Borges: A Low-Latency Distributed Shared Log on a CXL Memory/SSD Hybrid

Abstract

Fault-tolerant, distributed shared logs are a useful substrate for distributed applications, but existing designs coordinate shard servers, sequencers, and replicas over the network, so append and replay latency is dominated by message exchange. We present Borges, to our knowledge the first fault-tolerant shared log whose data and ordering state reside in a DRAM/SSD hybrid device accessed with Compute Express Link (CXL). Because current CXL pods lack inter-host cache coherence, Borges assigns every shared metadata word a single writer, atomically publishes each committed prefix as a matching global cut and stream index pair, and tracks each stream with compact byte offsets, so replay stays cheap even when a stream spans shards. We evaluate Borges on Samsung's CMM-H device against CXL ports of Scalog and Boki, using a linearizable lock, a key-value store, and a fault-tolerant workflow with exactly-once semantics. Borges achieves 2.87× lower append p99 latency and 2.16× higher throughput than the Scalog port, and for the Retwis workflow application it lowers p50 latency by 5.7×/22.5× relative to the Scalog/Boki ports.