[ClusterLabs] multi-state constraints

[Alastair Basden]

>> Single location constraint may have multiple rules, I would assume pcs
>> supports it. It is certainly supported by crmsh.
>
> Yes, it is supported by pcs. First, create a location rule constraint
> with 'pcs constraint location ... rule'. Then you can add more rules to
> it with 'pcs constraint rule add' command.

So:
pcs constraint location resourceClone rule role=master score=100 \#uname eq node1
pcs constraint location resourceClone rule add role=master score=50 \#uname eq node2

Is that the same as:
pcs constraint location resourceClone rule role=master score=100 \#uname eq node1
pcs constraint location resourceClone rule role=master score=50 \#uname eq node2
?


>>> 
>>> On Tue, 11 May 2021, Andrei Borzenkov wrote:
>>> 
>>>> [EXTERNAL EMAIL]
>>>> 
>>>> On Tue, May 11, 2021 at 10:50 AM Alastair Basden
>>>> <a.g.basden at durham.ac.uk> wrote:
>>>>> 
>>>>> Hi Andrei, all,
>>>>> 
>>>>> So, what I want to achieve is that if both nodes are up, node1
>>>>> preferentially has drbd as master.  If that node fails, then node2
>>>>> should
>>>>> become master.  If node1 then comes back online, it should become master
>>>>> again.
>>>>> 
>>>>> I also want to avoid node3 and node4 ever running drbd, since they don't
>>>>> have the disks.
>>>>> 
>>>>> For the link below about promotion scores, what is the pcs command to
>>>>> achieve this?  I'm unfamiliar with where the xml goes...
>>>>> 
>>>> 
>>>> I do not normally use PCS so am not familiar with its syntax. I assume
>>>> there should be documentation that describes how to define location
>>>> constraints with rules. Maybe someone who is familiar with it can
>>>> provide an example.
>>>> 
>>>>> 
>>>>> 
>>>>> I notice that drbd9 has an auto promotion feature, perhaps that would
>>>>> help
>>>>> here, and so I can forget about configuring drbd in pacemaker?  Is that
>>>>> how it is supposed to work?  i.e. I can just concentrate on the
>>>>> overlying
>>>>> file system.
>>>>> 
>>>>> Sorry that I'm being a bit slow about all this.
>>>>> 
>>>>> Thanks,
>>>>> Alastair.
>>>>> 
>>>>> On Tue, 11 May 2021, Andrei Borzenkov wrote:
>>>>> 
>>>>>> [EXTERNAL EMAIL]
>>>>>> 
>>>>>> On 10.05.2021 20:36, Alastair Basden wrote:
>>>>>>> Hi Andrei,
>>>>>>> 
>>>>>>> Thanks.  So, in summary, I need to:
>>>>>>> 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
>>>>>>> 
>>>>>>> pcs constraint location resourcedrb0Clone prefers node1=100
>>>>>>> pcs constraint location resourcedrb0Clone prefers node2=50
>>>>>>> pcs constraint location resourcedrb0Clone avoids node3
>>>>>>> pcs constraint location resourcedrb0Clone avoids node4
>>>>>>> 
>>>>>>> Does this mean that it will prefer to run as master on node1, and
>>>>>>> slave
>>>>>>> on node2?
>>>>>> 
>>>>>> No. I already told you so.
>>>>>>
>>>>>>>    If not, how can I achieve that?
>>>>>>> 
>>>>>> 
>>>>>> DRBD resource agents sets master scores based on disk state. If you
>>>>>> statically override this decision you are risking promoting stale copy
>>>>>> which means data loss (I do not know if agent allows it, hopefully not;
>>>>>> but then it will continue to attempt to promote wrong copy and
>>>>>> eventually fail). But if you insist, it is documented:
>>>>>> 
>>>>>> https://clusterlabs.org/pacemaker/doc/en-US/Pacemaker/2.0/html/Pacemaker_Explained/s-promotion-scores.html
>>>>>> 
>>>>>> 
>>>>>> Also statically biasing one single node means workload will be
>>>>>> relocated
>>>>>> every time node becomes available, which usually implies additional
>>>>>> downtime. That is something normally avoided (which is why resource
>>>>>> stickiness exists).
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