Nonvolatile main memory (NVMM) technologies are coming out of the lab and onto the processor memory bus. When Intel releases 3D XPoint DIMMs, it will bring something new to memory and storage systems: A scalable, persistent, fast memory technology. NVMM's new capabilities bring enormous promise but also a raft of complications. The audacious vision for its eventual use would exploit its fine-grain access capabilities to create application-specific, highly -optimized persistent data structures. These data structures can be enormously fast, but building them to survive crashes is hard, 1 ' 3 so it will take some time before persistent data structures are ready for prime time.