I'm surprised application devs haven't done this, but you can "backup" a sqlite db to an in-memory sqlite db, then just "backup" the in-memory db every so often in the background.
The problem with running Pi-hole on the Pi 1 (original) is that it does not have enough physical memory after installing pi-hole stack (I used Nginx + php-fpm for the web UI), otherwise there is way to use utils like `vmtouch` to read the sqlite database file and keep it in page cache, even "lock" it.
On Pi 2 and 3 it's no longer an issue (even if you don't do anything about it) to a memory buff and better I/O (using faster micro SD).
Pi-hole provides a mechanism to backup and rotate the database from time to time, one can do that whatever way suits their use cases.
