Date: Mon, 6 Apr 2020 11:33:34 +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: <20200406093334.GJ1206@nol>
References: 
 <CAA4eK1KqiWqAgw9ETchcQf-z9ZY4aUN-4QvTh0R26+DpNnTDHg@mail.gmail.com>
 <CAA4eK1+sSXvf45x6CbXTEywzXLNcuwWUy-m1rg2KyJKcEXBPvw@mail.gmail.com>
 <20200404060303.n2q74lbccjm2swdc@nol>
 <CAA4eK1Kep_Wv1ZK+ODLhioPdP8VD4HuHx5d-C2h9s83yriYejg@mail.gmail.com>
 <20200404085423.GC1206@nol>
 <CAA4eK1JmoDH1zoxAi9BEszVe+5BNquJDN3E-RRADdrYHv3TCRQ@mail.gmail.com>
 <20200404092015.GE1206@nol>
 <CAA4eK1+OzW92A=nrJqNg6+M5jZ_fdpSAW45tBnFpdWv4035QRQ@mail.gmail.com>
 <20200406082307.GI1206@nol>
 <CAA4eK1JCkJFAx+hj3HS_MmM0nvnAphAZz1SubZMngOxNVy23=A@mail.gmail.com>
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On Mon, Apr 06, 2020 at 02:34:36PM +0530, Amit Kapila wrote:
> On Mon, Apr 6, 2020 at 1:53 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> >
> > On Mon, Apr 06, 2020 at 08:55:01AM +0530, Amit Kapila wrote:
> > >
> > > 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.
> >
> 
> I agree but we already have a way via pg_stat_statements to find it if
> the metric is so useful.
> 

Agreed.

> 
> >  That being said I totally agree with you that both
> > should be displayed.  Should I send a patch to also expose it?
> >
> 
> I think this should be a separate proposal.  Let's not add things
> unless they are really essential.  We can separately discuss of
> enhancing vacuum verbose for Buffer and WAL usage stats and see if
> others also find that information useful.  I think you can send a
> patch by removing the code I mentioned above if you agree.  Thanks for
> working on this.

Thanks!  v15 attached.

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From 9f4cbb1817372628a3fc7b3fffe82ec2a3942f91 Mon Sep 17 00:00:00 2001
From: Julien Rouhaud <julien.rouhaud@free.fr>
Date: Thu, 19 Mar 2020 16:08:47 +0100
Subject: [PATCH v15] Expose WAL usage counters in verbose (auto)vacuum output.

Author: Julien Rouhaud
Reviewed-by: Fuji Masao, Amit Kapila, Dilip Kumar
Discussion: https://postgr.es/m/CAB-hujrP8ZfUkvL5OYETipQwA=e3n7oqHFU=4ZLxWS_Cza3kQQ@mail.gmail.com
---
 src/backend/access/heap/vacuumlazy.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 3ca7f5d136..877512fae4 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -410,6 +410,8 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
 	int			nindexes;
 	PGRUsage	ru0;
 	TimestampTz starttime = 0;
+	WalUsage walusage_start = pgWalUsage;
+	WalUsage walusage = {0, 0, 0};
 	long		secs;
 	int			usecs;
 	double		read_rate,
@@ -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);
 
 			ereport(LOG,
 					(errmsg_internal("%s", buf.data)));
@@ -1727,6 +1738,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 									"%u pages are entirely empty.\n",
 									empty_pages),
 					 empty_pages);
+
 	appendStringInfo(&buf, _("%s."), pg_rusage_show(&ru0));
 
 	ereport(elevel,
-- 
2.20.1


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