Hi, On Mon, 27 Jan 2025 at 10:45, Bertrand Drouvot <bertranddrouvot...@gmail.com> wrote: > > Hi, > > On Fri, Jan 24, 2025 at 06:29:46PM +0300, Nazir Bilal Yavuz wrote: > > Hi, > > > > Thanks for looking into this! > > > > On Fri, 24 Jan 2025 at 17:20, Bertrand Drouvot > > <bertranddrouvot...@gmail.com> wrote: > > > > > > I did not look at the code yet but did a few tests. > > > I can see diff between pg_stat_wal and pg_stat_io, for example: > > > > > > " > > > postgres=# select pg_stat_reset_shared(); > > > pg_stat_reset_shared > > > ---------------------- > > > > > > (1 row) > > > > > > postgres=# insert into bdt select a from generate_series(1,200000) a ; > > > INSERT 0 200000 > > > > > > postgres=# select wal_bytes,stats_reset from pg_stat_wal; > > > wal_bytes | stats_reset > > > -----------+------------------------------- > > > 11800088 | 2025-01-24 14:17:28.507994+00 > > > (1 row) > > > > > > postgres=# select sum(write_bytes),stats_reset from pg_stat_io where > > > object = 'wal' group by stats_reset; > > > sum | stats_reset > > > ----------+------------------------------- > > > 12853248 | 2025-01-24 14:17:28.507988+00 > > > (1 row) > > > > > > Is that expected? > > > > I am not sure, I think they need to be the same. > > Yeah I think so (or at least we should document the reason(s) why (and how) > they could differ). > > > I could not > > understand the cause of the difference at first glance. I will look > > into this and will come back to you. > > Thanks.
Sorry for replying late. It turns out that the difference is expected. It is because pg_stat_wal.wal_bytes tracks the total amount of WAL generated, while pg_stat_io tracks the I/Os that are written to disk. >From the docs of pg_stat_io: pg_stat_io view will contain one row for each combination of backend type, target I/O object, and I/O context, showing cluster-wide I/O statistics. Combinations which do not make sense are omitted. >From the docs of pg_stat_wal.wal_bytes: Total amount of WAL generated in bytes. It is explicitly said that pg_stat_io tracks I/O operations and pg_stat_wal.wal_bytes tracks total amount of WAL generated in bytes. I think this is clear enough. Do you think we still need additional explanation? -- Regards, Nazir Bilal Yavuz Microsoft
