Re: wal stats questions

At Thu, 25 Mar 2021 19:01:23 +0900, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote in
> On 2021/03/25 16:37, Kyotaro Horiguchi wrote:
> > pgWalUsage was used without resetting and we (I) thought that that
> > behavior should be preserved. On second thought, as Andres suggested,
> > we can just reset pgWalUsage at sending since AFAICS no one takes
> > difference crossing pgstat_report_stat() calls.
>
> Yes, I agree that we can do that since there seems no such code for
> now.
> Also if we do that, we can check, for example "pgWalUsage.wal_records
> > 0"
> as you suggested, to easily determine whether there is pending WAL
> stats or not.
> Anyway I agree it's better to add comments about the design more.
...
> > If any wal activity has been recorded, wal_records is always positive
> > thus we can check for wal activity just by "pgWalUsage.wal_records >
> > 0, which should be fast enough.
>
> Maybe there is the case where a backend generates no WAL records,
> but just writes them because it needs to do write-ahead-logging
> when flush the table data? If yes, "pgWalUsage.wal_records > 0" is not
> enough.
> Probably other fields also need to be checked.

(I noticed I made the discussion above unconsciously premising
pgWalUsage reset.)

I may be misunderstanding or missing something, but the only place
where pgWalUsage counters are increased is XLogInsertRecrod. That is,
pgWalUsage counts wal insertions, not writes nor flushes. AFAICS
pgWalUsage.wal_records is always incremented when other counters are
increased. Looking from another side, we should refrain from adding
counters that incrases at a different time than
pgWalUsage.wal_recrods. (That should be written as a comment there.)

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center