You initiate a write to the journal, but do not sync it before ACKing to the client.
journal file was pre-alloacated and we use direct-io for journal write so no need to call fsync.
Again, it is not durably persisted before acking to the client. Like I said earlier, that might be fine for your durability model, but it is unusual.
We would wait for Bss data and journal DirectIO and the acking (sending response back to api_server) in the callback function. What you are implying is what s3 actually doing and you can get see from their paper[1] and we are stronger than that.