MongoDB Write Concern when Journaling is off I'm running mongo 3.4.7 (WiredTiger) replica set with two secondaries.

According to the docs, when journaling is enabled and write concern is {w: "majority", j: 0}, the ack is sent after the data is written to "In memory".

However the docs don't specify what happens when journaling is off on the server (--nojournal). What happens in that case? What is the write concern waiting for?
The answer to your question: Writing data change to disk.

When journaling is on, the first change is written to the journal, then it is written to data blocks.