A. No. Clusters exist in vSphere, but they're only there for high availability and automated distribution of resources. You can actually perform a migration of virtual machines (VMs) that aren't part of any cluster, as you can see here with vMotion.

Click to expand.

The only requirement is that the VMware hosts are running compatible processors and have access to the storage containing the VM, since you can't perform storage vMotion and normal vMotion at the same time.

Think of vMotion as a means to migrate active state without downtime. Think of clusters as features than can utilize vMotion to achieve their goals, primarily to move VMs between hosts for the purpose of balancing resources.

Remember that with vSphere, there are two uses for a cluster. (Both can be enabled for a single cluster.):

  • VMware HA (High Availability), which is used to detect failure and recover VMs. Obviously, in the event of a host failure, vMotion can't be used because there is no active state to migrate.
  • VMware DRS (Distributed Resource Scheduling), which is used to balance VMs over hosts to ensure even resource usage between all hosts. vMotion is key here to move VMs between hosts with zero downtime. Distributed Power Management (DPM) also uses the DRS cluster to turn off hosts and consolidate VMs when the resource need is low, to save power, and then turn hosts back on when they're needed.

As you can see, vMotion is really only used in the DRS scenario, not in a host failure situation.

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