But how much checking does it do to the ZFS structures themselves? They're validated as well. Everything has a cryptographic checksum that's stored in the pare

But how much checking does it do to the ZFS structures themselves? They're validated as well. Everything has a cryptographic checksum that's stored in the parent block, starting at the data and working all the way up to the top of the tree. Furthermore, the higher up the tree you go, the more redundant copies there are. The top of the tree has four copies, if I recall. This ignores support for mirroring and striping, which further improves data redundancy. But wait, there's more! Those four blocks aren't overwritten. ZFS is a copy-on-write filesystem (data and metadata) which behaves a lot like the persistent data structures that Clojure hackers are so fond of, so if the newest writes do not validate, it'll roll back to the newest valid commit of the tree. That's a nice way of saying that you're guessing given your experience with other filesystems. ZFS was a genuinely revolutionary filesystem, and doesn't behave like other filesystems, the sole OSS exception being BTRFS. Read up on it a bit, you'll find something interesting. :)