Yes, on reboot it will be gone. So what I do is I back it up once an hour to a remote machine that writes it on a hard disk, a little low power linux server I run 24/7. Then after reboot the dir must be recreated and the backup file copied into it and then the process restarted.
A bit of a pain but it is not writing to the ER's flash constantly and I keep data thru reboot. And I do not reboot often.