So I updated a backup server from 6 to 7 today (actually 2 servers but one uses BTRFS so this doesn't apply). Everything went fine. As I've read and also as the installer says at the end mdadm is now a plugin and has to be installed after the upgrade, from the plugins page.
Funny thing though, once I rebooted there was my raid array all mounted and accessible. I looked at the plugins page to see if somehow the plugin got installed automatically. It wasn't.
So, should I still install the plugin or would I be making thing worse?