Re: Add a pg_wal_preallocate() SQL function to eagerly create future WAL segments

Hi,

On Mon, 17 Aug 2026 at 05:08, Bharath Rupireddy <
bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:

> Hi,
>
> On Sun, Aug 16, 2026 at 8:09 AM Ayush Tiwari
> <ayushtiwari(dot)slg01(at)gmail(dot)com> wrote:
> >
> > Upon thinking more, I've removed the force option for now.
> > We can revisit/add it if and when need be, adding something like that
> > is easier than removing it later.
> >
> > v6 attached. Thoughts?
>
> Thanks for the new patch. It mostly looks good to me with a few minor
> comments (at the end). I marked the CF entry as RfC
> (https://commitfest.postgresql.org/patch/7052/) pgindent, make check,
> make check-world with both debug and release builds run fine. I did
> the following two tests to ensure it works as expected.
>
> 1/ Concurrent WAL file creation. A TAP test with an injection point
> where one backend is paused right before preallocating a segment while
> another creates it first with an INSERT query. The backend that
> preallocates creates fewer files than requested, returns without
> error, and the file ends up created only once.
>
> 2/ Preallocated 100 WAL files on the primary, then failed over to the
> standby and ran pg_rewind on the old primary to bring it back as a
> standby of the newly promoted primary. pg_rewind removed them [1]
> since they are beyond the divergence point.
>
> A few minor comments:
>
> 1/
> + nsegsadded = PreallocNXlogFiles(nsegs);
> +
> + PG_RETURN_INT64(nsegsadded);
>
> It might be good to return the WAL file names that were created for
> the record, but I don't see any strong use for that now. We can add it
> in future if needed.
>
> 2/
> + if (PG_ARGISNULL(0))
> + bytes = (int64) min_wal_size_mb * 1024 * 1024;
> + else
> + {
> + bytes = PG_GETARG_INT64(0);
> +
> + if (bytes < 0)
> + ereport(ERROR,
> + (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
> + errmsg("number of bytes to preallocate must not be negative")));
> + }
> +
>
> I'm fine with this, however, I will leave it to others and the
> committer to decide whether to auto-calculate from min_wal_size when
> user input is NULL.
>
> [1]
> pg_rewind: pg_wal/00000001000000000000006F (REMOVE)
> pg_rewind: pg_wal/00000001000000000000006E (REMOVE)
> ...
> ...
> pg_rewind: pg_wal/00000001000000000000000E (REMOVE)
> pg_rewind: pg_wal/00000001000000000000000D (REMOVE)
>
> --
> Bharath Rupireddy
> Amazon Web Services: https://aws.amazon.com

Rebased.

Regards,
Ayush