On Wed, 21 Jun 2023 at 13:04, Nazir Bilal Yavuz <byavu...@gmail.com> wrote: > > Hi, > > 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. The only path that I see in XLogWrite() that could potentially put anything into WAL is through RequestCheckpoint(), but that only writes out a checkpoint when it is not in a postmaster environment - in all other cases it will wake up the checkpointer and wait for that checkpoint to finish. 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. But on the topic of reporting the WAL stats in bgwriter; that seems like a good idea to fix, yes. +1 Kind regards, Matthias van de Meent Neon, Inc.
