https://rwmj.wordpress.com/2018/09/04/nbdkit-as-a-flexible-alternative-to-loopback-mounts/
This is a pretty cool writeup. I can vouch Btrfs will format mount, write to, scrub, and btrfs check works on an 8EiB (virtual) disk. The one thing I thought might cause a problem is the ndb device has a 1KiB sector size, but Btrfs (on x86_64) still uses 4096 byte "sector" and it all seems to work fine despite that. Anyway, maybe it's useful for some fstests instead of file backed losetup devices? -- Chris Murphy
