Mongo - What does the WriteConcern j function do when logging is disabled?

Question

The documentation is not clear what happens when the Write Concern j parameter is set to true and mongod runs without a log. Does this mean that the write operation will return after it has been confirmed that it was committed to the database? OR can a write operation return before the record is transferred to the database? Am I thinking about it wrong?

Answer 1

You think about it right. From the doc:

Changed in version 2.6: Indication of a problem with a record containing j: true for mongod or mongos working with the --nojournal parameter causes an error. Previous versions would ignore j: true.

You can only assume that you get some kind of error when you try to configure it as described.