Added GUC variable bgwriter_flush_method controlling the action

done by the background writer between writing dirty blocks and
napping.

    none (default)   no action
	sync             bgwriter calls smgrsync() causing a sync(2)

A global sync() is only good on dedicated database servers, so
more flush methods should be added in the future.

Jan

src/backend/storage/buffer/bufmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.152 2004/01/09 21:08:49 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.153 2004/01/24 20:00:45 wieck Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -55,6 +55,7 @@
 #include "storage/proc.h"
 #include "storage/smgr.h"
 #include "utils/relcache.h"
+#include "utils/guc.h"
 
 #include "pgstat.h"
 
@@ -65,9 +66,23 @@
 /* GUC variable */
 bool		zero_damaged_pages = false;
 
+#define	BGWRITER_FLUSH_NONE			0
+#define	BGWRITER_FLUSH_NONE_STR		"none"
+#define	BGWRITER_FLUSH_SYNC			1
+#define	BGWRITER_FLUSH_SYNC_STR		"sync"
+
+#define	BGWRITER_FLUSH_DEFAULT		BGWRITER_FLUSH_NONE
+#define	BGWRITER_FLUSH_DEFAULT_STR	BGWRITER_FLUSH_NONE_STR
+
 int			BgWriterDelay = 200;
 int			BgWriterPercent = 1;
 int			BgWriterMaxpages = 100;
+int			BgWriterFlushMethod = BGWRITER_FLUSH_NONE;
+char	   *BgWriterFlushMethod_str = NULL;
+const char	BgWriterFlushMethod_default[] = BGWRITER_FLUSH_DEFAULT_STR;
+
+const char *BgWriterAssignSyncMethod(const char *method,
+            bool doit, GucSource source);
 
 static void WaitIO(BufferDesc *buf);
 static void StartBufferIO(BufferDesc *buf, bool forInput);
@@ -1026,6 +1041,19 @@ BufferBackgroundWriter(void)
 		if (InterruptPending)
 			return;
 
+		/*
+		 * Perform the configured buffer flush method
+		 */
+		switch (BgWriterFlushMethod)
+		{
+			case BGWRITER_FLUSH_NONE:
+				break;
+
+			case BGWRITER_FLUSH_SYNC:
+				smgrsync();
+				break;
+		}
+
 		/*
 		 * Nap for the configured time or sleep for 10 seconds if
 		 * there was nothing to do at all.
@@ -1037,6 +1065,27 @@ BufferBackgroundWriter(void)
 	}
 }
 
+const char *
+BgWriterAssignSyncMethod(const char *method, bool doit, GucSource source)
+{
+	int		new_flush_method;
+
+	if (strcasecmp(method, BGWRITER_FLUSH_NONE_STR) == 0)
+		new_flush_method = BGWRITER_FLUSH_NONE;
+	else 
+	if (strcasecmp(method, BGWRITER_FLUSH_SYNC_STR) == 0)
+		new_flush_method = BGWRITER_FLUSH_SYNC;
+	else
+		return NULL;
+	
+	if (!doit)
+		return method;
+
+	BgWriterFlushMethod = new_flush_method;
+	return method;
+}
+            
+
 /*
  * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
  */

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.179 2004/01/23 23:54:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.180 2004/01/24 20:00:45 wieck Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -95,6 +95,8 @@ static const char *assign_msglvl(int *var, const char *newval,
 static const char *assign_log_error_verbosity(const char *newval, bool doit,
 						   GucSource source);
 static bool assign_phony_autocommit(bool newval, bool doit, GucSource source);
+extern const char *BgWriterAssignSyncMethod(const char *method,
+						   bool doit, GucSource source);
 
 
 /*
@@ -1689,6 +1691,15 @@ static struct config_string ConfigureNamesString[] =
 		XLOG_sync_method_default, assign_xlog_sync_method, NULL
 	},
 
+	{
+		{"bgwriter_flush_method", PGC_SIGHUP, RESOURCES,
+			gettext_noop("Selects the method used by the bgwriter for forcing writes out to disk."),
+			NULL
+		},
+		&BgWriterFlushMethod_str,
+		BgWriterFlushMethod_default, BgWriterAssignSyncMethod, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, NULL, NULL, NULL

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

@@ -61,9 +61,12 @@
 #debug_shared_buffers = 0	# 0-600 seconds
 
 # - Background writer -
+#debug_shared_buffers = 0	# 0-600 seconds interval (0 = off)
 #bgwriter_delay = 200		# 10-5000 milliseconds
 #bgwriter_percent = 1		# 0-100% of dirty buffers
 #bgwriter_maxpages = 100	# 1-1000 buffers max at once
+#bgwriter_flush_method = none # how the bgwriter flushes kernel buffers
+							# one of: none or sync
 
 # - Free Space Map -
 

src/include/storage/bufmgr.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.73 2003/12/14 00:34:47 neilc Exp $
+ * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.74 2004/01/24 20:00:46 wieck Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -31,6 +31,8 @@ extern bool zero_damaged_pages;
 extern int	BgWriterDelay;
 extern int	BgWriterPercent;
 extern int	BgWriterMaxpages;
+extern char *BgWriterFlushMethod_str;
+extern const char BgWriterFlushMethod_default[];
 
 
 /* in buf_init.c */
@@ -180,8 +182,6 @@ extern void AbortBufferIO(void);
 extern void BufmgrCommit(void);
 extern int	BufferSync(int percent, int maxpages);
 extern void BufferBackgroundWriter(void);
-extern const char *BgWriterAssignSyncMethod(const char *method,
-			bool doid, bool interactive);
 
 extern void InitLocalBuffer(void);