Breaking the Single-Entry Bottleneck: Dynamic Write Routing for Heterogeneous Key-Value Stores
Modern heterogeneous storage no longer follows a simple fast/slow hierarchy. Byte-addressable NVM provides fine-grained, low-latency persistence, whereas NVMe SSDs deliver high bandwidth under deep queues and offer greater capacity. Most LSM-tree key-value stores, however, send every foreground write through a designated performance tier; SSDs receive data only later through flush, compaction, or migration. When the NVM path saturates, this fixed entry point strands SSD write bandwidth. We present DuetKV, a heterogeneous key-value store that treats NVM and SSD as peer foreground write engines and selects a destination for each request. Saturation-aware routing favors NVM while it has headroom and shifts writes to SSD under pressure. A sync-async interleaved pipeline overlaps synchronous NVM persistence with asynchronous SSD I/O. Size-adaptive cross-tier compaction reclaims NVM space with low write amplification, while transparent read mirrors serve hot SSD-resident data from NVM. On our NVM–SSD prototype, DuetKV achieves up to 3.4 × the throughput of the strongest baseline on write-heavy workloads, while remaining competitive on read-heavy workloads.