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

On 2023-02-13 18:33:34 +0900, Kyotaro Horiguchi wrote:
> At Mon, 13 Feb 2023 10:15:03 +0530, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote in
> > On Sun, Feb 12, 2023 at 11:01 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > >
> > > On 2023-02-12 19:59:00 +0530, Bharath Rupireddy wrote:
> > > > /* 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.
>
> FWIW, I tried to use the "{[start .. end] = {}}" trick (GNU extension?
> [*1]) for constant array initialization, but individual members don't
> accept assigning a const value, thus I did deconstify as the follows.
>
> > static const struct iovec iov[PG_IOV_MAX] =
> > {[0 ... PG_IOV_MAX - 1] =
> > {
> > .iov_base = (void *)&zbuffer.data,
> > .iov_len = BLCKSZ
> > }
> > };
>
> I didn't checked the actual mapping, but if I tried an assignment
> "iov[0].iov_base = NULL", it failed as "assignment of member
> ‘iov_base’ in read-only object", so is it successfully placed in a
> read-only segment?
>
> Later code assigns iov[0].iov_len thus we need to provide a separate
> iov non-const variable, or can we use pwrite instead there? (I didn't
> find pg_pwrite_with_retry(), though)

Given that we need to do that, and given that we already need to loop to
handle writes that are longer than PG_IOV_MAX * BLCKSZ, it's probably not
worth avoiding iov initialization.

But I think it's worth limiting the initialization to blocks.

I'd also try to combine the first pg_writev_* with the second one.

Greetings,

Andres Freund