Re: WAL usage calculation patch

On Fri, Apr 3, 2020 at 9:35 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Fri, Apr 3, 2020 at 9:17 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Fri, Apr 3, 2020 at 9:02 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Apr 3, 2020 at 8:55 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > I think now I got the reason. Basically, both of these records are
> > > > storing the FPW, and FPW size can vary based on the hole size on the
> > > > page. If hold size is smaller the image length will be more, the
> > > > image_len= BLCKSZ-hole_size. So in subsequent records, the image size
> > > > is bigger.
> > > >
> > >
> > > This means if we always re-create the database or may be keep
> > > full_page_writes to off, then we should get consistent WAL usage data
> > > for all tests.
> >
> > With new database, it is always the same. But, with full-page write,
> > I could see one of the create index is writing extra wal and if we
> > change the older then the new create index at that place will write
> > extra wal. I guess that could be due to a non-in place update in some
> > of the system tables.
>
> I have analyzed the WAL and there could be multiple reasons for the
> same. With small data, I have noticed that while inserting in the
> system index there was a Page Split and that created extra WAL.
>

Thanks for the investigation. I think it is clear that we can't
expect the same WAL size even if we repeat the same operation unless
it is a fresh database.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com