On Thu, Mar 12, 2026 at 11:08 PM Anthonin Bonnefoy
<[email protected]> wrote:
>
> On Tue, Mar 10, 2026 at 6:11 PM Andres Freund <[email protected]> wrote:
> > I'm pretty sure this is not correct as-is, it suffers from the same issue as
> > https://postgr.es/m/vf4hbwrotvhbgcnknrqmfbqlu75oyjkmausvy66ic7x7vuhafx%40e4rvwavtjswo
> > I.e. it is not safe to use GetXLogInsertRecPtr() to determine up to where to
> > flush to, due to page boundaries.
Thanks for the report!
> I've managed to reproduce this issue by ensuring the FPI_FOR_HINT
> record finishes at the end of a page with the following script (might
> need some adjustment if the record sizes are different):
>
> DROP TABLE IF EXISTS test_insert_rec_ptr;
> CREATE TABLE test_insert_rec_ptr(aid int, data text) WITH
> (autovacuum_enabled = false);
> INSERT INTO test_insert_rec_ptr SELECT *, repeat('a', 100) FROM
> generate_series(0, 57);
> -- This should tag the page as full
> BEGIN; UPDATE test_insert_rec_ptr SET aid=2 where aid=1; ROLLBACK;
> CHECKPOINT;
> -- Start with a fresh file
> SELECT pg_switch_wal();
> -- Our FPI_FOR_HINT writes 8193 bytes
> -- With the long header, the first page has 8152 bytes available
> -- With the short header, the second page has 8168 bytes available
> -- We want our FPI_FOR_HINT to finish at the end of the second page
> (+/- 8 bytes of alignment)
> -- We need to write the first 25 bytes (or 32 with alignment) in the first
> page
> -- For that, we need to write 8120 bytes of WAL records
> BEGIN;
> -- 264 bytes of FPW
> INSERT INTO test_insert_rec_ptr VALUES(1);
> -- 74 * 104 bytes
> INSERT INTO test_insert_rec_ptr SELECT *, repeat('a', 44) FROM
> generate_series(1, 74);
> -- 108 bytes
> INSERT INTO test_insert_rec_ptr VALUES(1, repeat('a', 48));
> -- 46 bytes
> COMMIT;
> -- 264 + 74 * 104 + 46 + 108 = 8114 bytes, which will round up to 8120
> with alignment
> -- FPI_FOR_HINT record should be at 0x1FE0
> BEGIN; SELECT * FROM test_insert_rec_ptr WHERE aid=2; ROLLBACK;
>
> As far as I can tell, the only impact it has is to complain about the
> write request being too far:
> LOG: request to flush past end of generated WAL; request 0/01604018,
> current position 0/01604000
> ERROR: xlog flush request 0/01604018 is not satisfied --- flushed
> only to 0/01604000
>
> To avoid this issue, it sounds like we need something to use
> XLogBytePosToEndRecPtr instead of XLogBytePosToRecPtr to convert the
> byte position? With XLogBytePosToRecPtr(), the flush request would
> stop at 01604000 instead of going to the next page with 01604018.
>
> In the attached patch, I've added a GetXLogInsertEndRecPtr() function
> which is similar to GetXLogInsertRecPtr(), except it uses
> XLogBytePosToEndRecPtr() to stop at the page boundary.
> There was also another XLogFlush(GetXLogWriteRecPtr()) call in
> syncutils.c, so I replaced both calls with
> XLogFlush(GetXLogInsertEndRecPtr()).
Thanks for investigating the issue and making the patch!
It looks good to me.
Andres,
Do you have any comments on the proposed patch?
Regards,
--
Fujii Masao
