Re: WAL usage calculation patch

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(at)gmail(dot)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?