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From: Melanie Plageman <melanieplageman@gmail.com>
Date: Fri, 3 Apr 2026 12:45:50 -0400
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 <CAAKRu_bWPO-w3arE3pM5i5LArp7mA+Rg0chm-1WL-JxvDxv7Cg@mail.gmail.com>
Subject: Re: AIO / read stream heuristics adjustments for index prefetching
To: Andres Freund <andres@anarazel.de>
Cc: Nazir Bilal Yavuz <byavuz81@gmail.com>, pgsql-hackers@postgresql.org,
	Thomas Munro <thomas.munro@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
 Tomas Vondra <tv@fuzzy.cz>
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On Thu, Apr 2, 2026 at 9:33=E2=80=AFAM Andres Freund <andres@anarazel.de> w=
rote:
>
> > +                /*
> > +                 * XXX: Should we actually reduce this at any time oth=
er than
> > +                 * a reset? For now we have to, as this is also a cond=
ition
> > +                 * for re-enabling fast_path.
> > +                 */
> > +                if (stream->combine_distance > 1)
> > +                    stream->combine_distance--;
> >
> > I don't think we need to reduce this other than reset.
>
> Hm. I go back and forth on that one :)

Separate from the fast-path enablement, we also probably want to
decrease combine distance when we decrease readahead_distance because
there is a point where we still want to parallelize the IOs even when
the distance is lower and to do that, we need to make smaller IOs. I'm
not sure where this point is, but I wonder if a few 256kB IOs is
faster than 1 1MB IO (could test that with fio actually). I imagine
that there is some size where that is true because of peculiarities in
how drives (and cloud storage) issue/break up IOs after they are a
certain size, etc.

- Melanie