From 0914ef57c75e68a391ff7330e4d9bafaffec35e8 Mon Sep 17 00:00:00 2001
From: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Date: Fri, 29 Jan 2021 16:46:30 +0900
Subject: [PATCH 2/2] Makes the wal receiver report WAL statistics

This patch makes the WAL receiver report WAL statistics
and fundamentally changes how the stas collector's behaves
with regards to that function and how it interacts with
the WAL receiver.

Author: Masahiro Ikeda
Reviewed-By: Japin Li, Hayato Kuroda, Masahiko Sawada, David Johnston
Discussion: https://postgr.es/m/0509ad67b585a5b86a83d445dfa75392@oss.nttdata.com
---
 doc/src/sgml/monitoring.sgml          |  3 +-
 src/backend/replication/walreceiver.c | 40 +++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 2435f401db..da48e6f946 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -3494,7 +3494,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       <para>
        Number of times WAL buffers were written out to disk via 
        <function>XLogWrite</function>, which nomally called by an 
-       <function>XLogFlush</function> request(see <xref linkend="wal-configuration"/>)
+       <function>XLogFlush</function> request(see <xref linkend="wal-configuration"/>),
+       or WAL data written out to disk by WAL receiver.
       </para></entry>
      </row>
 
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index eaf5ec9a72..73435b616c 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -485,7 +485,11 @@ WalReceiverMain(void)
 
 				/* Check if we need to exit the streaming loop. */
 				if (endofwal)
+				{
+					/* Send WAL statistics to the stats collector */
+					pgstat_send_wal();
 					break;
+				}
 
 				/*
 				 * Ideally we would reuse a WaitEventSet object repeatedly
@@ -550,8 +554,16 @@ WalReceiverMain(void)
 														wal_receiver_timeout);
 
 						if (now >= timeout)
+						{
+							/*
+							 * Send WAL statistics to the stats collector
+							 * before terminating
+							 */
+							pgstat_send_wal();
+
 							ereport(ERROR,
 									(errmsg("terminating walreceiver due to timeout")));
+						}
 
 						/*
 						 * We didn't receive anything new, for half of
@@ -874,6 +886,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr)
 	while (nbytes > 0)
 	{
 		int			segbytes;
+		instr_time	start;
 
 		if (recvFile < 0 || !XLByteInSeg(recptr, recvSegNo, wal_segment_size))
 		{
@@ -910,6 +923,13 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr)
 					XLogArchiveForceDone(xlogfname);
 				else
 					XLogArchiveNotify(xlogfname);
+
+				/*
+				 * Send WAL statistics to the stats collector when finishing
+				 * the current WAL segment file to avoid overloading it.
+				 */
+				pgstat_send_wal();
+
 			}
 			recvFile = -1;
 
@@ -931,7 +951,27 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr)
 		/* OK to write the logs */
 		errno = 0;
 
+		/* Measure I/O timing to write WAL data */
+		if (track_wal_io_timing)
+			INSTR_TIME_SET_CURRENT(start);
+
 		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+
+		/*
+		 * Increment the I/O timing and the number of times WAL data were
+		 * written out to disk.
+		 */
+		if (track_wal_io_timing)
+		{
+			instr_time	duration;
+
+			INSTR_TIME_SET_CURRENT(duration);
+			INSTR_TIME_SUBTRACT(duration, start);
+			WalStats.m_wal_write_time = INSTR_TIME_GET_MICROSEC(duration);
+		}
+
+		WalStats.m_wal_write++;
+
 		if (byteswritten <= 0)
 		{
 			char		xlogfname[MAXFNAMELEN];
-- 
2.25.1