Hi, Thanks for the feedback.
On Mon, 20 Nov 2023 at 10:47, Michael Paquier <mich...@paquier.xyz> wrote: > > On Thu, Nov 09, 2023 at 02:39:26PM +0300, Nazir Bilal Yavuz wrote: > > There are some differences between pg_stat_wal and pg_stat_io while > > collecting WAL stats. For example in the XLogWrite() function in the > > xlog.c file, pg_stat_wal counts wal_writes as write system calls. This > > is not something we want for pg_stat_io since pg_stat_io counts the > > number of blocks rather than the system calls, so instead incremented > > pg_stat_io by npages. > > > > Could that cause a problem since pg_stat_wal's behaviour will be > > changed? Of course, as an alternative we could change pg_stat_io's > > behaviour but in the end either pg_stat_wal's or pg_stat_io's > > behaviour will be changed. > > Yep, that could be confusing for existing applications that track the > information of pg_stat_wal. The number of writes is not something > that can be correctly shared between both. The timings for the writes > and the syncs could be shared at least, right? Yes, the timings for the writes and the syncs should work. Another question I have in mind is the pg_stat_reset_shared() function. When we call it with 'io' it will reset pg_stat_wal's timings and when we call it with 'wal' it won't reset them, right? > > This slightly relates to pgstat_count_io_op_n() in your latest patch, > where it feels a bit weird to see an update of > PendingWalStats.wal_sync sit in the middle of a routine dedicated to > pg_stat_io.. I am not completely sure what's the right balance here, > but I would try to implement things so as pg_stat_io paths does not > need to know about PendingWalStats. Write has block vs system calls differentiation but it is the same for sync. Because of that I put PendingWalStats.wal_sync to pg_stat_io but I agree that it looks a bit weird. -- Regards, Nazir Bilal Yavuz Microsoft
