At Thu, 25 Mar 2021 19:01:23 +0900, Fujii Masao <masao.fu...@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
