Re: per backend I/O statistics

Hi,

On Tue, Oct 08, 2024 at 01:46:23PM +0900, Michael Paquier wrote:
> On Mon, Oct 07, 2024 at 09:54:21AM +0000, Bertrand Drouvot wrote:
> > On Fri, Sep 20, 2024 at 01:26:49PM +0900, Michael Paquier wrote:
> >> This would rely on the fact that we would use the ProcNumber for the
> >> dshash key, and this information is not provided in pg_stat_activity.
> >> Perhaps we should add this information in pg_stat_activity so as it
> >> would be easily possible to do joins with a SQL function that returns
> >> a SRF with all the stats associated with a given connection slot
> >> (auxiliary or backend process)?
> >
> > I'm not sure that's needed. What has been done in the previous versions is
> > to get the stats based on the pid (see pg_stat_get_backend_io()) where the
> > procnumber is retrieved with something like GetNumberFromPGProc(BackendPidGetProc(pid)).
>
> Ah, I see. So you could just have the proc number in the key to
> control the upper-bound on the number of possible stats entries in the
> dshash.

Yes.

> Assuming that none of this data is persisted to the stats file at
> shutdown and that the stats of a single entry are reset each time a
> new backend reuses a previous proc slot, that would be OK by me, I
> guess.
>
> >> The active PIDs of the live sessions are not stored in the active
> >> stats, why not?
> >
> > That was not needed. We can still retrieve the stats based on the pid thanks
> > to something like GetNumberFromPGProc(BackendPidGetProc(pid)) without having
> > to actually store the pid in the stats. I think that's fine because the pid
> > only matters at "display" time (pg_stat_get_backend_io()).
>
> Okay, per the above and the persistency of the stats.

Great, I'll work on an updated patch version then.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com