What tmp actually is usually is a ramdisk.
You'll see if defined as a tmpfs in your /etc/fstab file. That means all the info is kept in your ram, and is just mapped onto a folder. Every time your ram looses it's data (reboot) tmp will be automatically cleaned since it's data is gone.
You can change that from tmpfs to a normal partition easily, or just delete that line and it'll go under the / (root) partition.
It's always possible to change things, reinstalling all of linux should never happen
I think your problem is easily solved by mounting what's currently mounted as tmp with a new name. Create the directory in / first. Haven't used raid before though, but give it a try.