Re: [BUG]: the walsender does not update its IO statistics until it exits

Hi,

On Wed, Feb 26, 2025 at 05:08:17AM -0500, Andres Freund wrote:
> Hi,
>
> On 2025-02-26 15:37:10 +0900, Michael Paquier wrote:
> > That's bad, worse for a logical WAL sender, because it means that we
> > have no idea what kind of I/O happens in this process until it exits,
> > and logical WAL senders could loop forever, since v16 where we've
> > begun tracking I/O.
>
> FWIW, I think medium term we need to work on splitting stats flushing into two
> separate kinds of flushes:
> 1) non-transactional stats, which should be flushed at a regular interval,
> unless a process is completely idle
> 2) transaction stats, which can only be flushed at transaction boundaries,
> because before the transaction boundary we don't know if e.g. newly
> inserted rows should be counted as live or dead
>
> So far we have some timer logic for 2), but we have basically no support for
> 1). Which means we have weird ad-hoc logic in various kinds of
> non-plain-connection processes. And that will often have holes, as Bertrand
> noticed here.
>
> I think it's also bad that we don't have a solution for 1), even just for
> normal connections. If a backend causes a lot of IO we might want to know
> about that long before the longrunning transaction commits.
>
> I suspect the right design here would be to have a generalized form of the
> timeout mechanism we have for 2).
>
> For that we'd need to make sure that pgstat_report_stat() can be safely called
> inside a transaction.

I can see an issue with GetCurrentTransactionStopTimestamp(), any other issue
/concern you have in mind?

Regards,

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