Notably, this is going to manage your data in it's native format (i.e. you can actually read-write the files out of the S3 bucket as if they were actual objects, mapping 1:1 to each file). The ZFS backend is (almost certainly) a block-based format that is persisted to S3 (meaning that you cannot use it for existing data in S3, and you cannot access data written through ZFS via S3).