What is a log-structured merge tree (LSM tree)?

A log-structured merge tree (LSM tree) is a tiered, write-optimized data structure that organizes key-value pairs in multiple components, residing partly in memory and partly on disk. Each component is maintained as an append-only sequence, so new records and updates are written sequentially rather than issuing costly random writes to persistent media. LSM trees excel where raw ingest rate matters above all, but for applications that also demand strict, predictable read latency and flash endurance, products like Aerospike offer a more efficient alternative. LSM trees include concepts such as write operations, SSTables, and point lookup, which turn random writes into orderly, bulk transfers and then tidy up the results in the background. However, they can have trade-offs, such as compaction, read amplification, and heavy metadata overhead. Aerospike's Hybrid Memory Architecture (HMA) solves these limits by writing unsorted blocks once, keeping the primary index in DRAM, and using a lightweight block-level garbage collector instead of multi-stage compaction. HMA eliminates most write amplification and guarantees at most one disk I/O per lookup, making it suitable for applications that require sub-millisecond decisions.

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