Iceberg format-v2 dynamic writer diagnostic

Release + mimalloc, 2M generated Arrow rows, one warmup and three measured pairs, alternating engine order. Both engines use a 4 GiB Java heap, parallelism four, four buckets per target, Zstd 3, one-second checkpoints and immediate metadata updates. Timing includes Java routing, native/stock data encoding, Java deletes, a non-draining canonical savepoint and terminal draining canonical savepoint. This writer-only diagnostic is separate from the final Nexmark matrix. Stock readback validates every ID/payload; keyed writes repeat 100k keys.

Mode	Stock median [range], s	Native median [range], s	Ratio
forward-hash	2.249 [2.179–2.301]	2.313 [2.303–2.322]	0.97×
keyed	3.206 [2.951–3.641]	3.371 [3.057–3.505]	0.95×

All 16 trials passed committed-output, native-data-footer and released-handle/reservation assertions. Format-v3 keyed trials require actual Java Puffin deletion vectors; native v3 reading remains excluded. Raw observations, graphs, final table metadata, checkpoint sizes, environment and compressed logs are retained alongside this report.