Date: Mon, 6 Apr 2020 10:23:07 +0200
From: Julien Rouhaud <rjuju123@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>,
	Kuntal Ghosh <kuntalghosh.2007@gmail.com>,
	Fujii Masao <masao.fujii@oss.nttdata.com>,
	Kirill Bychik <kirill.bychik@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: WAL usage calculation patch
Message-ID: <20200406082307.GI1206@nol>
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On Mon, Apr 06, 2020 at 08:55:01AM +0530, Amit Kapila wrote:
> On Sat, Apr 4, 2020 at 2:50 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> >
> I have pushed pg_stat_statements and Explain related patches.  I am
> now looking into (auto)vacuum patch and have few comments.
> 

Thanks!

> @@ -614,6 +616,9 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
> 
>   TimestampDifference(starttime, endtime, &secs, &usecs);
> 
> + memset(&walusage, 0, sizeof(WalUsage));
> + WalUsageAccumDiff(&walusage, &pgWalUsage, &walusage_start);
> +
>   read_rate = 0;
>   write_rate = 0;
>   if ((secs > 0) || (usecs > 0))
> @@ -666,7 +671,13 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
>   (long long) VacuumPageDirty);
>   appendStringInfo(&buf, _("avg read rate: %.3f MB/s, avg write rate:
> %.3f MB/s\n"),
>   read_rate, write_rate);
> - appendStringInfo(&buf, _("system usage: %s"), pg_rusage_show(&ru0));
> + appendStringInfo(&buf, _("system usage: %s\n"), pg_rusage_show(&ru0));
> + appendStringInfo(&buf,
> + _("WAL usage: %ld records, %ld full page writes, "
> +    UINT64_FORMAT " bytes"),
> + walusage.wal_records,
> + walusage.wal_num_fpw,
> + walusage.wal_bytes);
> 
> Here, we are not displaying Buffers related data, so why do we think
> it is important to display WAL data?  I see some point in displaying
> Buffers and WAL data in a vacuum (verbose), but I feel it is better to
> make a case for both the statistics together rather than just
> displaying one and leaving other.  I think the other change related to
> autovacuum stats seems okay to me.

One thing is that the amount of WAL, and more precisely FPW, is quite
unpredictable wrt. vacuum and even more anti-wraparound vacuum, so this is IMHO
a very useful metric.  That being said I totally agree with you that both
should be displayed.  Should I send a patch to also expose it?