Last night I tried upgrading a system clone in the spare disk. It failed miserably. So I reverted.
And I had another issue with docker while the plugin was offline. Suddenly my containers where not starting and then they disappeared. Without me doing anything, only rebooting. But in the end I managed to diagnose it.
I store the data from my containers in a ZFS array. Its mounted in the Z2 directory, and I refer it in my containers, for example: /Z2/docker-data/emby.
Well, since the docker plugin failed me, the docker service was starting before ZFS during boot. As the pool was not yet imported, there where no content in Z2, so docker decided to create the referenced folders. ZFS then attempted to mount the pool on Z2, but failed because the directory wasn't empty. So I missed the pool and containers because they couldn't boot.
I'll try to do a clean install on the spare disk, but we'll see how my arrays will be restored. Guys, that's not a nice situation. Now I'm thinking on the several installations I've done to some customers and where left alone at the datacenter.