Why VCF Management Domain Needs 4 Hosts

The question “Can the Management Domain be deployed with just 3 hosts in VCF?” is from my experience a question that is often asked online and in the VCF design workshops. Recently, there has been a number of these questions on Reddit and on the Broadcom VCF Community, so I decided to provide a bit more insight to this question.

The short answer

No, you cannot deploy a VCF Management Domain with just 3 servers. VCF requires a minimum of 4 hosts in the Management Domain. This requirement is closely tied to vSAN, which is used as the principal storage solution within the Management Domain.

Note: there are some tricks that you can apply during VCF bring-up process to deploy VCF even with 1 host but it is just for lab and testing purposes. NOT supported for production!

Why 4 hosts are required

The reasoning behind this requirement lies in vSAN’s architecture and the need for operational stability during maintenance or upgrades. Here’s a breakdown:

  • vSAN Storage and Resilience: While vSAN can technically run on a minimum of 2 or 3 hosts, the addition of a 4th host provides the necessary redundancy to ensure that storage remains available during maintenance operations. With only 3 hosts, taking one host offline (for patching or upgrades) would reduce the cluster to just 2 hosts, potentially compromising vSAN quorum and making the storage unavailable.
  • Maintenance and Upgrades: VCF leverages lifecycle management tools to streamline updates and patches. A 4 host cluster allows you to place one host in maintenance mode during these operations without disrupting vSAN or overall cluster operations.
  • Cluster Stability: The 4th host adds a layer of fault tolerance. Even if a single host experiences a failure, the cluster can continue to operate without immediate risk of losing vSAN quorum.
  • VCF vSAN Design Requirement: When designing VCF you must meet the vSAN Design Requirement VCF-VSAN-REQD-CFG-002 that dictates to provide required minimum number of hosts according to the cluster type, in this case Management. Justification to this requirement is to satisfy the requirements for vSAN storage availability.

Note: Requirements in a VCF design cannot be changed!

Hope this helps you understand why you need 4 ESXi hosts in the Management Workload Domain in VCF. Thoughts? Other considerations? Feel free to leave a comment below.

Cheers!

– Marek.Z

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