Re: Use pg_pwritev_with_retry() instead of write() in dir_open_for_write() to avoid partial writes? - Mailing list pgsql-hackers

On Sun, Feb 12, 2023 at 11:01 PM Andres Freund <andres@anarazel.de> wrote:
>
> On 2023-02-12 19:59:00 +0530, Bharath Rupireddy wrote:
> > Thanks for looking at it. We know that we don't change the zbuffer in
> > the function, so can we avoid static const and have just a static
> > variable, like the attached
> > v1-0001-Use-static-variable-to-avoid-memset-calls-in-pg_p.patch? Do
> > you see any problem with it?
>
> Making it static const is better, because it allows the memory for the
> variable to be put in a readonly section.

Okay.

> >       /* Prepare to write out a lot of copies of our zero buffer at once. */
> >       for (i = 0; i < lengthof(iov); ++i)
> >       {
> > -             iov[i].iov_base = zbuffer.data;
> > +             iov[i].iov_base = (void *) (unconstify(PGAlignedBlock *, &zbuffer)->data);
> >               iov[i].iov_len = zbuffer_sz;
> >       }
>
> Another thing: I think we should either avoid iterating over all the IOVs if
> we don't need them, or, even better, initialize the array as a constant, once.

How about like the attached patch? It makes the iovec static variable
and points the zero buffer only once/for the first time to iovec. This
avoids for-loop on every call.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com