v20-0003-Makes-the-wal-receiver-report-WAL-statistics.patch

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index db4b4e460c..281b13b9fa 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -3493,7 +3493,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Number of times WAL buffers were written out to disk via
-       <function>XLogWrite</function> request.
+       <function>XLogWrite</function> request and WAL data were written
+       out to disk by the WAL receiver process.
        See <xref linkend="wal-configuration"/> for more information about
        the internal WAL function <function>XLogWrite</function>.
       </para></entry>
@@ -3521,7 +3522,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Total amount of time spent writing WAL buffers to disk via
-       <function>XLogWrite</function> request, in milliseconds
+       <function>XLogWrite</function> request and WAL data to disk
+       by the WAL receiver process, in milliseconds
        (if <xref linkend="guc-track-wal-io-timing"/> is enabled,
        otherwise zero).  This includes the sync time when
        <varname>wal_sync_method</varname> is either
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index ae4a3c1293..39e7028c96 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -769,7 +769,7 @@
   </para>
 
   <para>
-   There are two internal functions to write WAL data to disk:
+   There are two internal functions to write generated WAL data to disk:
    <function>XLogWrite</function> and <function>issue_xlog_fsync</function>.
    When <xref linkend="guc-track-wal-io-timing"/> is enabled, the total
    amounts of time <function>XLogWrite</function> writes and
@@ -795,7 +795,19 @@
    <function>issue_xlog_fsync</function> syncs WAL data to disk are also
    counted as <literal>wal_write</literal> and <literal>wal_sync</literal>
    in <structname>pg_stat_wal</structname>, respectively.
-  </para>
+   To write replicated WAL data to disk by the WAL receiver is almost the same
+   as above except for some points. First, there is a dedicated code path for the
+   WAL receiver to write data although <function>issue_xlog_fsync</function> is
+   the same for syncing data.
+   Second, the WAL receiver writes replicated WAL data per bytes from the local
+   memory although the generated WAL data is written per WAL buffer pages.
+   The counters of <literal>wal_write</literal>, <literal>wal_sync</literal>,
+   <literal>wal_write_time</literal>, and <literal>wal_sync_time</literal> are
+   common statistics for writing/syncing both generated and replicated WAL data.
+   But, you can distinguish them because the generated WAL data is written/synced
+   in the primary server and the replicated WAL data is written/synced in
+   the standby server.
+   </para>
  </sect1>
 
  <sect1 id="wal-internals">
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index a7a94d2a83..df028c5039 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -771,6 +771,9 @@ WalRcvDie(int code, Datum arg)
 	/* Ensure that all WAL records received are flushed to disk */
 	XLogWalRcvFlush(true);
 
+	/* Send WAL statistics to the stats collector before terminating */
+	pgstat_send_wal(true);
+
 	/* Mark ourselves inactive in shared memory */
 	SpinLockAcquire(&walrcv->mutex);
 	Assert(walrcv->walRcvState == WALRCV_STREAMING ||
@@ -910,6 +913,12 @@ 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(false);
 			}
 			recvFile = -1;