As I answered @cmh714 suggestion, my intention was to reduce the micro-sd card writes, to avoid a failure. The backup is nice, but I want to prevent a corrupt SD (happened to me too occasionally).
I remember there was a discussion about this with instructions, but I can't find it, nor could I be sure this would work with your build.
It's already optimized to write to SD card only when necessary. Frequently changed data are written to /dev/shm for performance gain and no writing to SD card is a by-product. I have had only one corrupted SD card a few years back but none since then. Besides normal usage, my SD cards have been flashed and written quite a lot. They're so cheap these days even for good quality brands.