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

[Jan Friesse]

On 05/08/2021 00:11, Frank D. Engel, Jr. wrote:
> In theory if you could have an independent voting infrastructure among 
> the three clusters which serves to effectively create a second cluster 
> infrastructure interconnecting them to support resource D, you could 

Yes. It's called booth.

> have D running on one of the clusters so long as at least two of them 
> can communicate with each other.
> 
> 
> In other words, give each cluster one vote, then as long as two of them 
> can communicate there are two votes which makes quorum, thus resource D 
> can run on one of those two clusters.
> 
> If all three clusters lose contact with each other, then D still cannot 
> safely run.
> 
> 
> To keep the remaining resources working when contact is lost between the 
> clusters, the vote for this would need to be independent of the vote 
> within each individual cluster, effectively meaning that each node would 
> belong to two clusters at once: its own local cluster (A/B/C) plus a 
> "global" cluster spread across the three locations.  I don't know 
> offhand if that is readily possible to support with the current software.
> 
> 
> On 8/4/21 5:01 PM, Antony Stone wrote:
>> 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.
>>
> 
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