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To use reported AWUPF as a way to turn off MySQL double writes safely, I guess you need to be using direct IO (otherwise Linux's 4096 hardcoded buffer size kicks in) and you need to know that nothing in the IO stack can ever split up your writes in a way that could overwrite InnoDB pages non-atomically. How can we know that that is true of the whole IO stack? Is it even possible today, without something like the O_ATOMIC proposal?

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Can I have a reference for that hardcoded buffer size please?

See man pages for mkfs.xfs, mkfs.ext4. On other Unix systems block size is/was more variable, but on Linux you can't exceed the 4kB memory page size. So it is not possible to match database block size (typically 16kB for InnoDB, 8kB for PostgreSQL). (Exception being ZFS which has a completely different architecture and can do atomic writes.) Note that I am not saying buffered IO would necessarily be atomic at that level on (say) ancestral SGI XFS or FreeBSD UFS with block size matching the database's (I don't know if it might it some circumstances split writes on sector boundaries for some reason)! Just that it's already a non starter when the block size is too small.

direct i/o forces 4k buffers too :/

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