From: | Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
---|---|
To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
Cc: | Julien Rouhaud <rjuju123(at)gmail(dot)com>, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>, Kirill Bychik <kirill(dot)bychik(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
Subject: | Re: WAL usage calculation patch |
Date: | 2020-04-03 04:05:13 |
Message-ID: | CAFiTN-vAqkDV5Whxw2OvdbN9OU7xZyKzRrUG6m6dr8nvG-R=Yw@mail.gmail.com |
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Lists: | pgsql-hackers |
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.
--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com
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