"To deliver the highest write performance, Couchbase Server writes to an integrated, in-memory cache first. Then, it synchronizes the cache with the storage device for durability."

Comparing the write speed of non-durable writes against the write speed of durable writes is the same bullshit tactic that MongoDB first utilized a few years ago. Why don't we just add /dev/null in there too?

We're also apparently using 2 replicas however is the write being confirmed prior to the replica? From Couchbase documentation: "When a client application writes data to a node, that data will be placed in a replication queue and then a copy will be sent to another node. The replicated data will be available in RAM on the second node and will be placed in a disk write queue to be stored on disk at the second node."

So a write in Couchbase only guarantees it has been written to RAM on the 1st node. There's no fsync nor replica-writes occurring at the time of write_success=true