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

[Christine Caulfield]

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changes to the cluster config will cause it to be withdrawn.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.

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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