Is it possible to run Postgres or an app that uses Postgres + some extras (everything conternerized) on Fly.io that would access a ZFS pool? Or filesystem is something that Fly users cannot control?
Don’t think Fly supports BYOFS. The hint is in its init snapshot code which takes in a mount_path, a device_path and later unapologetically slams an ext4 on it: init-snapshot/main.rs at 48cf36ff7ac6229df66bf1de2b1e2b1c233b2e51 · superfly/init-snapshot · GitHub 
Ignoring perf implications, I wonder if there is a way to stack ZFS on ext4 on Fly: You can probably achieve what you're looking for by stacking a few filesystems. ... | Hacker News