Re: wal stats questions - Mailing list pgsql-hackers

From Kyotaro Horiguchi
Subject Re: wal stats questions
Date
Msg-id 20210326.100828.1578001749705763697.horikyota.ntt@gmail.com
Whole thread Raw
In response to Re: wal stats questions  (Fujii Masao <masao.fujii@oss.nttdata.com>)
Responses Re: wal stats questions
List pgsql-hackers
At Thu, 25 Mar 2021 19:01:23 +0900, Fujii Masao <masao.fujii@oss.nttdata.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



pgsql-hackers by date:

Previous
From: Fujii Masao
Date:
Subject: Re: make the stats collector shutdown without writing the statsfiles if the immediate shutdown is requested.
Next
From: Fujii Masao
Date:
Subject: Re: Stronger safeguard for archive recovery not to miss data