Fix inconsistent reporting of checkpointer stats.

Previously, the pg_stat_checkpointer view and the checkpoint completion
log message could show different numbers for buffers written
during checkpoints. The view only counted shared buffers,
while the log message included both shared and SLRU buffers,
causing inconsistencies.

This commit resolves the issue by updating both the view and the log message
to separately report shared and SLRU buffers written during checkpoints.
A new slru_written column is added to the pg_stat_checkpointer view
to track SLRU buffers, while the existing buffers_written column now
tracks only shared buffers. This change would help users distinguish
between the two types of buffers, in the pg_stat_checkpointer view and
the checkpoint complete log message, respectively.

Bump catalog version.

Author: Nitin Jadhav
Reviewed-by: Bharath Rupireddy, Michael Paquier, Kyotaro Horiguchi, Robert Haas
Reviewed-by: Andres Freund, vignesh C, Fujii Masao
Discussion: https://postgr.es/m/CAMm1aWb18EpT0whJrjG+-nyhNouXET6ZUw0pNYYAe+NezpvsAA@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -3127,7 +3127,16 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        <structfield>buffers_written</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of buffers written during checkpoints and restartpoints
+       Number of shared buffers written during checkpoints and restartpoints
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slru_written</structfield> <type>bigint</type>
+      </para>
+      <para>
+        Number of SLRU buffers written during checkpoints and restartpoints
       </para></entry>
      </row>
 

src/backend/access/transam/slru.c

@@ -716,9 +716,12 @@ SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata)
 	if (!ok)
 		SlruReportIOError(ctl, pageno, InvalidTransactionId);
 
-	/* If part of a checkpoint, count this as a buffer written. */
+	/* If part of a checkpoint, count this as a SLRU buffer written. */
 	if (fdata)
-		CheckpointStats.ckpt_bufs_written++;
+	{
+		CheckpointStats.ckpt_slru_written++;
+		PendingCheckpointerStats.slru_written++;
+	}
 }
 
 /*

src/backend/access/transam/xlog.c

@@ -6727,14 +6727,15 @@ LogCheckpointEnd(bool restartpoint)
 	 */
 	if (restartpoint)
 		ereport(LOG,
-				(errmsg("restartpoint complete: wrote %d buffers (%.1f%%); "
+				(errmsg("restartpoint complete: wrote %d buffers (%.1f%%), "
-						"%d WAL file(s) added, %d removed, %d recycled; "
+						"wrote %d SLRU buffers; %d WAL file(s) added, "
-						"write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
+						"%d removed, %d recycled; write=%ld.%03d s, "
-						"sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
+						"sync=%ld.%03d s, total=%ld.%03d s; sync files=%d, "
-						"distance=%d kB, estimate=%d kB; "
+						"longest=%ld.%03d s, average=%ld.%03d s; distance=%d kB, "
-						"lsn=%X/%X, redo lsn=%X/%X",
+						"estimate=%d kB; lsn=%X/%X, redo lsn=%X/%X",
 						CheckpointStats.ckpt_bufs_written,
 						(double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
+						CheckpointStats.ckpt_slru_written,
 						CheckpointStats.ckpt_segs_added,
 						CheckpointStats.ckpt_segs_removed,
 						CheckpointStats.ckpt_segs_recycled,
@@ -6750,14 +6751,15 @@ LogCheckpointEnd(bool restartpoint)
 						LSN_FORMAT_ARGS(ControlFile->checkPointCopy.redo))));
 	else
 		ereport(LOG,
-				(errmsg("checkpoint complete: wrote %d buffers (%.1f%%); "
+				(errmsg("checkpoint complete: wrote %d buffers (%.1f%%), "
-						"%d WAL file(s) added, %d removed, %d recycled; "
+						"wrote %d SLRU buffers; %d WAL file(s) added, "
-						"write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
+						"%d removed, %d recycled; write=%ld.%03d s, "
-						"sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
+						"sync=%ld.%03d s, total=%ld.%03d s; sync files=%d, "
-						"distance=%d kB, estimate=%d kB; "
+						"longest=%ld.%03d s, average=%ld.%03d s; distance=%d kB, "
-						"lsn=%X/%X, redo lsn=%X/%X",
+						"estimate=%d kB; lsn=%X/%X, redo lsn=%X/%X",
 						CheckpointStats.ckpt_bufs_written,
 						(double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
+						CheckpointStats.ckpt_slru_written,
 						CheckpointStats.ckpt_segs_added,
 						CheckpointStats.ckpt_segs_removed,
 						CheckpointStats.ckpt_segs_recycled,

src/backend/catalog/system_views.sql

@@ -1145,6 +1145,7 @@ CREATE VIEW pg_stat_checkpointer AS
         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_slru_written() AS slru_written,
         pg_stat_get_checkpointer_stat_reset_time() AS stats_reset;
 
 CREATE VIEW pg_stat_io AS

src/backend/utils/activity/pgstat_checkpointer.c

@@ -56,6 +56,7 @@ pgstat_report_checkpointer(void)
 	CHECKPOINTER_ACC(write_time);
 	CHECKPOINTER_ACC(sync_time);
 	CHECKPOINTER_ACC(buffers_written);
+	CHECKPOINTER_ACC(slru_written);
 #undef CHECKPOINTER_ACC
 
 	pgstat_end_changecount_write(&stats_shmem->changecount);
@@ -135,5 +136,6 @@ pgstat_checkpointer_snapshot_cb(void)
 	CHECKPOINTER_COMP(write_time);
 	CHECKPOINTER_COMP(sync_time);
 	CHECKPOINTER_COMP(buffers_written);
+	CHECKPOINTER_COMP(slru_written);
 #undef CHECKPOINTER_COMP
 }

src/backend/utils/adt/pgstatfuncs.c

@@ -1221,6 +1221,12 @@ pg_stat_get_checkpointer_buffers_written(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->buffers_written);
 }
 
+Datum
+pg_stat_get_checkpointer_slru_written(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_INT64(pgstat_fetch_stat_checkpointer()->slru_written);
+}
+
 Datum
 pg_stat_get_bgwriter_buf_written_clean(PG_FUNCTION_ARGS)
 {

src/include/access/xlog.h

@@ -167,6 +167,7 @@ typedef struct CheckpointStatsData
 	TimestampTz ckpt_end_t;		/* end of checkpoint */
 
 	int			ckpt_bufs_written;	/* # of buffers written */
+	int			ckpt_slru_written;	/* # of SLRU buffers written */
 
 	int			ckpt_segs_added;	/* # of new xlog segments created */
 	int			ckpt_segs_removed;	/* # of xlog segments deleted */

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202409302
+#define CATALOG_VERSION_NO	202410021
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5847,6 +5847,11 @@
   proname => 'pg_stat_get_checkpointer_buffers_written', provolatile => 's',
   proparallel => 'r', prorettype => 'int8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpointer_buffers_written' },
+{ oid => '8573',
+  descr => 'statistics: number of SLRU buffers written during checkpoints and restartpoints',
+  proname => 'pg_stat_get_checkpointer_slru_written', provolatile => 's',
+  proparallel => 'r', prorettype => 'int8', proargtypes => '',
+  prosrc => 'pg_stat_get_checkpointer_slru_written' },
 { oid => '6314', descr => 'statistics: last reset for the checkpointer',
   proname => 'pg_stat_get_checkpointer_stat_reset_time', provolatile => 's',
   proparallel => 'r', prorettype => 'timestamptz', proargtypes => '',

src/include/pgstat.h

@@ -301,6 +301,7 @@ typedef struct PgStat_CheckpointerStats
 	PgStat_Counter write_time;	/* times in milliseconds */
 	PgStat_Counter sync_time;
 	PgStat_Counter buffers_written;
+	PgStat_Counter slru_written;
 	TimestampTz stat_reset_timestamp;
 } PgStat_CheckpointerStats;
 

src/test/regress/expected/rules.out

@@ -1831,6 +1831,7 @@ pg_stat_checkpointer| SELECT pg_stat_get_checkpointer_num_timed() AS num_timed,
     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_slru_written() AS slru_written,
     pg_stat_get_checkpointer_stat_reset_time() AS stats_reset;
 pg_stat_database| SELECT oid AS datid,
     datname,