Hello bytes, bye blocks: Pcie storage meets compute express link for memory expansion (cxl-ssd)

Compute express link (CXL) is the first open multi-protocol method to support cache coherent interconnect for different processors, accelerators, and memory device types. Even though CXL manages data coherency mainly between CPU memory spaces and memory on attached devices, we argue that it can also be useful to reform existing block storage as cost-efficient, large-scale working memory. Specifically, this paper examines three different sub-protocols of CXL from a memory expander viewpoint. It then suggests which device type can be the best option for PCIe storage to bridge its block semantics to memory-compatible, byte semantics. We then discuss how to integrate a storage-integrated memory expander into an existing system and speculate how much effect it does have on the system performance. Lastly, we visit various CXL network topologies and explore a new opportunity to efficiently manage the storage-integrated, CXL-based memory expansion.