What's the advantage of having a quorum disk in a two-node cluster?

[pietrek]

Hi,

The title says it all. I'd like to know why is it recommended to have a quorum in this guide: https://pve.proxmox.com/wiki/Two-Node_High_Availability_Cluster

What benefits does it provide in such setup? I mean, AFAIK quorum, is used to share cluster's current state and configuration between nodes, but then every cluster can have it's own local quorum synchronized between cluster members, right?

Could abybody please enlighten me a bit?

Also, as a small side-question. Is it posssible to a two nodes + shared NFS storage based cluster with automatic failover functionality in case one of them suddenly burns to the ground without any warning, or is that impossible and requires third cluster member?

 

[WaltervdSchee]

Quorum is not a shared state on the cluster configuration, it is a 'shared' disk that allows a cluster to have more 'votes'.
Ever single server within a cluster is seen as having a single vote to make sure a cluster is 'quorate' enough to be considered 'alive' to start services (read: VMs)
In a two host cluster, this means the cluster must be set to have a quorate of 1 vote to be considered alive.
However, since there might be some switch port problem and/or network switch problem, this would also mean, since the quorate number is set to a single vote, even a single server, without network connectivity would consider the cluster to be 'alive'.
This would cause all sorts of problems.

To 'fix' this, we introduce the concept of a quorum disk, which is network-shared between both servers in a two-server cluster, which allows a quorate of '2' to be set on the cluster.
If a single server is up, but can't reach the quorum disk, (network issues) this machine will consider the cluster 'not quorate'.
The second server, should it have a correct network connection to the qorum disk, it would have '2' votes, allowing the cluster services to start.

The 'fencing' devices would then trigger the first server to be physically switch off power, to allow for maintainance to intervene.