It's easy enough to jam enough disks into a rack with these 90disk 4u super micro jbods. The thing that always scared me (ESP with rsync) is how do you get performant metadata for a few billion files? Or even tens of millions.

And raidz3 resilver must be horrible at those densities!

Again. Just curious. Email at jmancuso@expandrive if you feel like chatting. I know we offer a similar product, but we are about to leave zfs for the above concerns. Wouldn't mind sending some business your way.