Add pg_stat_wal statistics view.

This view shows the statistics about WAL activity. Currently it has only
two columns: wal_buffers_full and stats_reset. wal_buffers_full column
indicates the number of times WAL data was written to the disk because
WAL buffers got full. This information is useful when tuning wal_buffers.
stats_reset column indicates the time at which these statistics were
last reset.

pg_stat_wal view is also the basic infrastructure to expose other
various statistics about WAL activity later.

Bump PGSTAT_FILE_FORMAT_ID due to the change in pgstat format.

Bump catalog version.

Author: Masahiro Ikeda
Reviewed-by: Takayuki Tsunakawa, Kyotaro Horiguchi, Amit Kapila, Fujii Masao
Discussion: https://postgr.es/m/188bd3f2d2233cf97753b5ced02bb050@oss.nttdata.com

Author: MasaoFujii

doc/src/sgml/monitoring.sgml

@@ -424,6 +424,14 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
      </entry>
      </row>
 
+     <row>
+      <entry><structname>pg_stat_wal</structname><indexterm><primary>pg_stat_wal</primary></indexterm></entry>
+      <entry>One row only, showing statistics about WAL activity. See
+       <link linkend="monitoring-pg-stat-wal-view">
+       <structname>pg_stat_wal</structname></link> for details.
+      </entry>
+     </row>
+
      <row>
       <entry><structname>pg_stat_database</structname><indexterm><primary>pg_stat_database</primary></indexterm></entry>
       <entry>One row per database, showing database-wide statistics. See
@@ -3280,6 +3288,56 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
 
  </sect2>
 
+ <sect2 id="monitoring-pg-stat-wal-view">
+   <title><structname>pg_stat_wal</structname></title>
+
+  <indexterm>
+   <primary>pg_stat_wal</primary>
+  </indexterm>
+
+  <para>
+   The <structname>pg_stat_wal</structname> view will always have a
+   single row, containing data about WAL activity of the cluster.
+  </para>
+
+  <table id="pg-stat-wal-view" xreflabel="pg_stat_wal">
+   <title><structname>pg_stat_wal</structname> View</title>
+   <tgroup cols="1">
+    <thead>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       Column Type
+      </para>
+      <para>
+       Description
+      </para></entry>
+     </row>
+    </thead>
+
+    <tbody>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_buffers_full</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Number of times WAL data was written to the disk because WAL buffers got full
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
+      </para>
+      <para>
+       Time at which these statistics were last reset
+      </para></entry>
+     </row>
+     </tbody>
+   </tgroup>
+  </table>
+
+</sect2>
+
  <sect2 id="monitoring-pg-stat-database-view">
   <title><structname>pg_stat_database</structname></title>
 
@@ -4668,8 +4726,9 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
         argument.  The argument can be <literal>bgwriter</literal> to reset
         all the counters shown in
         the <structname>pg_stat_bgwriter</structname>
-        view, or <literal>archiver</literal> to reset all the counters shown in
-        the <structname>pg_stat_archiver</structname> view.
+        view, <literal>archiver</literal> to reset all the counters shown in
+        the <structname>pg_stat_archiver</structname> view or <literal>wal</literal>
+        to reset all the counters shown in the <structname>pg_stat_wal</structname> view.
        </para>
        <para>
         This function is restricted to superusers by default, but other users

src/backend/access/transam/xlog.c

@@ -2196,6 +2196,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 					WriteRqst.Flush = 0;
 					XLogWrite(WriteRqst, false);
 					LWLockRelease(WALWriteLock);
+					WalStats.m_wal_buffers_full++;
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */

src/backend/catalog/system_views.sql

@@ -979,6 +979,12 @@ CREATE VIEW pg_stat_bgwriter AS
         pg_stat_get_buf_alloc() AS buffers_alloc,
         pg_stat_get_bgwriter_stat_reset_time() AS stats_reset;
 
+CREATE VIEW pg_stat_wal AS
+    SELECT
+        w.wal_buffers_full,
+        w.stats_reset
+    FROM pg_stat_get_wal() w;
+
 CREATE VIEW pg_stat_progress_analyze AS
     SELECT
         S.pid AS pid, S.datid AS datid, D.datname AS datname,

src/backend/postmaster/checkpointer.c

@@ -504,6 +504,9 @@ CheckpointerMain(void)
 		 */
 		pgstat_send_bgwriter();
 
+		/* Send WAL statistics to the stats collector. */
+		pgstat_send_wal();
+
 		/*
 		 * If any checkpoint flags have been set, redo the loop to handle the
 		 * checkpoint without sleeping.

src/backend/postmaster/pgstat.c

@@ -135,11 +135,12 @@ char	   *pgstat_stat_filename = NULL;
 char	   *pgstat_stat_tmpname = NULL;
 
 /*
- * BgWriter global statistics counters (unused in other processes).
- * Stored directly in a stats message structure so it can be sent
- * without needing to copy things around.  We assume this inits to zeroes.
+ * BgWriter and WAL global statistics counters.
+ * Stored directly in a stats message structure so they can be sent
+ * without needing to copy things around.  We assume these init to zeroes.
  */
 PgStat_MsgBgWriter BgWriterStats;
+PgStat_MsgWal WalStats;
 
 /*
  * List of SLRU names that we keep stats for.  There is no central registry of
@@ -281,6 +282,7 @@ static int	localNumBackends = 0;
  */
 static PgStat_ArchiverStats archiverStats;
 static PgStat_GlobalStats globalStats;
+static PgStat_WalStats walStats;
 static PgStat_SLRUStats slruStats[SLRU_NUM_ELEMENTS];
 
 /*
@@ -353,6 +355,7 @@ static void pgstat_recv_vacuum(PgStat_MsgVacuum *msg, int len);
 static void pgstat_recv_analyze(PgStat_MsgAnalyze *msg, int len);
 static void pgstat_recv_archiver(PgStat_MsgArchiver *msg, int len);
 static void pgstat_recv_bgwriter(PgStat_MsgBgWriter *msg, int len);
+static void pgstat_recv_wal(PgStat_MsgWal *msg, int len);
 static void pgstat_recv_slru(PgStat_MsgSLRU *msg, int len);
 static void pgstat_recv_funcstat(PgStat_MsgFuncstat *msg, int len);
 static void pgstat_recv_funcpurge(PgStat_MsgFuncpurge *msg, int len);
@@ -938,6 +941,9 @@ pgstat_report_stat(bool force)
 	/* Now, send function statistics */
 	pgstat_send_funcstats();
 
+	/* Send WAL statistics */
+	pgstat_send_wal();
+
 	/* Finally send SLRU statistics */
 	pgstat_send_slru();
 }
@@ -1370,11 +1376,13 @@ pgstat_reset_shared_counters(const char *target)
 		msg.m_resettarget = RESET_ARCHIVER;
 	else if (strcmp(target, "bgwriter") == 0)
 		msg.m_resettarget = RESET_BGWRITER;
+	else if (strcmp(target, "wal") == 0)
+		msg.m_resettarget = RESET_WAL;
 	else
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("unrecognized reset target: \"%s\"", target),
-				 errhint("Target must be \"archiver\" or \"bgwriter\".")));
+				 errhint("Target must be \"archiver\", \"bgwriter\" or \"wal\".")));
 
 	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RESETSHAREDCOUNTER);
 	pgstat_send(&msg, sizeof(msg));
@@ -2674,6 +2682,21 @@ pgstat_fetch_global(void)
 	return &globalStats;
 }
 
+/*
+ * ---------
+ * pgstat_fetch_stat_wal() -
+ *
+ *	Support function for the SQL-callable pgstat* functions. Returns
+ *	a pointer to the WAL statistics struct.
+ * ---------
+ */
+PgStat_WalStats *
+pgstat_fetch_stat_wal(void)
+{
+	backend_read_statsfile();
+
+	return &walStats;
+}
 
 /*
  * ---------
@@ -4419,6 +4442,38 @@ pgstat_send_bgwriter(void)
 	MemSet(&BgWriterStats, 0, sizeof(BgWriterStats));
 }
 
+/* ----------
+ * pgstat_send_wal() -
+ *
+ *		Send WAL statistics to the collector
+ * ----------
+ */
+void
+pgstat_send_wal(void)
+{
+	/* We assume this initializes to zeroes */
+	static const PgStat_MsgWal all_zeroes;
+
+	/*
+	 * This function can be called even if nothing at all has happened. In
+	 * this case, avoid sending a completely empty message to the stats
+	 * collector.
+	 */
+	if (memcmp(&WalStats, &all_zeroes, sizeof(PgStat_MsgWal)) == 0)
+		return;
+
+	/*
+	 * Prepare and send the message
+	 */
+	pgstat_setheader(&WalStats.m_hdr, PGSTAT_MTYPE_WAL);
+	pgstat_send(&WalStats, sizeof(WalStats));
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&WalStats, 0, sizeof(WalStats));
+}
+
 /* ----------
  * pgstat_send_slru() -
  *
@@ -4658,6 +4713,10 @@ PgstatCollectorMain(int argc, char *argv[])
 					pgstat_recv_bgwriter(&msg.msg_bgwriter, len);
 					break;
 
+				case PGSTAT_MTYPE_WAL:
+					pgstat_recv_wal(&msg.msg_wal, len);
+					break;
+
 				case PGSTAT_MTYPE_SLRU:
 					pgstat_recv_slru(&msg.msg_slru, len);
 					break;
@@ -4927,6 +4986,12 @@ pgstat_write_statsfiles(bool permanent, bool allDbs)
 	rc = fwrite(&archiverStats, sizeof(archiverStats), 1, fpout);
 	(void) rc;					/* we'll check for error with ferror */
 
+	/*
+	 * Write WAL stats struct
+	 */
+	rc = fwrite(&walStats, sizeof(walStats), 1, fpout);
+	(void) rc;					/* we'll check for error with ferror */
+
 	/*
 	 * Write SLRU stats struct
 	 */
@@ -5186,11 +5251,12 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 						 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
 
 	/*
-	 * Clear out global and archiver statistics so they start from zero in
-	 * case we can't load an existing statsfile.
+	 * Clear out global, archiver, WAL and SLRU statistics so they start from
+	 * zero in case we can't load an existing statsfile.
 	 */
 	memset(&globalStats, 0, sizeof(globalStats));
 	memset(&archiverStats, 0, sizeof(archiverStats));
+	memset(&walStats, 0, sizeof(walStats));
 	memset(&slruStats, 0, sizeof(slruStats));
 
 	/*
@@ -5199,6 +5265,7 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 	 */
 	globalStats.stat_reset_timestamp = GetCurrentTimestamp();
 	archiverStats.stat_reset_timestamp = globalStats.stat_reset_timestamp;
+	walStats.stat_reset_timestamp = globalStats.stat_reset_timestamp;
 
 	/*
 	 * Set the same reset timestamp for all SLRU items too.
@@ -5268,6 +5335,17 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 		goto done;
 	}
 
+	/*
+	 * Read WAL stats struct
+	 */
+	if (fread(&walStats, 1, sizeof(walStats), fpin) != sizeof(walStats))
+	{
+		ereport(pgStatRunningInCollector ? LOG : WARNING,
+				(errmsg("corrupted statistics file \"%s\"", statfile)));
+		memset(&walStats, 0, sizeof(walStats));
+		goto done;
+	}
+
 	/*
 	 * Read SLRU stats struct
 	 */
@@ -5578,6 +5656,7 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 	PgStat_StatDBEntry dbentry;
 	PgStat_GlobalStats myGlobalStats;
 	PgStat_ArchiverStats myArchiverStats;
+	PgStat_WalStats myWalStats;
 	PgStat_SLRUStats mySLRUStats[SLRU_NUM_ELEMENTS];
 	FILE	   *fpin;
 	int32		format_id;
@@ -5633,6 +5712,17 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 		return false;
 	}
 
+	/*
+	 * Read WAL stats struct
+	 */
+	if (fread(&myWalStats, 1, sizeof(myWalStats), fpin) != sizeof(myWalStats))
+	{
+		ereport(pgStatRunningInCollector ? LOG : WARNING,
+				(errmsg("corrupted statistics file \"%s\"", statfile)));
+		FreeFile(fpin);
+		return false;
+	}
+
 	/*
 	 * Read SLRU stats struct
 	 */
@@ -6213,6 +6303,12 @@ pgstat_recv_resetsharedcounter(PgStat_MsgResetsharedcounter *msg, int len)
 		memset(&archiverStats, 0, sizeof(archiverStats));
 		archiverStats.stat_reset_timestamp = GetCurrentTimestamp();
 	}
+	else if (msg->m_resettarget == RESET_WAL)
+	{
+		/* Reset the WAL statistics for the cluster. */
+		memset(&walStats, 0, sizeof(walStats));
+		walStats.stat_reset_timestamp = GetCurrentTimestamp();
+	}
 
 	/*
 	 * Presumably the sender of this message validated the target, don't
@@ -6427,6 +6523,18 @@ pgstat_recv_bgwriter(PgStat_MsgBgWriter *msg, int len)
 	globalStats.buf_alloc += msg->m_buf_alloc;
 }
 
+/* ----------
+ * pgstat_recv_wal() -
+ *
+ *	Process a WAL message.
+ * ----------
+ */
+static void
+pgstat_recv_wal(PgStat_MsgWal *msg, int len)
+{
+	walStats.wal_buffers_full += msg->m_wal_buffers_full;
+}
+
 /* ----------
  * pgstat_recv_slru() -
  *

src/backend/utils/adt/pgstatfuncs.c

@@ -1697,6 +1697,42 @@ pg_stat_get_buf_alloc(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(pgstat_fetch_global()->buf_alloc);
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+#define PG_STAT_GET_WAL_COLS	2
+	TupleDesc	tupdesc;
+	Datum		values[PG_STAT_GET_WAL_COLS];
+	bool		nulls[PG_STAT_GET_WAL_COLS];
+	PgStat_WalStats *wal_stats;
+
+	/* Initialise values and NULL flags arrays */
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_buffers_full",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "stats_reset",
+					   TIMESTAMPTZOID, -1, 0);
+
+	BlessTupleDesc(tupdesc);
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	/* Fill values and NULLs */
+	values[0] = Int64GetDatum(wal_stats->wal_buffers_full);
+	values[1] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */