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

[Andrew Beekhof]

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) :)

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 :-)