Re: per backend I/O statistics

Hi,

On Tue, 17 Sept 2024 at 16:07, Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> On Tue, Sep 17, 2024 at 02:52:01PM +0300, Nazir Bilal Yavuz wrote:
> > Could we remove pg_stat_get_my_io() completely and use
> > pg_stat_get_backend_io() with the current backend's pid to get the
> > current backend's stats?
>
> The reason why I keep pg_stat_get_my_io() is because (as mentioned in [1]), the
> statistics snapshot is build for "my backend stats" (means it depends of the
> stats_fetch_consistency value). I can see use cases for that.
>
> OTOH, pg_stat_get_backend_io() behaves as if stats_fetch_consistency is set to
> none (each execution re-fetches counters from shared memory) (see [2]). Indeed,
> the snapshot is not build in each backend to copy all the others backends stats,
> as 1/ I think that there is no use case (there is no need to get others backends
> I/O statistics while taking care of the stats_fetch_consistency) and 2/ that
> could be memory expensive depending of the number of max connections.
>
> So I think it's better to keep both functions as they behave differently.
>
> Thoughts?

Yes, that is correct. Sorry, you already had explained it and I had
agreed with it but I forgot.

> > If you meant the same thing, please don't
> > mind it.
>
> What I meant to say is to try to remove the duplicate code that we can find in
> those 3 functions (say creating a new function that would contain the duplicate
> code and make use of this new function in the 3 others). Did not look at it in
> details yet.

I got it, thanks for the explanation.

--
Regards,
Nazir Bilal Yavuz
Microsoft