NATS is a popular distributed streaming system. Jepsen tested NATS 2.12.1, focusing on its durable JetStream subsystem, and found that it could lose data or get stuck in persistent split-brain in response to simulated node failures. This data loss was caused in part by a default fsync policy which flushed data to disk once every two minutes, rather than before acknowledgement. Even a single kernel crash or power failure, combined with process pauses or network partitions, could cause NATS replicas to lose acknowledged messages...