[ClusterLabs] multi-state constraints

[Andrei Borzenkov]

On 10.05.2021 19:17, Andrei Borzenkov wrote:
> On 10.05.2021 16:50, Alastair Basden wrote:
>> Hi,
>>
>> We have a 4 node cluster, on which we want a drbd service to run
>> master/slave on 2 nodes (with 1 of those preferred as the master).  It
>> should not be started at all on the other 2 nodes.
>>
>> To set this up, we've done:
>> pcs resource create resourcedrbd0 ocf:linbit:drbd drbd_resource=disk0 op
>> monitor interval=60s
>> pcs resource master resourcedrbd0Clone resourcedrbd0 master-max=1
>> master-node-max=1 clone-max=2 clone-node-max=1 notify=true
>>
>> I think we need to constrain the resource:
>> pcs constraint location resourcedrbd0 prefers node1=100
>> pcs constraint location resourcedrbd0 prefers node2=50
>> pcs constraint location resourcedrbd0 avoids node3
>> pcs constraint location resourcedrbd0 avoids node4
>>
>> Do we also need to constrain the clone?:
> 
> No, only one set is needed. I am not sure what is preferred, but you can
> use either primitive or clone in resource constraints.
> 

Probably I was wrong and you have to use clone constraints. According to
documentation, "You should never reference the name of a clone’s child
(the primitive or group resource being cloned)".

>> pcs constraint location resourcedrb0Clone prefers node1=100
>> pcs constraint location resourcedrb0Clone prefers node2=50
> 
> Those two constraints are redundant and they do not do what you expect.
> Location constraints are not related to master selection. They just
> define which nodes are eligible to run clone instance; which instance
> will be promoted is defined by master score which is usually set by
> resource agent.
> 
> You can define rules to set master score, but I do not know how it will
> interact with explicit scores set by other means.
> 
>> pcs constraint location resourcedrb0Clone avoids node3
>> pcs constraint location resourcedrb0Clone avoids node4
>>
>> Is this correct, or should I be using a colocation somehow?
>>
>>
>> Do I also need:
>> pcs constraint order demote resourcedrbd0 then promote resourcedrbd0    ?
>> I think probably not.
>>
>> For the file system resource that will then I think we should use this:
>> pcs constraint colocation add resourceFilesystem with resourcedrbd0Clone
>> INFINITY with-rsc-role=Master
>> pcs constraint order promote resourcedrbd0Clone then start
>> resourceFilesystem
>>
>> Is that correct?  I'm not sure about the with-rsc-role=Master part.
>>
>> Thanks,
>> Alastair.
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>