Distributed Persistence Domain for Persistent Memory Pooling
Compute Express Link (CXL) enables memory pooling over disaggregated memory, offering the potential to improve resource utilization in persistent memory systems. However, integrating persistence semantics into CXL-based memory pooling introduces substantial latency, which limits system scalability. This overhead arises because persist operations must traverse the entire CXL fabric, including switches, links, and protocol layers, before reaching remote persistent memory. To this end, we argue that extending CXL switches with persistence support is a promising direction for improving the scalability of persistent memory pooling. However, moving persistence support into the network breaks the traditional correctness assumptions of centralized persistence domains. In particular, enabling persistence within distributed structures, such as CXL switches, can introduce stale reads and writes if not carefully coordinated. In this paper, we propose Distributed Persistence Domain (DPD), a new abstraction for persistent memory pooling that enables persistence support at the CXL switch level. We first formalize the concept of a distributed persistence domain and use DPD as a framework to identify the correctness hazards that arise when persistence structures are distributed across the CXL fabric. Based on this analysis, we derive the design requirements needed to guarantee correctness. Building on these insights, we present Persistent CXL Switch, a CXL switch architecture that incorporates persistence support to significantly reduce persist latency, enable read forwarding, and coalesce writes, while preserving correctness and crash consistency. We evaluated our system design using both SPLASH-4 and YCSB benchmarks. Simulation results show an average speedup of 33% over volatile CXL switches, and up to 36% speedup with read forwarding optimization across all workloads.