[ClusterLabs] multi-state constraints

[Andrei Borzenkov]

On 11.05.2021 18:20, Alastair Basden wrote:
> Hi,
> 
> So, I think the following would do it:
> pcs constraint location resourceClone rule role=master score=100 uname
> eq node1
> pcs constraint location resourceClone rule role=master score=50 uname eq
> node2
> 

Single location constraint may have multiple rules, I would assume pcs
supports it. It is certainly supported by crmsh.

> But, I'm unsure about uname.  In the xml example you showed, it had
> #uname.  Is that correct, or do I use uname without the hash?
> 

This should be #uname - it is special attribute.

> So perhaps:
> pcs constraint location resourceClone rule role=master score=50 \#uname
> eq node2
> 
> Cheers,
> Alastair.
> 
> 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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