> The problem I have with this analysis is that any reads of these
> files should be satisfied from the buffer cache too; it should not
> matter if the data is on disk yet or not.

You missed the part of my description where I said, "if the machines
boot uncleanly after this process."

Thanks for the standards quote about fsync().  I think for the long
term I will write a little fsync program to help with safely rewriting
files inside shell scripts.