Re: WAL usage calculation patch

Hello.

The v13 patch seems failing to apply on the master.

At Fri, 3 Apr 2020 06:37:21 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> On Thu, Apr 2, 2020 at 8:06 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Apr 2, 2020 at 6:41 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > 4.
> > > /* # of WAL full page image generated */
> > > Can we change it to "/* # of WAL full page image records generated */"?
> >
> > IMHO, "# of WAL full-page image records" seems like the number of wal
> > record which contains the full-page image.
> >
>
> I think this resembles what you have written here.
>
> > But, actually, this is the
> > total number of the full-page images, not the number of records that
> > have a full-page image.
> >
>
> We count this when forming WAL records. As per my understanding, all
> three counters are about WAL records. This counter tells how many
> records have full page images and one of the purposes of having this
> counter is to check what percentage of records contain full page
> image.

Aside from which is desirable or useful, acutually XLogRecordAssemble
in v13-0001 counts the number of attached images then XLogInsertRecord
sums up the number of images in pgWalUsage.wal_num_fpw.

FWIW, it seems to me that the main concern here is the source of WAL
size. If it is the case I think that the number of full page image is
more useful.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center