I'm running EC2 EBS images on instances with instance stores on /xvdb and /xvdc. I have managed to create a script to raid these two drives on first boot and mount them as /mnt. I am doing this as a OneTime service after systemd-remount-fs.service (I have no idea if that is the appropriate time).
What I really want to do is raid them, copy over the EBS's /var, and then mount them as /var before continuing booting for real. I'm assuming that I'll need to do this in the ram disk image during early boot. I'm quite out of touch with systemd and boot order - I last got dirty with building ram disks in 2009. What is the "proper" way to integrate a script that runs during the first boot and does the necessary setup?