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

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
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