What happens during the LSM compaction process?
As MemTables fill, they flush to immutable SSTables on disk. Eventually, multiple overlapping SSTables exist at the same level; compaction merges them into fewer, larger SSTables at the next level, removing duplicates and deleted keys. This keeps read paths efficient but costs CPU and write amplification.
Answered in
LSM-Trees vs B-Trees: Why Cassandra Chose Sequential WritesB-trees seek random disk positions. LSM-trees buffer in memory and flush sequentially, converting random I/O into sequential writes for millions of ops/sec.
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