In that case, you use copy OR what you can can also do, if you really care about disk usage, is use symbolic links between the drives. have a .venv sym link on drive A (raid 1) point to the uv_cache_dir's venv on drive B (raid 0). I have not tested though what happens when you unmount and sync.

... your hourly backups aren't also using the same hard-linking strategy?

They’re ZFS snapshots, so no. That’s why I’m forced to keep my cache directory in a different dataset.

Ironically, if my backup scheme were using hard links, then I could simply exclude the cache directory from backup, so I’d have no reason to do that mountpoint spiel, and uv’s hard links would work normally.

... ZFS snapshots don't produce copies of the underlying file data, do they? (Yeah, I'm still a dinosaur on ext4...)

Nothing wrong with ext4! Having choices and preferences is a good thing.

You’re correct, ZFS snapshots don’t produce copies, at least not at the time they’re being created. They work a little like copy-on-write.