Re: Use pg_pwritev_with_retry() instead of write() in dir_open_for_write() to avoid partial writes?

On Sun, Feb 12, 2023 at 4:14 AM Andres Freund <andres@anarazel.de> wrote:
>
> > +ssize_t
> > +pg_pwrite_zeros(int fd, size_t size)
> > +{
> > +     PGAlignedBlock  zbuffer;
> > +     size_t  zbuffer_sz;
> > +     struct iovec    iov[PG_IOV_MAX];
> > +     int             blocks;
> > +     size_t  remaining_size = 0;
> > +     int             i;
> > +     ssize_t written;
> > +     ssize_t total_written = 0;
> > +
> > +     zbuffer_sz = sizeof(zbuffer.data);
> > +
> > +     /* Zero-fill the buffer. */
> > +     memset(zbuffer.data, 0, zbuffer_sz);
>
> I previously commented on this - why are we memseting a buffer on every call
> to this?  That's not at all free.
>
> Something like
>     static const PGAlignedBlock zerobuf = {0};
> would do the trick.  You do need to cast the const away, to assign to
> iov_base, but that's not too ugly.

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?

FWIW, it comes out like the attached
v1-0001-Use-static-const-variable-to-avoid-memset-calls-i.patch with
static const.

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