Re: per backend WAL statistics

Hi,

On Fri, Mar 07, 2025 at 02:42:13PM +0900, Michael Paquier wrote:
> On Thu, Mar 06, 2025 at 10:33:52AM +0000, Bertrand Drouvot wrote:
> > Indeed, there is no reason for pgstat_backend_have_pending_cb() to return true if
> > pgstat_tracks_backend_bktype() is not satisfied.
> >
> > So it deserves a dedicated patch to fix this already existing issue:
> > 0001 attached.
>
> pgstat_backend_have_pending_cb(void)
> {
> - return (!pg_memory_is_all_zeros(&PendingBackendStats,
> - sizeof(struct PgStat_BackendPending)));
> + if (!pgstat_tracks_backend_bktype(MyBackendType))
> + return false;
> + else
> + return (!pg_memory_is_all_zeros(&PendingBackendStats,
> + sizeof(struct PgStat_BackendPending)));
>
> So, if I understand your point correctly, it is not a problem on HEAD
> because we are never going to update PendingBackendStats in the
> checkpointer as pgstat_count_backend_io_op[_time]() blocks any attempt
> to do so.

I think this is wrong on HEAD because we initialize PendingBackendStats to
zeros in pgstat_create_backend() based on the backend type (pgstat_tracks_backend_bktype()).

But when it's time to flush, then pgstat_backend_have_pending_cb() checks
for zeros in PendingBackendStats *without* any check on the backend type.

I think the issue is "masked" on HEAD because PendingBackendStats is
probably automatically initialized with zeros (as being a static variable at
file scope).

Regards,

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