That's for concurrent reads, where the "atomicity" guarantees as defined in POSIX don't mean what anyone expects.

Yes I misinterpreted that, thanks. I wonder if a later spec requires sector atomic writes because I always worry something will be optimized later while still meeting reqs. I thought this had happened for XFS on linux but the sector guarantee remained )(only the stronger SGI behavior was dropped to reduce latency) - or how now more C compiler optimizations trip-up people inadvertently hitting UB. It really does seem 256 bytes like how used in this HN post is safe on all common systems, but I'd still be afraid it could break unless I see it spelled-out in a spec and man page.