Add more monitoring data for WAL writes in the WAL receiver

michaelpq · michaelpq · commit 7f7f324eb5a4 · 2025-03-06T09:41:37.000+09:00
This commit adds two improvements related to the monitoring of WAL writes for the WAL receiver. First, write counts and timings are now counted in pg_stat_io for the WAL receiver. These have been discarded from pg_stat_wal in ff99918 due to performance concerns, related to the fact that we still relied on an on-disk file for the stats back then, even with track_wal_io_timing to avoid the overhead of the timestamp calculations. This implementation is simpler than the original proposal as it is possible to rely on the APIs of pgstat_io.c to do the job. Like the fsync and read data, track_wal_io_timing needs to be enabled to track the timings. Second, a wait event is added around the pg_pwrite() call in charge of the writes, using the exiting WAIT_EVENT_WAL_WRITE. This is useful as the WAL receiver data is tracked in pg_stat_activity. Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com> Discussion: https://postgr.es/m/Z8gFnH4o3jBm5BRz@ip-10-97-1-34.eu-west-3.compute.internal
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
@@ -922,6 +922,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 {
 	int			startoff;
 	int			byteswritten;
+	instr_time	start;
 
 	Assert(tli != 0);
 
@@ -952,7 +953,18 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		/* OK to write the logs */
 		errno = 0;
 
+		/*
+		 * Measure I/O timing to write WAL data, for pg_stat_io.
+		 */
+		start = pgstat_prepare_io_time(track_wal_io_timing);
+
+		pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
 		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		pgstat_report_wait_end();
+
+		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
+								IOOP_WRITE, start, 1, byteswritten);
+
 		if (byteswritten <= 0)
 		{
 			char		xlogfname[MAXFNAMELEN];
