Right, but their building block is 48 disks x whatever is best profitable (a mix of 4,6,8,10, and 12TB currently or similar). So each system is 500TB or less,

Right, but their building block is 48 disks x whatever is best profitable (a mix of 4,6,8,10, and 12TB currently or similar). So each system is 500TB or less, zfs is a filesystem for a single node not a distributed filesystem. Even if we ignore that factor of 800 or so (400PB spread across 500 nodes), they still would need another factor of 128,000 before they would need ZFS's 65th bit. Keep in mind it's 2^128 blocks, not 2^128 bytes. Backblaze would need to consume the entire planets supply of disks for a very long time AND put all those disks connected to a single linux, freebsd, or solaris box. Only then would they need the 65th bit that ZFS has for addressing blocks.