Mongo v6.0.0 immediately exits with featureCompatibilityVersion error

[Altay (Altay Hunter)]

@Doug_Duncan

If you want to save your data, you can fix this, by downloading the compressed archive of 5.0.x and extract it. From this extracted folder you can run ./bin/mongod --dbpath <current database path> . Connect to this instance with mongosh and run db.adminCommand( { setFeatureCompatibilityVersion: "5.0" } ) . This will change the FCV for you. You can then exit mongosh and then shutdown the mongod instance and finally start your version 6.0.0 server. You will want to change the FCV here as well to be 6.0 .

Thanks! This worked like a charm. Since I’m on Arch Linux I just needed to install mongodb50-bin from the AUR to downgrade, ran the adminCommand, then installed mongodb-bin again to upgrade my binary back to 6.0.0.

Is it good practice to update the FCV after each major release of mongo? Is there a reason it doesn’t happen automatically?

@Doug_Duncan

If you don’t care about any of your data, you can just delete the entire database directory that the current data is stored in.

Good to know in case I run into similar issues in the future.