Add wait event for fsync of WAL segments

This has been visibly a forgotten spot in the first implementation of
wait events for I/O added by 249cf07, and what has been missing is a
fsync call for WAL segments which is a wrapper reacting on the value of
GUC wal_sync_method.

Reported-by: Konstantin Knizhnik
Author: Konstantin Knizhnik
Reviewed-by: Craig Ringer, Michael Paquier
Discussion: https://postgr.es/m/4a243897-0ad8-f471-aa40-242591f2476e@postgrespro.ru

doc/src/sgml/monitoring.sgml

@@ -1674,6 +1674,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>WALSenderTimelineHistoryRead</literal></entry>
          <entry>Waiting for a read from a timeline history file during walsender timeline command.</entry>
         </row>
+        <row>
+         <entry><literal>WALSync</literal></entry>
+         <entry>Waiting for a WAL file to reach stable storage.</entry>
+        </row>
         <row>
          <entry><literal>WALSyncMethodAssign</literal></entry>
          <entry>Waiting for data to reach stable storage while assigning WAL sync method.</entry>

src/backend/access/transam/xlog.c

@@ -10156,6 +10156,7 @@ assign_xlog_sync_method(int new_sync_method, void *extra)
 void
 issue_xlog_fsync(int fd, XLogSegNo segno)
 {
+	pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC);
 	switch (sync_method)
 	{
 		case SYNC_METHOD_FSYNC:
@@ -10191,6 +10192,7 @@ issue_xlog_fsync(int fd, XLogSegNo segno)
 			elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
 			break;
 	}
+	pgstat_report_wait_end();
 }
 
 /*

src/backend/postmaster/pgstat.c

@@ -3925,6 +3925,9 @@ pgstat_get_wait_io(WaitEventIO w)
 		case WAIT_EVENT_WAL_READ:
 			event_name = "WALRead";
 			break;
+		case WAIT_EVENT_WAL_SYNC:
+			event_name = "WALSync";
+			break;
 		case WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN:
 			event_name = "WALSyncMethodAssign";
 			break;

src/include/pgstat.h

@@ -921,6 +921,7 @@ typedef enum
 	WAIT_EVENT_WAL_INIT_SYNC,
 	WAIT_EVENT_WAL_INIT_WRITE,
 	WAIT_EVENT_WAL_READ,
+	WAIT_EVENT_WAL_SYNC,
 	WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN,
 	WAIT_EVENT_WAL_WRITE
 } WaitEventIO;