Re: Show WAL write and fsync stats in pg_stat_io

Hi,

Thanks for looking into this!

On Fri, 24 Jan 2025 at 17:20, Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)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. I could not
understand the cause of the difference at first glance. I will look
into this and will come back to you.

Quick note: WAL stats in pg_stat_io view includes WAL initialization
stats under object = 'wal' and context = 'init', your query may count
these initialization stats too. So the correct query is:

'select sum(write_bytes),stats_reset from pg_stat_io where object =
'wal' and context = 'normal' group by stats_reset;'.

By saying that, this does not solve the problem; there is still a
difference although you omit WAL initialization stats from the
pg_stat_io.

--
Regards,
Nazir Bilal Yavuz
Microsoft