[Pacemaker] [RFC] [Patch] DC node preferences (dc-priority)

[Lars Ellenberg]

On Fri, May 25, 2012 at 10:50:25AM +1000, Andrew Beekhof wrote:
> On Fri, May 25, 2012 at 10:04 AM, Lars Ellenberg
> <lars.ellenberg at linbit.com> wrote:
> > On Sun, May 06, 2012 at 09:45:09PM +1000, Andrew Beekhof wrote:
> >> On Thu, May 3, 2012 at 5:38 PM, Lars Ellenberg
> >> <lars.ellenberg at linbit.com> wrote:
> >> >
> >> > People sometimes think they have a use case
> >> > for influencing which node will be the DC.
> >>
> >> Agreed :-)
> >>
> >> >
> >> > Sometimes it is latency (certain cli commands work faster
> >> > when done on the DC),
> >>
> >> Config changes can be run against any node, there is no reason to go
> >> to the one on the DC.
> >>
> >> > sometimes they add a "mostly quorum"
> >> > node which may be not quite up to the task of being DC.
> >>
> >> I'm not sure I buy that.  Most of the load would comes from the
> >> resources themselves.
> >>
> >> > Prohibiting a node from becoming DC completely would
> >> > mean it can not even be cleanly shutdown (with 1.0.x, no MCP),
> >> > or act on its own resources for certain no-quorum policies.
> >> >
> >> > So here is a patch I have been asked to present for discussion,
> >>
> >> May one ask where it originated?
> >>
> >> > against Pacemaker 1.0, that introduces a "dc-prio" configuration
> >> > parameter, which will add some skew to the election algorithm.
> >> >
> >> >
> >> > Open questions:
> >> >  * does it make sense at all?
> >>
> >> Doubtful :-)
> >>
> >> >
> >> >  * election algorithm compatibility, stability:
> >> >   will the election be correct if some nodes have this patch,
> >> >   and some don't ?
> >>
> >> Unlikely, but you could easily make it so by placing it after the
> >> version check (and bumping said version in the patch)
> >>
> >> >  * How can it be improved so that a node with dc-prio=0 will
> >> >   "give up" its DC-role as soon as there is at least one other node
> >> >   with dc-prio > 0?
> >>
> >> Short of causing an election every time a node joins... I doubt it.
> >
> > Where would be a suitable place in the code/fsa to do so?
> 
> Just after the call to exit(0) :)

Just what I thought ;-)

> I'd do it at the end of do_started() but only if dc-priority* > 0.
> That way you only cause an election if someone who is likely to win it starts.
> And people that don't enable this feature are unaffected.
>
> * Not dc-prio, its 2012, there's no need to save the extra 4 chars :-)

Thanks,

-- 
: Lars Ellenberg
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