Re: per backend I/O statistics

Hi,

On Wed, Nov 20, 2024 at 09:01:26AM +0900, Michael Paquier wrote:
> On Tue, Nov 19, 2024 at 04:28:55PM +0000, Bertrand Drouvot wrote:
> > So, for the startup process only, what about?
> >
> > - don't call pgstat_create_backend_stat() in pgstat_beinit()...
> > - but call it in StartupXLOG() instead (after the stats are discarded or restored).
> >
> > That way we could get rid of the changes linked to the assertion and still handle
> > the startup process particular case. Thoughts?
>
> Hmm. That may prove to be a good idea in the long-term. The startup
> process is a specific path kicked in at a very early stage, so it is
> also a bit weird that we'd try to insert statistics while we are going
> to reset them anyway a bit later.

Exactly.

> That may also be relevant for
> custom statistics, actually, especially if some calls happen in some
> hook paths taken before the reset is done. This could happen for
> injection point stats when loading it with shared_preload_libraries,
> actually, if you load, attach or run a callback at this early stage.

Right. I did not had in mind to go that far here (for the per backend stats
needs). My idea was "just" to move the new pgstat_create_backend_stat() (which
is related to per backend stats only) call at the right place in StartupXLOG()
for the startup process only. As that's the startup process that will reset
or restore the stats I think that makes sense.

It looks like that what you have in mind is much more generic, what about:

- Focus on this thread first and then move the call as proposed above
- Think about a more generic idea later on (on the per-backend I/O stats is
in).

Thoughts?

Regards,

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