ZFS can do this. Just set copies=2 on a dataset, and it'll always keep 2 copies of everything written thenceforth (with ~twice the storage usage).

And since it's a dataset setting instead of a pool setting, it can be applied selectively on a storage device.

"Oh, those photos are also backed up to Google Photos. It's maybe not ideal, but it's a lot of data and not worth keeping 2 copies those locally, so I'll leave those at the default of copies=1. But the this collection of code and short stories and I've written? That's pretty important to me, and it's actually kind of small -- I'll set that as copies=2. Actually, maybe copies=3 would be even better..."

The setting will stick with it between machines/sessions/whatevers without further user input. Things with copies=2 get mounted the same way as anything else is with ZFS in a given environment.

ZFS already detects bitrot very well even with copies=1, but it can't fix bitrot with chechsums alone. With copies=2, it has a non-zero chance at actually fixing it (or at least being completely readable so the data can be transitioned to the next storage device, completely intact).

https://docs.oracle.com/cd/E19253-01/819-5461/gevpg/index.ht...