[rekoil]

Sounds like what bcache does? https://bcache.evilpiepirate.org/

This is what bcachefs is based on.

[M95D]

From kernel Documentation/bcache.txt:

> You'll need make-bcache from the bcache-tools repository. Both the cache device and backing device must be formatted before use.

So, it's far from overlayfs. I could accept formatting the cache device, but not the backing storage.