With btrfs, you can freely create subvolumes and snapshots anywhere (including nested inside of each other), you can have thousands of them without any noticeable performance impact, and you can easily convert a snapshot to a writable subvolume. I don't have much experience with ZFS, but from reading another post [0], my impression is that this isn't really doable with ZFS. And based off of rift's Readme, I think that these features are required for it to work. But I'm not an expert, so I may be mistaken about something here.

[0]: https://news.ycombinator.com/item?id=45077119

The AI instructions in the `specs.md` file gave me a couple clues, e.g. `at` is the directory where the worktree should be created.

And it's clearly using btrfs subvolumes for managing a collection of related Git working trees; there's a concept of "parent" and "child" worktrees.

I don't yet understand why it's better than worktrees, other than being theoretically instant to create new ones (which could, I suppose, be a noticeable speedup if your repo is very very large).

But yeah, some more hand-written instructions in the README would definitely be helpful. I'd be particularly interested to learn whether some of the common "gotchas" one can run into with worktrees are solved by Rift or not. (E.g., I've never needed to move my "root" git repo, but apparently that causes problems because the worktrees then can't find the root repo; does Rift deal with that situation correctly?)

No. XFS too - `cp --reflink=auto`. or LVM / ZFS snapshots if you will