Sources
CommitLog. MemTable flush (SSTable creation). Compaction (re-write same data through levels). Repair streaming. Hints.
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Measuring
Disk write throughput / logical write throughput. STCS: 2-5x. LCS: 5-15x. TWCS on time-series: 1.5-3x.
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Tuning knobs
Compaction throughput (nodetool setcompactionthroughput). Reduces peak IO but stretches over time. Balance CPU + IO.