Which logical entity is used to group Storage Data Server (SDS) nodes for fault isolation...

The Protection Domain (PD) is the highest level of logical aggregation for storage nodes in PowerFlex.

Fault Isolation: The Protection Domain acts as a "walled garden" for failures. If a catastrophic failure occurs (e.g., a power outage affecting a specific rack), the impact is contained within that Protection Domain. Data replication and rebuilding occur within the boundaries of the PD. An SDS in Protection Domain 1 cannot mirror data to an SDS in Protection Domain 2.

Resource Management: Administrators manage resources at the PD level to balance the cluster. For example, a cluster might have one Protection Domain for "High Performance Flash" nodes and another PD for "High Capacity HDD" nodes.

Differentiation:

Storage Pools exist inside a Protection Domain to group disks.

Fault Sets exist inside a Protection Domain to further granularize failure zones (e.g., preventing mirroring between nodes in the same rack).