| Thank you for your thoughtful critique. To clarify what our numbers actually mean and address your main question of "what does that number actually mean": 1- The 600ns figure represents precisely what you described - an in-memory "write done" where memory structures are updated and the data becomes globally readable to all processes. This is indeed comparable to what Redis without persistence or memcached provides. Even at this comparable measurement basis (which isn't our marketing gimmick, but the same standard used by in-memory stores), we're still 2-6x faster than Redis depending on access patterns. For full persistence guarantees, our mean latency increases to 2582ns per record (600ns in-memory operation + 1982ns disk commit) for our benchmark scenario with 1M records and 100-byte values. This represents the complete durability cycle. This needs to be compared with for example Redis with AOF enabled. 2- I agree that the meaning of "write done" requires clear context. We've been focusing on the in-memory performance advantages in our communications without adequately distinguishing between in-memory and persistence guarantees. We weren't trying to hide the disk persistence number, we simply used "write done" because in our comparison we compared with Redis without persistence. but mentioning the persistence made an understandable confusion. that was bad on our part. Based on your feedback, we'll update our documentation to provide more precise metrics that clearly separate these operational phases and their respective guarantees. UPDATE: clarification on mean disk write measurement: the mean value is calculated from the total time of flushing the whole write buffer (parallel processing depending on the number of available cpu cores) divided by the number of records. so the total time for processing and writing 1M records as described above was 1982ms which makes the mean write time for each record 1982ns. |
By the way, this set of numbers also makes you look stupid, and you should consider redoing those measurements. No disk out there has less than 10 microseconds of write latency, and the ones in the cloud are closer to 50 us. Citing 2 micros here makes your 600 ns number also look 10x too optimistic.
I would suggest taking this whole thread as less of an opportunity to do marketing "damage control" and more of an opportunity to get honest feedback about your engineering and measurement practices. From the outside, they don't look good.