Heap sizing
8-16GB typical. 32GB max (larger = compressed oops off). Off-heap for MemTable + Bloom + row cache = massive savings.
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G1GC
Default modern Cassandra. Region-based. Predictable pauses (MaxGCPauseMillis=200 default). Tune -XX:G1RSetUpdatingPauseTimePercent for large heaps.
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ZGC
Cassandra 4.1+ supports ZGC. Sub-ms pauses even with large heap. Higher CPU cost. Great for latency-sensitive.