Re: Odd off-by-one dirty buffers and checkpoint buffers written
| От | David G. Johnston |
|---|---|
| Тема | Re: Odd off-by-one dirty buffers and checkpoint buffers written |
| Дата | |
| Msg-id | CAKFQuwYQub49cNg_vpfJew3QS0Jgji2tx=AE8=_nx83XkzQs+w@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Odd off-by-one dirty buffers and checkpoint buffers written (Nathan Bossart <nathandbossart@gmail.com>) |
| Ответы |
Re: Odd off-by-one dirty buffers and checkpoint buffers written
|
| Список | pgsql-hackers |
On Tue, Apr 19, 2022 at 4:36 PM Nathan Bossart <nathandbossart@gmail.com> wrote:
On Tue, Apr 19, 2022 at 04:21:21PM -0700, David G. Johnston wrote:
> I've done this four times in a row and while the number of dirty buffers
> shown each time vary (see below) I see that "wrote N buffers" is always
> exactly one more than the total count of dirty buffers. I'm just curious
> if anyone has a quick answer for this unusual correspondence.
I see that SlruInternalWritePage() increments ckpt_bufs_written, so my
first guess would be that it's due to something like CheckPointCLOG().
I peeked at pg_stat_bgwriter and see an increase in buffers_checkpoint matching the dirty buffers number.
I also looked at pg_stat_slru to try and find the corresponding change caused by:
slru.c:766 (SlruPhysicalWritePage)
pgstat_count_slru_page_written(shared->slru_stats_idx);
I do see (Xact) blks_hit change during this process (after the update/commit, not the checkpoint, though) but it increases by 2 when dirty buffers is 4. I was expecting 4, thinking that blocks and buffers and pages are basically the same things (which [1] seems to affirm).
David J.
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