Re: WAL usage calculation patch

On Thu, Apr 23, 2020 at 2:35 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Apr 23, 2020 at 12:16 PM Peter Eisentraut
> <peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
>
> > The internal symbol for the WAL record is
> > XLOG_FPI and xlogdesc.c prints it as "FPI".
> >
>
> That is just one way/reason we log the page. There are others as
> well. I thought here we are computing the number of full-page writes
> happened in the system due to various reasons like (a) a page is
> operated upon first time after the checkpoint, (b) log the XLOG_FPI
> record, (c) Guc for WAL consistency checker is on, etc. If we see in
> XLogRecordAssemble where we decide to log this information, there is a
> comment " .... log a full-page write for the current block." and there
> was an existing variable with 'fpw_lsn' which indicates to an extent
> that what we are computing in this patch is full-page writes. But
> there is a reference to full-page image as well. I think as
> full_page_writes is an exposed variable that is well understood so
> exposing information with similar name via this patch doesn't sound
> illogical to me. Whatever we use here we need to be consistent all
> throughout, even pg_stat_statements need to name exposed variable as
> wal_fpi instead of wal_fpw.
>
> To me, full-page writes sound more appealing with other WAL usage
> variables like records and bytes. I might be more used to this term as
> 'fpw' that is why it occurred better to me. OTOH, if most of us think
> that a full-page image is better suited here, I am fine with changing
> it at all places.
>

Julien, Peter, others do you have any opinion here? I think it is
better if we decide on one of FPW or FPI and make the changes at all
places for this patch.

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