Island clustering in a virtualized environment

Island Cluster

In this example  6 ESXi hosts are split into 3 cluster supporting 12 VMs

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The above example is used in a large environment where a Large number of VMs need

In large environments where numerous VMs need to be limited to two ESXi hosts without administration, utilizing clusters can streamline operations. The production cluster typically includes all application servers like SQL and Exchange, while the management cluster houses AD, DNS, DHCP, SCCM, VCenter, and similar services. The non-production cluster often comprises older hardware running on VMware’s 90-day evaluation licensing.

Separating management and production into distinct clusters can simplify disaster recovery (DR) processes. For example, you can shut down the production cluster first, allowing you to start up the management cluster after a power down. This ensures that network services are restored and tested before the production cluster comes online.

The use of island clustering is driven by specific needs such as security—keeping two VMs from sharing the same host—or performance, ensuring that two large VMs remain separate. Licensing benefits significantly from defining that a VM has only ever been hosted within a specific cluster and on only two hosts. Some licensing agreements do not accept DR rules due to their potential for changes, which could breach licensing agreements if more than two ESXi hosts are in a cluster.

As an alternative to island clustering, you could implement DR rules, but this would require an audit log of all DR moves, regularly checked and archived for the VM's lifespan as proof of compliance with licensing. This process could be automated by third-party products, though at an additional cost. It is crucial to get written advice on whether DR rules are an acceptable form of VM control to prevent them from migrating onto the wrong host.

While island clustering might be more costly initially, it can offer long-term savings due to the reduced administrative overhead compared to not using an island cluster solution.