Hi there,
No native backup mechanism offered by SonarQube, simply because nothing specific is needed. Like you said: backup your database, and you've got 99% of your backup right there. The other 1% being: your sonar.properties configuration file, and potentially the list of plugins you're running (though maybe you just run an out-of-the-box Edition).
You shouldn't have anything to do with rsync/hardlinking, nor to care about ElasticSearch data either. If you need to restore backup then you can simply re-deploy the SonarQube version you were using, re-apply sonar.properties , deploy plugins, and you're good to go (ElasticSearch index will be rebuilt at startup).
Best regards,
Nicolas