VoltDB is a probably a better example of an in-memory database with ACID semantics. Redis usually won't be deployed in a way where it fsyncs after each write op

VoltDB is a probably a better example of an in-memory database with ACID semantics. Redis usually won't be deployed in a way where it fsyncs after each write operation because it runs too slow. You need to do some tricks like batching fsyncs to get decent performance and I don't think Redis has support for this. However, SSD fsync performance seems very high. I remember benchmarking an EC2 i3 instance that was giving 20000 fsync/s but if redis is giving you like 80k writes/s then even fsync this fast is going to be a bottleneck. [https://redis.io/topics/benchmarks https://redis.io/topics/benchmarks] I think people who are deploying redis are willing to tolerate lower durability guarantees for the extra performance. A lot of the time redis is some kind of cache and there is a way of reconstructing the real data from more durable storage in the case of failure. Or people are willing to lose a second of data or whatever fsync interval people are using.