Re: WAL usage calculation patch

On Thu, Mar 5, 2020 at 8:55 PM Kirill Bychik <kirill(dot)bychik(at)gmail(dot)com> wrote:
>
> > While at it, did you consider adding a full-page image counter in the WalUsage?
> > That's something I'd really like to have and it doesn't seem hard to integrate.
>
> Well, not sure I understand you 100%, being new to Postgres dev. Do
> you want a separate counter for pages written whenever doPageWrites is
> true? I can do that, if needed. Please confirm.

Yes, I meant a separate 3rd counter for the number of full page images
written. However after a quick look I think that a FPI should be
detected with (doPageWrites && fpw_lsn != InvalidXLogRecPtr && fpw_lsn
<= RedoRecPtr).

> > Another point is that this patch won't help to see autovacuum activity.
> > As an example, I did a quick te.....
> > ...LONG QUOTE...
> > but that may seem strange to only account for (auto)vacuum activity, rather
> > than globally, grouping per RmgrId or CommandTag for instance. We could then
> > see the complete WAL usage per-database. What do you think?
>
> I wanted to keep the patch small and simple, and fit to practical
> needs. This patch is supposed to provide tuning assistance, catching
> an io heavy query in commit-bound situation.
> Total WAL usage per DB can be assessed rather easily using other means.
> Let's get this change into the codebase and then work on connecting
> WAL usage to (auto)vacuum stats.

I agree that having a view of the full activity is a way bigger scope,
so it could be done later (and at this point in pg14), but I'm still
hoping that we can get insight of other backend WAL activity, such as
autovacuum, in pg13.