Q: If I'm protecting my Hyper-V virtual machines using System Center Data Protection Manager, do I need to install the DPM agent on all Hyper-V servers or just one?

A: You need to install the Data Protection Manager (DPM) agent on all Hyper-V servers in the cluster and add all Hyper-V servers in the cluster to be managed by DPM. When you add virtual machines to be protected, DPM will show only the virtual machines from the cluster object, which it detects automatically, and not via the individual Hyper-V servers, as the figure below shows.

Notice that when you look at a specific Hyper-V server (SAVDALHV21), no virtual machines actually show. But when you look at the cluster object, each virtual machine is visible as a cluster resource, which then contains the actual virtual machine to be protected.

When the actual backup is taken, DPM is smart enough to automatically perform the backup via the node that's running the actual virtual machine.