RDBMS have consistency guarantees that require the engine to fsync at each commit. This ensures that if the system crashes after a transaction no data is lost,

RDBMS have consistency guarantees that require the engine to fsync at each commit. This ensures that if the system crashes after a transaction no data is lost, but the transaction throughput is limited by the disk writes. Conversely Redis (with BGSAVE or diskstore) and other systems only ensure integrity, which means that if there is a crash you don't end up with a corrupted database, but you may lose the latest data that you wrote in the DB because they haven't yet been committed to disk. Relaxing these guarantees completely changes the rules of the game, so the tradeoffs that work well with RDBMS, such as WAL, may not be the best fit in this world.