[ClusterLabs] Antw: [EXT] Re: Sub‑clusters / super‑clusters?

[Antony Stone]

On Wednesday 04 August 2021 at 22:06:39, Frank D. Engel, Jr. wrote:

> There is no safe way to do what you are trying to do.
> 
> If the resource is on cluster A and contact is lost between clusters A
> and B due to a network failure, how does cluster B know if the resource
> is still running on cluster A or not?
>
> It has no way of knowing if cluster A is even up and running.
> 
> In that situation it cannot safely start the resource.

I am perfectly happy to have an additional machine at a third location in 
order to avoid this split-brain between two clusters.

However, what I cannot have is for the resources which should be running on 
cluster A to get started on cluster B.

If cluster A is down, then its resources should simply not run - as happens 
right now with two independent clusters.

Suppose for a moment I had three clusters at three locations: A, B and C.

Is there a method by which I can have:

1. Cluster A resources running on cluster A if cluster A is functional and not 
running anywhere if cluster A is non-functional.

2. Cluster B resources running on cluster B if cluster B is functional and not 
running anywhere if cluster B is non-functional.

3. Cluster C resources running on cluster C if cluster C is functional and not 
running anywhere if cluster C is non-functional.

4. Resource D running _somewhere_ on clusters A, B or C, but only a single 
instance of D at a single location at any time.

Requirements 1, 2 and 3 are easy to achieve - don't connect the clusters.

Requirement 4 is the one I'm stuck with how to implement.

If the three nodes comprising cluster A can manage resources such that they 
run on only one of the three nodes at any time, surely there must be a way of 
doing the same thing with a resource running on one of three clusters?


Antony.

-- 
I don't know, maybe if we all waited then cosmic rays would write all our 
software for us. Of course it might take a while.

 - Ron Minnich, Los Alamos National Laboratory

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