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

Hi,

On Fri, 25 Sept 2026 at 20:54, Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com> wrote:
> On Thu, 27 Aug 2026 at 17:48, Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com> wrote:
> >
> > 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)
> >>
> >

Rebased.

Regards,
Ayush