[ClusterLabs] Reliability questions on the new QDevices in uneven node count Setups

[Christine Caulfield]

On 25/07/16 14:51, Christine Caulfield wrote:
> On 25/07/16 14:29, Thomas Lamprecht wrote:
>> Hi all,
>>
>> I'm currently testing the new features of corosync 2.4, especially
>> qdevices.
>> First tests show quite nice results, like having quorum on a single node
>> left out of a three node cluster.
>>
>> But what I'm a bit worrying about is what happens if the server where
>> qnetd runs, or the qdevice daemon fails,
>> in this case the cluster cannot afford any other loss of a node in my
>> three node setup as votes expected are
>> 5 and thus 3 are needed for quorum, which I cannot fulfill if the qnetd
>> does not run run or failed.
> 
> We're looking into ways of making this more resilient. It might be
> possible to cluster a qnetd (though this is not currently supported) in
> a separate cluster from the arbitrated one, obviously.
> 
> The LMS algorithm is quite smart about how it doles out its vote and can
> handle isolation from the main qnetd provided that the main core of the
> cluster (the majority in a split) retains quorum, but any more serious
> changes to the cluster config will cause it to be withdrawn. So in this
> case you should find that your 3 node cluster will continue to work in
> the absence of the qnetd server or link, provided you don't lose any nodes.
> 

I should have also said that you'll need to enable 'wait_for_all' for
this to work.

Chrissie

> In a 3 node setup obviously LMS is more appropriate than ffsplit anyway.
> 
> Chrissie
> 
>>
>> So in this case I'm bound to the reliability of the server providing the
>> qnetd service,
>> if it fails I cannot afford to loose any other node in my three node
>> example,
>> or also in any other example with uneven node count as the qdevice vote
>> subsystems provides node count -1 votes.
>>
>> So if I see it correctly QDevices make only sense in case of even node
>> counts,
>> maybe especially 2 node setups as if qnetd works we have on more node
>> which may fail and if qnetd failed
>> we are as good as without it as qnted provides only one vote here.
>>
>> Am I missing something, or any thoughts to that?
>>
>>
>>
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> 
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