With many workloads, there's a tradeoff between the granularity of durability and the overall performance.

If a workload has many small writes (some of our product workloads do), then syncing each write can cause write amplification and massively affect overall throughput and latency. Suppose I do a 100B write, this causes a 4KiB page write to sync, which is 40x write amp. Suddenly a 2GiB/sec SSD can effectively only write 50MiB/sec. Similarly, the per-write latency goes from <5us to 10us (with the fastest Optane SSDs) or 150us (with flash SSDs).

So storage systems tend to offer a range of durability guarantees. Some systems have a special sync operation for applications to ensure that all writes are durable.

RocksDB offers a fairly weak guarantee by default too, writing to the write-ahead-log (WAL), but not performing fsyncs (https://github.com/facebook/rocksdb/wiki/WAL-Performance). They make a similar write amplification argument too (https://github.com/facebook/rocksdb/wiki/WAL-Performance#wri...).