Re: per backend WAL statistics

Hi,

On Mon, Mar 10, 2025 at 03:08:49PM +0300, Nazir Bilal Yavuz wrote:
> Hi,
>
> Thank you for working on this!
>
> I just started reading the code and have a couple of questions.

Thanks for looking at it!

> I think that every time we flush IO or WAL stats, we want(?) to flush
> backend stats as well,

Yeah, I think that's happening anyway.

> so would it make sense to move
> pgstat_flush_backend() calls to inside of pgstat_flush_io() and
> pgstat_wal_flush_cb()?

I don't think so because pgstat_flush_backend() still needs to be called by the
pgstat_backend_flush_cb() (i.e flush_static_cb) callback (I mean I think this
makes sense to keep this callback around and that it does "really" something).

So for example, for the WAL case, that would mean the backend WAL stats would be
flushed twice: one time from pgstat_wal_flush_cb() (i.e flush_static_cb) callback
and one time from the pgstat_backend_flush_cb() (another flush_static_cb) callback.

I think it's better to keep them separate and reason as they are distinct
types of stats (which they really are). I think we had the same kind of reasoning
while working on [1].

> I see that backend statistics are not collected
> for some of the backend types but that is already checked in the
> pgstat_flush_backend() with pgstat_tracks_backend_bktype().

Sorry, I don't get it. Do you have a question around that?

> Also, is there a chance that wal_bytes gets incremented without
> wal_records getting incremented? I searched the code and did not find
> any example of that but I just wanted to be sure. If there is a case
> like that, then pgstat_backend_wal_have_pending() needs to check
> wal_bytes instead of wal_records.

I think that's fine. That's also how pgstat_wal_have_pending_cb() has been
re-factored in 2421e9a51d2.

[1]: https://www.postgresql.org/message-id/flat/Z0QjeIkwC0HNI16K%40ip-10-97-1-34.eu-west-3.compute.internal#16762c1767ffb168318e7c3734fa5f64

Regards,

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