Well, it's better when this write barrier happens (which is not true on ext4, iirc), but we rely mostly on the the fact that operating systems flush automatical

Well, it's better when this write barrier happens (which is not true on ext4, iirc), but we rely mostly on the the fact that operating systems flush automatically within 20 seconds (I don't remember the actual numbers, but I'm almost sure it's actually within at most 5 second in practice). Checksums/signatures would be more precise, indeed.