Keep reading. You can use persistent memory for the ramdisk so it'll be maintained between system reboots https://memcached.org/blog/persistent-memory/

Yup, but you can copy that file (+ the .meta file) to disk, reboot, copy back, etc. It'll take a long time for a large cache unless your disk is fast.

If you have some kind of pmem device and a dax mount, those can survive reboots on their own.