[Linux-cluster] CS4 Update 2 / question about quorum

[Patrick Caulfield]

Alain Moulle wrote:
> Hi
> 
> A question a little bit theoretical for my understanding :
> for a cluster with 8 nodes, I understand that each node
> has by default a Quorum Votes value = 1 , so does that
> mean that until 3 nodes are failed , services are failovered
> by others, and that at the 4th failed one, the cluster
> is stalled in the current state ?
> And in which cases would it be judicious to set the
> Quorum Vote for some nodes at 2 or more ?
> Or is there a way to modify the % to define if
> the cluster is quorate or note ?
> For example, let's suppose that on the 8 nodes cluster,
> whatever 2 nodes are able (in term of capacity/perf ...)
> to run all HA services of the 8 nodes, is-it possible
> to configure the cs4 such as the failover will be possible
> even if 6 nodes are failed ?

Quorum is not really related to failover, it's to prevent "split-brain" so
that (eg) a service doesn't end up running on two nodes that can't talk to
each other or (more importantly) that a GFS filesystem doesn't get corrupted
by two non-cooperating systems.

Yes, it's possible to set votes on some machines higher than others but you
need to be very careful that you do your calculations correctly such that you
can't get into a split brain situation if two higher-rated nodes split off
into two separate clusters.

Fiddling with the node votes is most useful where you have server (perhaps
gnbd) nodes in the cluster without which the satellites can't work.

patrick