Re: bgwriter doesn't flush WAL stats
| От | Nazir Bilal Yavuz |
|---|---|
| Тема | Re: bgwriter doesn't flush WAL stats |
| Дата | |
| Msg-id | CAN55FZ08ZzbzMGaLWPpFgbfO+WWzTd+7U=z06dzfPY1XQAbcsg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: bgwriter doesn't flush WAL stats (Matthias van de Meent <boekewurm+postgres@gmail.com>) |
| Ответы |
Re: bgwriter doesn't flush WAL stats
|
| Список | pgsql-hackers |
Hi,
Thanks for the explanation.
On Wed, 21 Jun 2023 at 18:03, Matthias van de Meent <boekewurm+postgres@gmail.com> wrote:
>
> On Wed, 21 Jun 2023 at 13:04, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> > I was trying to add WAL stats to pg_stat_io. While doing that I was comparing pg_stat_wal and pg_stat_io's WAL stats and there was some inequality between the total number of WALs. I found that the difference comes from bgwriter's WALs. bgwriter generates WAL but it doesn't flush them because the pgstat_report_wal() function isn't called in bgwriter. I attached a small patch for calling the pgstat_report_wal() function in bgwriter.
> >
> > bgwriter generates WAL by calling functions in this order:
> > bgwriter.c -> BackgroundWriterMain() -> BgBufferSync() -> SyncOneBuffer() -> FlushBuffer() -> XLogFlush() -> XLogWrite()
>
> I was quite confused here, as XLogWrite() does not generate any WAL;
> it only writes existing WAL from buffers to disk.
> In a running PostgreSQL instance, WAL is only generated through
> XLogInsert(xloginsert.c) and serialized / written to buffers in its
> call to XLogInsertRecord(xlog.c); XLogFlush and XLogWrite are only
> responsible for writing those buffers to disk.
Yes, you are right. Correct explanation should be "bgwriter writes existing WAL from buffers to disk but pg_stat_wal doesn't count them because bgwriter doesn't call pgstat_report_wal() to update WAL statistics".
> I also got confused with your included views; they're not included in
> the patch and the current master branch doesn't emit object=wal, so I
> can't really check that the patch works as intended.
I attached a WIP patch for showing WAL stats in pg_stat_io.
Thanks for the explanation.
On Wed, 21 Jun 2023 at 18:03, Matthias van de Meent <boekewurm+postgres@gmail.com> wrote:
>
> On Wed, 21 Jun 2023 at 13:04, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> > I was trying to add WAL stats to pg_stat_io. While doing that I was comparing pg_stat_wal and pg_stat_io's WAL stats and there was some inequality between the total number of WALs. I found that the difference comes from bgwriter's WALs. bgwriter generates WAL but it doesn't flush them because the pgstat_report_wal() function isn't called in bgwriter. I attached a small patch for calling the pgstat_report_wal() function in bgwriter.
> >
> > bgwriter generates WAL by calling functions in this order:
> > bgwriter.c -> BackgroundWriterMain() -> BgBufferSync() -> SyncOneBuffer() -> FlushBuffer() -> XLogFlush() -> XLogWrite()
>
> I was quite confused here, as XLogWrite() does not generate any WAL;
> it only writes existing WAL from buffers to disk.
> In a running PostgreSQL instance, WAL is only generated through
> XLogInsert(xloginsert.c) and serialized / written to buffers in its
> call to XLogInsertRecord(xlog.c); XLogFlush and XLogWrite are only
> responsible for writing those buffers to disk.
Yes, you are right. Correct explanation should be "bgwriter writes existing WAL from buffers to disk but pg_stat_wal doesn't count them because bgwriter doesn't call pgstat_report_wal() to update WAL statistics".
> I also got confused with your included views; they're not included in
> the patch and the current master branch doesn't emit object=wal, so I
> can't really check that the patch works as intended.
I attached a WIP patch for showing WAL stats in pg_stat_io.
After applying patch, I used these queries for the getting views I shared in the first mail;
Query for the first view:SELECT
'pg_stat_wal' AS view_name,
SUM(wal_write) AS total_wal_write
FROM
pg_stat_wal
UNION ALL
SELECT
'pg_stat_io' AS view_name,
SUM(writes) AS total_wal_write
FROM
pg_stat_io
WHERE
object = 'wal';
SELECT backend_type, object, writes FROM pg_stat_io where object = 'wal';
I also changed the description on the patch file and attached it.
Regards,
Nazir Bilal Yavuz
Microsoft
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