How to specify the right device names when using Ignition for installing Flatcar Linux?

I am currently using [Container Linux](https://coreos.com/os/eol/) for my home server and I want to upgrade to [Flatcar Linux](https://www.flatcar-linux.org/) . To do this I plan to use [Ignition](https://docs.flatcar-linux.org/os/root-filesystem-placement/) and configure RAID 1 for the root filesystem. I have two disks dedicated for the root filesystem and the rest form a BTRFS array for extra storage, so I want to make sure not to overwrite the BTRFS disks. The root disks for Container Linux currently register as /dev/sda and /dev/sdb. My understanding is that the names of these disks depends on which Linux kernel you are using and other random boot factors. How can I make sure I pick the right disks in my Ignition configuration? I guess this is a very common issue but I can't find anything specific to Container Linux/Flatcar Linux.