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Question

Can I or should I cluster two virtual appliances?

Answer

While it is technically possible to cluster two virtual appliances, in most cases it is counterproductive, and we thereby strongly discourage it.
The main use case for clustering is to avoid hardware outages. In a virtualized environment the hypervisor should take care of that.
There are several reasons to not cluster two virtual appliances:

  1.  It makes no sense to do clustering on top of virtualization, as the virtualization provides the necessary hardware fault tolerance. For example, vSphere provides two features, depending on your needs: HA or FT. Others will have similar approaches.
  2. When creating snapshots of the appliance, the OS is frozen briefly, which can trigger the failover of the cluster, which in turn will leave you in a split-brain situation.
  3. You add a level of complexity into an already complex environment, which in general you want to avoid.

For the virtual appliance the clustering feature is only there to use the virtual appliance as a secondary node with a physical appliance as the primary. In this scenario the virtual appliance will take over only in case of hardware failure of the primary node. As soon as the hardware is up and running again, you should make it the primary again.