Introduce pg_stat_checkpointer

Historically, the statistics of the checkpointer have been always part
of pg_stat_bgwriter.  This commit removes a few columns from
pg_stat_bgwriter, and introduces pg_stat_checkpointer with equivalent,
renamed columns (plus a new one for the reset timestamp):
- checkpoints_timed -> num_timed
- checkpoints_req -> num_requested
- checkpoint_write_time -> write_time
- checkpoint_sync_time -> sync_time
- buffers_checkpoint -> buffers_written

The fields of PgStat_CheckpointerStats and its SQL functions are renamed
to match with the new field names, for consistency.  Note that
background writer and checkpointer have been split into two different
processes in commits 806a2ae and bf405ba.  The pgstat
structures were already split, making this change straight-forward.

Bump catalog version.

Author: Bharath Rupireddy
Reviewed-by: Bertrand Drouvot, Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACVxX2ii=66RypXRweZe2EsBRiPMj0aHfRfHUeXJcC7kHg@mail.gmail.com

Author: michaelpq

doc/src/sgml/monitoring.sgml

@@ -451,6 +451,15 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
      </entry>
      </row>
 
+     <row>
+      <entry><structname>pg_stat_checkpointer</structname><indexterm><primary>pg_stat_checkpointer</primary></indexterm></entry>
+      <entry>One row only, showing statistics about the
+       checkpointer process's activity. See
+       <link linkend="monitoring-pg-stat-checkpointer-view">
+       <structname>pg_stat_checkpointer</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
@@ -2868,7 +2877,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
 
   <para>
    The <structname>pg_stat_bgwriter</structname> view will always have a
-   single row, containing global data for the cluster.
+   single row, containing data about the background writer of the cluster.
   </para>
 
   <table id="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
@@ -2888,77 +2897,118 @@ description | Waiting for a newly initialized WAL file to reach durable storage
     <tbody>
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>checkpoints_timed</structfield> <type>bigint</type>
+       <structfield>buffers_clean</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of scheduled checkpoints that have been performed
+       Number of buffers written by the background writer
       </para></entry>
      </row>
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>checkpoints_req</structfield> <type>bigint</type>
+       <structfield>maxwritten_clean</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of requested checkpoints that have been performed
+       Number of times the background writer stopped a cleaning
+       scan because it had written too many buffers
       </para></entry>
      </row>
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>checkpoint_write_time</structfield> <type>double precision</type>
+       <structfield>buffers_alloc</structfield> <type>bigint</type>
       </para>
       <para>
-       Total amount of time that has been spent in the portion of
-       checkpoint processing where files are written to disk, in milliseconds
+       Number of buffers allocated
       </para></entry>
      </row>
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>checkpoint_sync_time</structfield> <type>double precision</type>
+       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
-       Total amount of time that has been spent in the portion of
-       checkpoint processing where files are synchronized to disk, in
-       milliseconds
+       Time at which these statistics were last reset
       </para></entry>
      </row>
+    </tbody>
+   </tgroup>
+  </table>
+
+ </sect2>
+
+ <sect2 id="monitoring-pg-stat-checkpointer-view">
+  <title><structname>pg_stat_checkpointer</structname></title>
 
+  <indexterm>
+   <primary>pg_stat_checkpointer</primary>
+  </indexterm>
+
+  <para>
+   The <structname>pg_stat_checkpointer</structname> view will always have a
+   single row, containing data about the checkpointer process of the cluster.
+  </para>
+
+  <table id="pg-stat-checkpointer-view" xreflabel="pg_stat_checkpointer">
+   <title><structname>pg_stat_checkpointer</structname> View</title>
+   <tgroup cols="1">
+    <thead>
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>buffers_checkpoint</structfield> <type>bigint</type>
+       Column Type
       </para>
       <para>
-       Number of buffers written during checkpoints
+       Description
       </para></entry>
      </row>
+    </thead>
 
+    <tbody>
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>buffers_clean</structfield> <type>bigint</type>
+       <structfield>num_timed</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of buffers written by the background writer
+       Number of scheduled checkpoints that have been performed
       </para></entry>
      </row>
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>maxwritten_clean</structfield> <type>bigint</type>
+       <structfield>num_requested</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of times the background writer stopped a cleaning
-       scan because it had written too many buffers
+       Number of requested checkpoints that have been performed
       </para></entry>
      </row>
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>buffers_alloc</structfield> <type>bigint</type>
+       <structfield>write_time</structfield> <type>double precision</type>
       </para>
       <para>
-       Number of buffers allocated
+       Total amount of time that has been spent in the portion of
+       checkpoint processing where files are written to disk, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>sync_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total amount of time that has been spent in the portion of
+       checkpoint processing where files are synchronized to disk, in
+       milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>buffers_written</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Number of buffers written during checkpoints
       </para></entry>
      </row>
 
@@ -4669,8 +4719,10 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         Resets some cluster-wide statistics counters to zero, depending on the
         argument.  The argument can be <literal>bgwriter</literal> to reset
         all the counters shown in
-        the <structname>pg_stat_bgwriter</structname>
-        view, <literal>archiver</literal> to reset all the counters shown in
+        the <structname>pg_stat_bgwriter</structname> view,
+        <literal>checkpointer</literal> to reset all the counters shown in
+        the <structname>pg_stat_checkpointer</structname> view,
+        <literal>archiver</literal> to reset all the counters shown in
         the <structname>pg_stat_archiver</structname> view,
         <literal>io</literal> to reset all the counters shown in the
         <structname>pg_stat_io</structname> view,

src/backend/access/transam/xlog.c

@@ -6349,8 +6349,8 @@ LogCheckpointEnd(bool restartpoint)
 												 CheckpointStats.ckpt_sync_end_t);
 
 	/* Accumulate checkpoint timing summary data, in milliseconds. */
-	PendingCheckpointerStats.checkpoint_write_time += write_msecs;
-	PendingCheckpointerStats.checkpoint_sync_time += sync_msecs;
+	PendingCheckpointerStats.write_time += write_msecs;
+	PendingCheckpointerStats.sync_time += sync_msecs;
 
 	/*
 	 * All of the published timing statistics are accounted for.  Only

src/backend/catalog/system_views.sql

@@ -1111,16 +1111,20 @@ CREATE VIEW pg_stat_archiver AS
 
 CREATE VIEW pg_stat_bgwriter AS
     SELECT
-        pg_stat_get_bgwriter_timed_checkpoints() AS checkpoints_timed,
-        pg_stat_get_bgwriter_requested_checkpoints() AS checkpoints_req,
-        pg_stat_get_checkpoint_write_time() AS checkpoint_write_time,
-        pg_stat_get_checkpoint_sync_time() AS checkpoint_sync_time,
-        pg_stat_get_bgwriter_buf_written_checkpoints() AS buffers_checkpoint,
         pg_stat_get_bgwriter_buf_written_clean() AS buffers_clean,
         pg_stat_get_bgwriter_maxwritten_clean() AS maxwritten_clean,
         pg_stat_get_buf_alloc() AS buffers_alloc,
         pg_stat_get_bgwriter_stat_reset_time() AS stats_reset;
 
+CREATE VIEW pg_stat_checkpointer AS
+    SELECT
+        pg_stat_get_checkpointer_num_timed() AS num_timed,
+        pg_stat_get_checkpointer_num_requested() AS num_requested,
+        pg_stat_get_checkpointer_write_time() AS write_time,
+        pg_stat_get_checkpointer_sync_time() AS sync_time,
+        pg_stat_get_checkpointer_buffers_written() AS buffers_written,
+        pg_stat_get_checkpointer_stat_reset_time() AS stats_reset;
+
 CREATE VIEW pg_stat_io AS
 SELECT
        b.backend_type,

src/backend/postmaster/checkpointer.c

@@ -358,7 +358,7 @@ CheckpointerMain(void)
 		if (((volatile CheckpointerShmemStruct *) CheckpointerShmem)->ckpt_flags)
 		{
 			do_checkpoint = true;
-			PendingCheckpointerStats.requested_checkpoints++;
+			PendingCheckpointerStats.num_requested++;
 		}
 
 		/*
@@ -372,7 +372,7 @@ CheckpointerMain(void)
 		if (elapsed_secs >= CheckPointTimeout)
 		{
 			if (!do_checkpoint)
-				PendingCheckpointerStats.timed_checkpoints++;
+				PendingCheckpointerStats.num_timed++;
 			do_checkpoint = true;
 			flags |= CHECKPOINT_CAUSE_TIME;
 		}
@@ -569,7 +569,7 @@ HandleCheckpointerInterrupts(void)
 		 * updates the statistics, increment the checkpoint request and flush
 		 * out pending statistic.
 		 */
-		PendingCheckpointerStats.requested_checkpoints++;
+		PendingCheckpointerStats.num_requested++;
 		ShutdownXLOG(0, 0);
 		pgstat_report_checkpointer();
 		pgstat_report_wal(true);

src/backend/storage/buffer/bufmgr.c

@@ -2751,7 +2751,7 @@ BufferSync(int flags)
 			if (SyncOneBuffer(buf_id, false, &wb_context) & BUF_WRITTEN)
 			{
 				TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
-				PendingCheckpointerStats.buf_written_checkpoints++;
+				PendingCheckpointerStats.buffers_written++;
 				num_written++;
 			}
 		}

src/backend/utils/activity/pgstat_checkpointer.c

@@ -47,11 +47,11 @@ pgstat_report_checkpointer(void)
 	pgstat_begin_changecount_write(&stats_shmem->changecount);
 
 #define CHECKPOINTER_ACC(fld) stats_shmem->stats.fld += PendingCheckpointerStats.fld
-	CHECKPOINTER_ACC(timed_checkpoints);
-	CHECKPOINTER_ACC(requested_checkpoints);
-	CHECKPOINTER_ACC(checkpoint_write_time);
-	CHECKPOINTER_ACC(checkpoint_sync_time);
-	CHECKPOINTER_ACC(buf_written_checkpoints);
+	CHECKPOINTER_ACC(num_timed);
+	CHECKPOINTER_ACC(num_requested);
+	CHECKPOINTER_ACC(write_time);
+	CHECKPOINTER_ACC(sync_time);
+	CHECKPOINTER_ACC(buffers_written);
 #undef CHECKPOINTER_ACC
 
 	pgstat_end_changecount_write(&stats_shmem->changecount);
@@ -92,6 +92,7 @@ pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 									&stats_shmem->stats,
 									sizeof(stats_shmem->stats),
 									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
 	LWLockRelease(&stats_shmem->lock);
 }
 
@@ -113,10 +114,10 @@ pgstat_checkpointer_snapshot_cb(void)
 
 	/* compensate by reset offsets */
 #define CHECKPOINTER_COMP(fld) pgStatLocal.snapshot.checkpointer.fld -= reset.fld;
-	CHECKPOINTER_COMP(timed_checkpoints);
-	CHECKPOINTER_COMP(requested_checkpoints);
-	CHECKPOINTER_COMP(checkpoint_write_time);
-	CHECKPOINTER_COMP(checkpoint_sync_time);
-	CHECKPOINTER_COMP(buf_written_checkpoints);
+	CHECKPOINTER_COMP(num_timed);
+	CHECKPOINTER_COMP(num_requested);
+	CHECKPOINTER_COMP(write_time);
+	CHECKPOINTER_COMP(sync_time);
+	CHECKPOINTER_COMP(buffers_written);
 #undef CHECKPOINTER_COMP
 }

src/backend/utils/adt/pgstatfuncs.c

@@ -1182,21 +1182,21 @@ PG_STAT_GET_DBENTRY_FLOAT8_MS(idle_in_transaction_time)
 PG_STAT_GET_DBENTRY_FLOAT8_MS(session_time)
 
 Datum
-pg_stat_get_bgwriter_timed_checkpoints(PG_FUNCTION_ARGS)
+pg_stat_get_checkpointer_num_timed(PG_FUNCTION_ARGS)
 {
-	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->timed_checkpoints);
+	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->num_timed);
 }
 
 Datum
-pg_stat_get_bgwriter_requested_checkpoints(PG_FUNCTION_ARGS)
+pg_stat_get_checkpointer_num_requested(PG_FUNCTION_ARGS)
 {
-	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->requested_checkpoints);
+	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->num_requested);
 }
 
 Datum
-pg_stat_get_bgwriter_buf_written_checkpoints(PG_FUNCTION_ARGS)
+pg_stat_get_checkpointer_buffers_written(PG_FUNCTION_ARGS)
 {
-	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->buf_written_checkpoints);
+	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->buffers_written);
 }
 
 Datum
@@ -1212,19 +1212,25 @@ pg_stat_get_bgwriter_maxwritten_clean(PG_FUNCTION_ARGS)
 }
 
 Datum
-pg_stat_get_checkpoint_write_time(PG_FUNCTION_ARGS)
+pg_stat_get_checkpointer_write_time(PG_FUNCTION_ARGS)
 {
 	/* time is already in msec, just convert to double for presentation */
 	PG_RETURN_FLOAT8((double)
-					 pgstat_fetch_stat_checkpointer()->checkpoint_write_time);
+					 pgstat_fetch_stat_checkpointer()->write_time);
 }
 
 Datum
-pg_stat_get_checkpoint_sync_time(PG_FUNCTION_ARGS)
+pg_stat_get_checkpointer_sync_time(PG_FUNCTION_ARGS)
 {
 	/* time is already in msec, just convert to double for presentation */
 	PG_RETURN_FLOAT8((double)
-					 pgstat_fetch_stat_checkpointer()->checkpoint_sync_time);
+					 pgstat_fetch_stat_checkpointer()->sync_time);
+}
+
+Datum
+pg_stat_get_checkpointer_stat_reset_time(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_TIMESTAMPTZ(pgstat_fetch_stat_checkpointer()->stat_reset_timestamp);
 }
 
 Datum
@@ -1684,14 +1690,9 @@ pg_stat_reset_shared(PG_FUNCTION_ARGS)
 	if (strcmp(target, "archiver") == 0)
 		pgstat_reset_of_kind(PGSTAT_KIND_ARCHIVER);
 	else if (strcmp(target, "bgwriter") == 0)
-	{
-		/*
-		 * Historically checkpointer was part of bgwriter, continue to reset
-		 * both for now.
-		 */
 		pgstat_reset_of_kind(PGSTAT_KIND_BGWRITER);
+	else if (strcmp(target, "checkpointer") == 0)
 		pgstat_reset_of_kind(PGSTAT_KIND_CHECKPOINTER);
-	}
 	else if (strcmp(target, "io") == 0)
 		pgstat_reset_of_kind(PGSTAT_KIND_IO);
 	else if (strcmp(target, "recovery_prefetch") == 0)
@@ -1702,7 +1703,7 @@ pg_stat_reset_shared(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("unrecognized reset target: \"%s\"", target),
-				 errhint("Target must be \"archiver\", \"bgwriter\", \"io\", \"recovery_prefetch\", or \"wal\".")));
+				 errhint("Target must be \"archiver\", \"bgwriter\", \"checkpointer\", \"io\", \"recovery_prefetch\", or \"wal\".")));
 
 	PG_RETURN_VOID();
 }

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202310271
+#define CATALOG_VERSION_NO	202310301
 
 #endif