Re: pg_stat_io for the startup process

Hi,

On 2023-05-22 13:36:42 +0900, Kyotaro Horiguchi wrote:
> At Sun, 21 May 2023 15:14:23 -0700, Andres Freund <andres@anarazel.de> wrote in 
> > Hi,
> > 
> > I don't really feel confident we're going to get the timeout approach solid
> > enough for something going into the tree around beta 1.
> > 
> > How about this, imo a lot simpler, approach: We flush stats whenever replaying
> > a XLOG_RUNNING_XACTS record. Unless the primary is idle, it will log those at
> > a regular interval. If the primary is idle, we don't need to flush stats in
> > the startup process, because we'll not have done any io.
> > 
> > We only log XLOG_RUNNING_XACTS when wal_level >= replica, so stats wouldn't
> > get regularly flushed if wal_level = minimal - but in that case the stats are
> > also not accessible, so that's not a problem.
> 
> I concur with the three aspects, interval regularity, reliability and
> about the case of the minimal wal_level. While I can't confidently
> claim it is the best, its simplicilty gives us a solid reason to
> proceed with it.  If necessary, we can switch to alternative timing
> sources in the future without causing major disruptions for users, I
> believe.
> 
> > It's not the prettiest solution, but I think the simplicity is worth a lot.
> > 
> > Greetings,
> 
> +1.

Attached a patch implementing this approach.

It's imo always a separate bug that we were using
GetCurrentTransactionStopTimestamp() when force was passed in - that timestamp
can be quite out of date in some cases. But I don't immediately see a
noticeable consequence, so ...

Greetings,

Andres Freund