In context to VM migration, what feature ensures it is done without downtime during a cluster upgrade?

The feature that ensures virtual machine migration without downtime during a cluster upgrade is vSphere vMotion. This technology allows the live migration of running virtual machines from one physical server to another without any disruption to the VM's operation. This means that while the underlying infrastructure, such as hosts or clusters, may be undergoing maintenance or upgrades, the VMs can continue to run seamlessly.

vSphere vMotion works by transferring the memory and execution state of the VM over the network, enabling the VM to retain its connectivity and state throughout the process. This capability is particularly valuable during cluster upgrades, as it allows for planned maintenance without affecting the availability of applications running on those VMs.

In contrast, vSphere Replication is used primarily for disaster recovery purposes, copying VM data to a secondary location rather than live migration. Storage Dead Paths Management handles issues related to storage path failures, but does not facilitate live migration. Distributed Resource Scheduler automates VM distribution and resource allocation across hosts in a cluster for load balancing but is not specifically designed for zero-downtime migration. Thus, vSphere vMotion stands out as the key feature for enabling uninterrupted VM operation during cluster upgrades.