VMware vCenter Affinity and Anti Affinity Rules Virtual Machines VMs.

VMware vCenter Affinity Rules control where Virtual Machines (VMs) run on physical hosts in a cluster, using Affinity to keep related VMs together (e.g., app & DB) and Anti-Affinity to separate them for high availability (e.g., domain controllers), overriding Distributed Resource Scheduler (DRS) to ensure critical dependencies are met or failure domains are respected, configured as "Must" (hard rule) or "Should" (soft preference).

|| Core Concepts
__Goal: Manage VM placement for performance, resource optimization, high availability, and compliance.

__Applies To: Can link VM groups to host groups or manage VM-to-VM relationships within a cluster.

|| Types of Rules
__Affinity (Keep Together): Ensures specific VMs (like a web server and database) run on the same host, ideal for clustered applications with high interdependency.

__Anti-Affinity (Separate): Prevents VMs from running on the same host, crucial for redundancy (e.g., keeping two domain controllers on different physical servers).

__VM-Host Affinity: Links a group of VMs to a specific group of hosts.

|| Rule Types (Mandatory vs. Preferential)
__"Must" (Mandatory): A hard rule that must be obeyed; DRS and HA will try to enforce it, but it can break in extreme HA scenarios.

__"Should" (Preferential): A soft rule that DRS recommends, but can be violated to maintain overall cluster balance or availability.

|| How They Work with DRS
__DRS (Distributed Resource Scheduler) automatically balances VMs across hosts.

__Affinity rules override DRS, telling it where VMs must or should be placed, giving administrators fine-grained control beyond simple load balancing.

|| Configuration Example
__Create VM Groups and Host Groups in the cluster.

__Define an Affinity Rule: "Must keep VM Group A on Host Group B" or "Should keep VM Group C and VM Group D on different hosts".

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