I have too problem with upgrading to sonarqube 7.9 (from 6.7…). I get basically the same log messages as in the previous post, but also “max file descriptors  for elasticsearch process is too low, increase to at least ”. I have increased, ulimit -n says “65535” but ElasticSearch doesn’t see that. You also write about MySQL, I have MySQL installed, but I don’t just want to uninstall it?
If your logs say that SonarQube/Elasticsearch doesn’t have access to enough processes, then that’s actually the problem. I know from personal experience that getting this set correctly can be tricky, but it’s really a Linux sysadmin thing at this point.
Regarding MySQL, you don’t have to uninstall it from your system, you just can’t use it with SonarQube anymore.