Does coreos-installer format /var?

I see a lot of discussion about the goals of coreos and how reprovisioning a machine is the recommended way to change the config. What I see neither in those discussions nor in the coreos or coreos-installer documentation, is what happens to existing data during a reprovisioning.

I’m aware, that I could just have /var on a different device, but my question is if that’s the expected/recommended way to do this or if coreos-installer actually guarantees that I don’t loose any data.

Hmm. I think you’ve highlighted a gap in our docs. coreos-installer supports the --save-partlabel or --save-partindex options to preserve data on partitions you don’t want to blow away. See the coreos-install install man page. As always, I recommend backing up any data you can’t live without before any sort of reprovision operation.

We probably need to mention this in Installing CoreOS on Bare Metal :: Fedora Docs or link to an example with more details…

And to answer this explicitly… when you coreos-installer install to a disk the default is to wipe it completely (essentially like a dd).