Re: New statistics for tuning WAL buffer size

On Mon, Sep 28, 2020 at 7:00 AM Masahiro Ikeda <ikedamsh(at)oss(dot)nttdata(dot)com> wrote:
>
> On 2020-09-26 19:18, Amit Kapila wrote
>
> > This makes sense to me. I think even if such background processes have
> > to write WAL due to wal_buffers, it will be accounted next time the
> > backend sends the stats.
>
> Thanks for your comments.
>
> IIUC, since each process counts WalStats.m_wal_buffers_full,
> backend can't send the counter which other background processes have to
> write WAL due to wal_buffers.
>

Right, I misunderstood it.

> Although we can't track all WAL activity, the impact on the statistics
> is minimal so we can ignore it.
>

Yeah, that is probably true.

> > One minor point, don't we need to reset the counter
> > WalStats.m_wal_buffers_full once we sent the stats, otherwise the same
> > stats will be accounted multiple times.
>
> Now, the counter is reset in pgstat_send_wal.
> Isn't it enough?
>

That should be enough.

One other thing that occurred to me today is can't we keep this as
part of PgStat_GlobalStats? We can use pg_stat_reset_shared('wal'); to
reset it. It seems to me this is a cluster-wide stats and somewhat
similar to some of the other stats we maintain there.

--
With Regards,
Amit Kapila.