Too late to edit, but just as a note for anyone else who gets confused by my post: I was not paying careful enough attention and missed/misread the "backups" bit in the parent post, completely my fault. As far as I can tell from reading through the (quite pleasant!) documentation [0], Sylve does not (at least for now) support any sort of network storage for direct use as the VM backing store, though as it is FreeBSD underneath it's presumably doable to get something going from the command line. I'd thought they'd somehow managed to set something up so you could directly use another ZFS system via SSH as the primary backing store with management which would be pretty awesome. It still looks like a beautiful design, but since I'm pretty invested right now in separating out storage into its own hardware vs where compute happens it'd be hard to setup nodes as AIO for the near future at least here.
Still an awesome project to learn about and I hope it's successful.
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0: https://sylve.io/docs/