Enhancing LSM-Tree Key-Value Stores for Read-Modify-Writes via Key-Delta Separation

Read-modify-writes (RMWs) are increasingly observed in practical key-value (KV) storage workloads to support fine-grained updates. To make RMWs efficient, one approach is to write deltas (i.e., changes to current values) to the log-structured merge-tree (LSM-tree), yet it increases the read overhead caused by retrieving and combining a chain of deltas. We propose a notion called key-delta (KD) separation to support efficient reads and RMWs in LSM-tree KV stores under RMW-intensive workloads. KD separation aims to store deltas in separate storage areas and group deltas into storage units called buckets, such that all deltas of a key are kept in the same bucket and can be all together accessed in subsequent reads. To this end, we build KDSep, a middleware layer that realizes KD separation and integrates KDSep into state-of-the-art LSM-tree KV stores (e.g., RocksDB and BlobDB). We show that KDSep achieves significant I/O throughput gains and read latency reduction under RMW-intensive workloads while preserving the efficiency in general workloads.