Re: AIO v2.5

On Fri, Mar 14, 2025 at 3:43 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Open items:
>
> - Right now effective_io_concurrency cannot be set > 0 on Windows and other
> platforms that lack posix_fadvise. But with AIO we can read ahead without
> posix_fadvise().
>
> It'd not really make anything worse than today to not remove the limit, but
> it'd be pretty weird to prevent windows etc from benefiting from AIO. Need
> to look around and see whether it would require anything other than doc
> changes.

I've attached a patch that removes the limit for
effective_io_concurrency and maintenance_io_concurrency. I tested both
GUCs with fadvise manually disabled on my system and I think it is
working for those read stream users I tried (vacuum and BHS).

I checked around to make sure no one was using only the value of the
guc to guard prefetches, and it seems like we're safe.

The one thing I am wondering about with the docs is whether or not we
need to make it more clear that only a subset of the "simultaneous
I/O" behavior controlled by eic/mic is available if your system
doesn't have fadvise. I tried to do that a bit, but I avoided getting
into too many details.

- Melanie