004.pg_checksums.optimize.writes.and.always.sync.patch

diff --git a/src/bin/pg_checksums/pg_checksums.c b/src/bin/pg_checksums/pg_checksums.c
index 831cf42d3a..a13cf22b57 100644
--- a/src/bin/pg_checksums/pg_checksums.c
+++ b/src/bin/pg_checksums/pg_checksums.c
@@ -31,8 +31,10 @@
 #include "storage/checksum_impl.h"
 
 
-static int64 files = 0;
-static int64 blocks = 0;
+static int64 files_scanned = 0;
+static int64 files_written = 0;
+static int64 blocks_scanned = 0;
+static int64 blocks_written = 0;
 static int64 badblocks = 0;
 static ControlFileData *ControlFile;
 
@@ -195,6 +197,7 @@ scan_file(const char *fn, BlockNumber segmentno)
 	int			f;
 	BlockNumber blockno;
 	int			flags;
+	int64		blocks_written_in_file = 0;
 
 	Assert(mode == PG_MODE_ENABLE ||
 		   mode == PG_MODE_CHECK);
@@ -208,7 +211,7 @@ scan_file(const char *fn, BlockNumber segmentno)
 		exit(1);
 	}
 
-	files++;
+	files_scanned++;
 
 	for (blockno = 0;; blockno++)
 	{
@@ -227,7 +230,7 @@ scan_file(const char *fn, BlockNumber segmentno)
 							 blockno, fn, r, BLCKSZ);
 			exit(1);
 		}
-		blocks++;
+		blocks_scanned++;
 
 		/*
 		 * Since the file size is counted as total_size for progress status
@@ -256,6 +259,12 @@ scan_file(const char *fn, BlockNumber segmentno)
 		{
 			int			w;
 
+			/* Do not rewrite if the checksum is already set to the expected value */
+			if (header->pd_checksum == csum)
+				continue;
+
+			blocks_written_in_file++;
+
 			/* Set checksum in page header */
 			header->pd_checksum = csum;
 
@@ -292,6 +301,13 @@ scan_file(const char *fn, BlockNumber segmentno)
 			pg_log_info("checksums enabled in file \"%s\"", fn);
 	}
 
+	/* Update write counters if any write activity has happened */
+	if (blocks_written_in_file > 0)
+	{
+		files_written++;
+		blocks_written += blocks_written_in_file;
+	}
+
 	close(f);
 }
 
@@ -637,8 +653,8 @@ main(int argc, char *argv[])
 			progress_report(true);
 
 		printf(_("Checksum operation completed\n"));
-		printf(_("Files scanned:  %s\n"), psprintf(INT64_FORMAT, files));
-		printf(_("Blocks scanned: %s\n"), psprintf(INT64_FORMAT, blocks));
+		printf(_("Files scanned:   %s\n"), psprintf(INT64_FORMAT, files_scanned));
+		printf(_("Blocks scanned:  %s\n"), psprintf(INT64_FORMAT, blocks_scanned));
 		if (mode == PG_MODE_CHECK)
 		{
 			printf(_("Bad checksums:  %s\n"), psprintf(INT64_FORMAT, badblocks));
@@ -647,6 +663,11 @@ main(int argc, char *argv[])
 			if (badblocks > 0)
 				exit(1);
 		}
+		else if (mode == PG_MODE_ENABLE)
+		{
+			printf(_("Files written:  %s\n"), psprintf(INT64_FORMAT, files_written));
+			printf(_("Blocks written: %s\n"), psprintf(INT64_FORMAT, blocks_written));
+		}
 	}
 
 	/*
diff --git a/doc/src/sgml/ref/pg_checksums.sgml b/doc/src/sgml/ref/pg_checksums.sgml
index c84bc5c5b2..6a6aebed51 100644
--- a/doc/src/sgml/ref/pg_checksums.sgml
+++ b/doc/src/sgml/ref/pg_checksums.sgml
@@ -47,7 +47,8 @@ PostgreSQL documentation
 
   <para>
    When verifying checksums, every file in the cluster is scanned. When
-   enabling checksums, every file in the cluster is rewritten in-place.
+   enabling checksums, each relation file block with a changed checksum is 
+   rewritten in-place.
    Disabling checksums only updates the file <filename>pg_control</filename>.
   </para>
  </refsect1>