Acheron: Persisting Tombstones in LSM Engines

Abstract

Modern NoSQL storage engines frequently employ log-structured merge (LSM) trees as their core data structures because they offer high ingestion rates and low latency for query processing. Client writes are captured in memory first and are gradually merged on disk in a level-wise manner. While this out-of-place paradigm sustains fast ingestion rates, it implements delete operations via inserting tombstones which logically invalidate older entries. Thus, obsolete data cannot be removed instantly and may be retained for an arbitrarily long time. Therefore, out-of-place deletion in LSM trees may, on the one hand, violate data privacy regulations (e.g., the right to be forgotten in EU's GDPR, right to delete in California's CCPA and CPRA), and on the other hand, it hurts performance.