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

Hi,

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.

Rebased.

Regards,
Ayush