New GUC, track_iotiming, to track I/O timings.

Currently, the only way to see the numbers this gathers is via
EXPLAIN (ANALYZE, BUFFERS), but the plan is to add visibility through
the stats collector and pg_stat_statements in subsequent patches.

Ants Aasma, reviewed by Greg Smith, with some further changes by me.

doc/src/sgml/config.sgml

@@ -4284,6 +4284,21 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-track-iotiming" xreflabel="track_iotiming">
+      <term><varname>track_iotiming</varname> (<type>boolean</type>)</term>
+      <indexterm>
+       <primary><varname>track_iotiming</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        Enables timing of database I/O calls.  This parameter is off by
+        default, because it will repeatedly query the operating system for
+        the current time, which may cause significant overhead on some
+        platforms.  Only superusers can change this setting.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-track-functions" xreflabel="track_functions">
       <term><varname>track_functions</varname> (<type>enum</type>)</term>
       <indexterm>

src/backend/commands/explain.c

@@ -1236,6 +1236,8 @@ ExplainNode(PlanState *planstate, List *ancestors,
 									 usage->local_blks_written > 0);
 			bool		has_temp = (usage->temp_blks_read > 0 ||
 									usage->temp_blks_written > 0);
+			bool		has_timing = (!INSTR_TIME_IS_ZERO(usage->time_read) ||
+									  !INSTR_TIME_IS_ZERO(usage->time_write));
 
 			/* Show only positive counter values. */
 			if (has_shared || has_local || has_temp)
@@ -1291,6 +1293,20 @@ ExplainNode(PlanState *planstate, List *ancestors,
 				}
 				appendStringInfoChar(es->str, '\n');
 			}
+
+			/* As above, show only positive counter values. */
+			if (has_timing)
+			{
+				appendStringInfoSpaces(es->str, es->indent * 2);
+				appendStringInfoString(es->str, "I/O Timings:");
+				if (!INSTR_TIME_IS_ZERO(usage->time_read))
+					appendStringInfo(es->str, " read=%0.2f",
+								 INSTR_TIME_GET_MILLISEC(usage->time_read));
+				if (!INSTR_TIME_IS_ZERO(usage->time_write))
+					appendStringInfo(es->str, " write=%0.2f",
+								 INSTR_TIME_GET_MILLISEC(usage->time_write));
+				appendStringInfoChar(es->str, '\n');
+			}
 		}
 		else
 		{
@@ -1304,6 +1320,8 @@ ExplainNode(PlanState *planstate, List *ancestors,
 			ExplainPropertyLong("Local Written Blocks", usage->local_blks_written, es);
 			ExplainPropertyLong("Temp Read Blocks", usage->temp_blks_read, es);
 			ExplainPropertyLong("Temp Written Blocks", usage->temp_blks_written, es);
+			ExplainPropertyFloat("I/O Read Time", INSTR_TIME_GET_MILLISEC(usage->time_read), 3, es);
+			ExplainPropertyFloat("I/O Write Time", INSTR_TIME_GET_MILLISEC(usage->time_write), 3, es);
 		}
 	}
 

src/backend/executor/instrument.c

@@ -145,4 +145,6 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+	INSTR_TIME_ACCUM_DIFF(dst->time_read, add->time_read, sub->time_read);
+	INSTR_TIME_ACCUM_DIFF(dst->time_write, add->time_write, sub->time_write);
 }

src/backend/storage/buffer/bufmgr.c

@@ -67,6 +67,7 @@
 bool		zero_damaged_pages = false;
 int			bgwriter_lru_maxpages = 100;
 double		bgwriter_lru_multiplier = 2.0;
+bool		track_iotiming = false;
 
 /*
  * How many buffers PrefetchBuffer callers should try to stay ahead of their
@@ -437,8 +438,21 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			MemSet((char *) bufBlock, 0, BLCKSZ);
 		else
 		{
+			instr_time	io_start,
+						io_time;
+
+			if (track_iotiming)
+				INSTR_TIME_SET_CURRENT(io_start);
+
 			smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
 
+			if (track_iotiming)
+			{
+				INSTR_TIME_SET_CURRENT(io_time);
+				INSTR_TIME_SUBTRACT(io_time, io_start);
+				INSTR_TIME_ADD(pgBufferUsage.time_read, io_time);
+			}
+
 			/* check for garbage data */
 			if (!PageHeaderIsValid((PageHeader) bufBlock))
 			{
@@ -1874,6 +1888,7 @@ FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
 {
 	XLogRecPtr	recptr;
 	ErrorContextCallback errcontext;
+	instr_time io_start, io_end;
 
 	/*
 	 * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
@@ -1921,12 +1936,21 @@ FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
 	buf->flags &= ~BM_JUST_DIRTIED;
 	UnlockBufHdr(buf);
 
+	if (track_iotiming)
+		INSTR_TIME_SET_CURRENT(io_start);
+
 	smgrwrite(reln,
 			  buf->tag.forkNum,
 			  buf->tag.blockNum,
 			  (char *) BufHdrGetBlock(buf),
 			  false);
 
+	if (track_iotiming)
+	{
+		INSTR_TIME_SET_CURRENT(io_end);
+		INSTR_TIME_ACCUM_DIFF(pgBufferUsage.time_write, io_end, io_start);
+	}
+
 	pgBufferUsage.shared_blks_written++;
 
 	/*

src/backend/utils/misc/guc.c

@@ -1018,6 +1018,15 @@ static struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+	{
+		{"track_iotiming", PGC_SUSET, STATS_COLLECTOR,
+			gettext_noop("Collects timing information for database IO activity."),
+			NULL
+		},
+		&track_iotiming,
+		false,
+		NULL, NULL, NULL
+	},
 
 	{
 		{"update_process_title", PGC_SUSET, STATS_COLLECTOR,

src/backend/utils/misc/postgresql.conf.sample

@@ -424,6 +424,7 @@
 
 #track_activities = on
 #track_counts = on
+#track_iotiming = off
 #track_functions = none			# none, pl, all
 #track_activity_query_size = 1024 	# (change requires restart)
 #update_process_title = on

src/include/executor/instrument.h

@@ -28,6 +28,8 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+	instr_time	time_read;				/* time spent reading */
+	instr_time	time_write;				/* time spent writing */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */

src/include/storage/bufmgr.h

@@ -48,6 +48,7 @@ extern PGDLLIMPORT int NBuffers;
 extern bool zero_damaged_pages;
 extern int	bgwriter_lru_maxpages;
 extern double bgwriter_lru_multiplier;
+extern bool track_iotiming;
 extern int	target_prefetch_pages;
 
 /* in buf_init.c */