Re-add GUC track_wal_io_timing

This commit is a rework of 2421e9a51d20, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads.  Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths.  The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal.  pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled.  The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu

3 files changed, 7 insertions, 6 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 919314f8258..799fc739e18 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -134,6 +134,7 @@ int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 int			wal_retrieve_retry_interval = 5000;
 int			max_slot_wal_keep_size_mb = -1;
 int			wal_decode_buffer_size = 512 * 1024;
+bool		track_wal_io_timing = false;
 
 #ifdef WAL_DEBUG
 bool		XLOG_DEBUG = false;
@@ -2437,7 +2438,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				/*
 				 * Measure I/O timing to write WAL data, for pg_stat_io.
 				 */
-				start = pgstat_prepare_io_time();
+				start = pgstat_prepare_io_time(track_wal_io_timing);
 
 				pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
 				written = pg_pwrite(openLogFile, from, nleft, startoffset);
@@ -3248,7 +3249,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 				 errmsg("could not create file \"%s\": %m", tmppath)));
 
 	/* Measure I/O timing when initializing segment */
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
 	save_errno = 0;
@@ -3310,7 +3311,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 	}
 
 	/* Measure I/O timing when flushing segment */
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC);
 	if (pg_fsync(fd) != 0)
@@ -8744,7 +8745,7 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	/*
 	 * Measure I/O timing to sync the WAL file for pg_stat_io.
 	 */
-	start = pgstat_prepare_io_time();
+	start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC);
 	switch (wal_sync_method)
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 995e4f7c26f..5483d4f0dd2 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1557,7 +1557,7 @@ WALRead(XLogReaderState *state,
 
 #ifndef FRONTEND
 		/* Measure I/O timing when reading segment */
-		io_start = pgstat_prepare_io_time();
+		io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 		pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
 #endif
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 915cb330295..c5475aaa97a 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3401,7 +3401,7 @@ retry:
 	readOff = targetPageOff;
 
 	/* Measure I/O timing when reading segment */
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
 	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);