RE: pg_preadv() and pg_pwritev()

> > I'm drawing a blank on trivial candidate uses for preadv(), without
> > infrastructure from later patches.
>
> Can't immediately think of something either.

This might be not that trivial , but maybe acquire_sample_rows() from analyze.c ?

Please note however there's patch https://www.postgresql.org/message-id/20201109180644.GJ16415%40tamriel.snowman.net (
https://commitfest.postgresql.org/30/2799/) for adding fadvise, but maybe those two could be even combined so you would
bedoing e.g. 16x fadvise() and then grab 8 pages in one preadv() call ?  I'm find unlikely however that preadv give any
additionalperformance benefit there after having fadvise, but clearly a potential place to test. 

-J.