> >> detection etc.
> >>     Feature wise it looks pretty similar to what we would want.
> >>     2. Has anyone done anything similar to this?
> >>     3. Any pointers on where/how to add additional failure detection
> >> inputs
> >>     to pacemaker?
> >>     4.
> >>     5. For a new design would you go with pacemaker+corosync,
> >>     pacemaker+corosync+knet or something different?
> >>
> >
> >
> > I will just share my point of view about Corosync side.
> >
> > Corosync is using it's own mechanism for detecting failure, based on
> > token rotation. Default timeout for detecting lost of token is 1
> > second, so detecting failure takes hugely more than 50ms. It can be
> > lowered, but that is not really tested.
> >
> > That means it's not currently possible to use different signaling
> > mechanism without significant Corosync change.
> >
> > So I don't think Corosync can be really used for described scenario.
> >
> > Honza
>
> On the other hand if a fail-over is triggered by loosing a node or anything
> that is being detected by corosync this is probably already the fast-path
> in a pacemaker-cluster.
>
> Detection of other types of failures (like a resource failing on
> an otherwise functional node) is probably even way slower.
> When a failure is detected by corosync, pacemaker has some kind of
> an event driven way to react on that.
> We even have to add some delay to the mere corosync detection time
> mentioned by Honza as pacemaker will have to run e.g. a selection
> cycle for the designated coordinator to be able to do decisions again.
>
> For other failures the base principle is rather probing a resource at a
> fixed rate (multiple seconds usually) for detection of failures instead
> of an event-driven mechanism.
> There might be trickery possible though using attributes to achieve
> event-driven-like reaction on certain failures. But I haven't done
> anything concrete to exploit these possibilities. Others might have
> more info (which I personally would be interested in as well ;-) ).
>
> Approaches to realize event-driven mechanisms for resource-failure-
> detection are under investigation/development (systemd-resources,
> IP resources sitting on interfaces, ...) but afaik there is nothing
> available out of the box by now.
>
> Having that all said I can add some personal experiences from
> having implemented an embedded product based on a
> pacemaker-cluster myself in the past:
>
> As reaction time based on pacemaker would be too slow for e.g.
> many communication-protocols (e.g. things like SIP) or realtime-
> streams it seems advisable to solve these issues on the
> application-layer inside a service (respectively distributed service
> in a cluster).
> Pacemaker and it's decision engine can then be used to bring
> up this distributed service in a cluster in some kind of an ordered
> way.
> Any additional services that might be less demanding regarding
> switch-over timeout can be made available via pacemaker
> directly.
>
> Otherwise pacemaker configuration is very flexible so that you
> can implement merely anything. It might be advisable to avoid
> certain approaches which are common in cases where a cluster
> is operated by somebody who can be informed quickly and
> has to react under certain SLAs. Thinking of e.g. fencing a node
> to be switched off instead of rebooting it might not be desirable
> with kind of an appliance that is expected to just sit there and
> work without merely any admin effort/expense at all.
> But that is of course just an example and configuration (incl.
> configuration concept) has to be tailored to your requirements.
>
> Regards,
> Klaus
>
> >
> >>
> >> Thanks
> >>
> >> David
> >>
> >>
> >>
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