[Pacemaker] Initial quorum

[Andrew Beekhof]

On Thu, Jul 21, 2011 at 4:13 PM, pskrap <pskrap at hotmail.com> wrote:
> Devin Reade <gdr at ...> writes:
>
>>
>> --On Wednesday, July 20, 2011 09:19:33 AM +0000 pskrap <pskrap at ...>
>> wrote:
>>
>> > I have a cluster where some of the resources cannot run on the same node.
>> > All  resources must be running to provide a functioning service. This
>> > means that a  certain amount of nodes needs to be up before it makes
>> > sense for the cluster to start any resources.
>>
>> Without knowing anything about your application, I would tend to question
>> this statement.  Is it true that you must not start *any* resources before
>> you have enough nodes, or is sufficient to say that the application
>> is not considered up until all resources are started?  It may not
>> make sense to run any, but does it do any harm?
>>
>> If you *can* start at least some resources before all nodes are available,
>> then I would expect that you could get by with defining colocation
>> constraints to ensure that some resources don't run on the same nodes,
>> perhaps augmenting things with some order constraints if necessary.
>>
>> If your applications die or do other horrible stuff when only some subset
>> are running then I'd have a talk with your application developers
>> as it sounds like a larger robustness problem.
>>
>> Devin
>>
>
> No, there are no crash issues etc when all resources are not running. The
> application is just not usable until all resources are started.
>
> As for the harm, the resources which have constraints preventing them from
> running will fail,

Are you talking about constraints in the pacemaker config or some other kind?

> but I guess they will recover as more nodes are added. The
> harm is mostly in the fact that starting nodes one by one will cause the
> resources to be unevenly distributed over the nodes since everything will start
> on the nodes in the order they are installed. I know I can give a preferred
> node to a resource and allow it to relocate when it becomes available. However,
> this application provides a real-time service so I only want resources to
> relocate when it is absolutely necessary. Therefore I have given the resources
> a preferred node, but do not allow them to relocate when it becomes available.
>
> So I guess the overall harm is limited even though it exists. I was just
> looking for a cleaner startup for the system. Since you did not mention any way
> to do what my question was about I assume it is currently not possible to do
> what I asked for. I do think such an option would be useful though. Logically
> it does not make sense for the cluster to be starting resources for an
> application before the cluster have enough nodes for the application to be able
> to run.
>
>
>
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