Support syncing WAL log to disk using either fsync(), fdatasync(),

O_SYNC, or O_DSYNC (as available on a given platform).  Add GUC parameter
to control sync method.
Also, add defense to XLogWrite to prevent it from going nuts if passed
a target write position that's past the end of the buffers so far filled
by XLogInsert.

doc/src/sgml/runtime.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.56 2001/03/13 01:17:05 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.57 2001/03/16 05:44:33 tgl Exp $
 -->
 
 <Chapter Id="runtime">
@@ -1224,8 +1224,8 @@ env PGOPTIONS='-c geqo=off' psql
       <term>WAL_BUFFERS (<type>integer</type>)</term>
       <listitem>
        <para>
-        Number of disk-page buffers for WAL log.  This option can only be set
-	at server start.
+        Number of disk-page buffers in shared memory for WAL log.
+	This option can only be set at server start.
        </para>
       </listitem>
      </varlistentry>
@@ -1250,6 +1250,23 @@ env PGOPTIONS='-c geqo=off' psql
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry>
+      <term>WAL_SYNC_METHOD (<type>string</type>)</term>
+      <listitem>
+       <para>
+        Method used for forcing WAL updates out to disk.  Possible
+	values are
+	<literal>FSYNC</> (call fsync() at each commit),
+	<literal>FDATASYNC</> (call fdatasync() at each commit),
+	<literal>OPEN_SYNC</> (write WAL files with open() option O_SYNC), or
+	<literal>OPEN_DATASYNC</> (write WAL files with open() option O_DSYNC).
+	Not all of these choices are available on all platforms.
+        This option can only be set at server start or in the
+	<filename>postgresql.conf</filename> file.
+       </para>
+      </listitem>
+     </varlistentry>
     </variablelist>
     </para>
    </sect2>

doc/src/sgml/wal.sgml

-<!-- $Header: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v 1.4 2001/03/13 01:17:05 tgl Exp $ -->
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v 1.5 2001/03/16 05:44:33 tgl Exp $ -->
 
 <chapter id="wal">
  <title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
@@ -281,15 +281,6 @@
    <command>CHECKPOINT</command>.
   </para>
 
-  <para>
-   Setting the <varname>WAL_DEBUG</varname> parameter to any non-zero
-   value will result in each <function>LogInsert</function> and
-   <function>LogFlush</function> <acronym>WAL</acronym> call being
-   logged to standard error.  At present, it makes no difference what
-   the non-zero value is.  This option may be replaced by a more
-   general mechanism in the future.
-  </para>
-
   <para>
    The <varname>COMMIT_DELAY</varname> parameter defines for how many
    microseconds the backend will sleep after writing a commit
@@ -304,6 +295,24 @@
    ten milliseconds, so that any nonzero <varname>COMMIT_DELAY</varname>
    setting between 1 and 10000 microseconds will have the same effect.
   </para>
+
+  <para>
+   The <varname>WAL_SYNC_METHOD</varname> parameter determines how
+   Postgres will ask the kernel to force WAL updates out to disk.
+   All the options should be the same as far as reliability goes,
+   but it's quite platform-specific which one will be the fastest.
+   Note that this parameter is irrelevant if <varname>FSYNC</varname>
+   has been turned off.
+  </para>
+
+  <para>
+   Setting the <varname>WAL_DEBUG</varname> parameter to any non-zero
+   value will result in each <function>LogInsert</function> and
+   <function>LogFlush</function> <acronym>WAL</acronym> call being
+   logged to standard error.  At present, it makes no difference what
+   the non-zero value is.  This option may be replaced by a more
+   general mechanism in the future.
+  </para>
  </sect1>
 </chapter>
 

src/backend/utils/misc/guc.c

@@ -4,7 +4,7 @@
  * Support for grand unified configuration scheme, including SET
  * command, configuration file, and command line options.
  *
- * $Header: /cvsroot/pgsql/src/backend/utils/misc/guc.c,v 1.32 2001/03/13 01:17:06 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/utils/misc/guc.c,v 1.33 2001/03/16 05:44:33 tgl Exp $
  *
  * Copyright 2000 by PostgreSQL Global Development Group
  * Written by Peter Eisentraut <peter_e@gmx.net>.
@@ -20,6 +20,7 @@
 
 #include "utils/guc.h"
 
+#include "access/xlog.h"
 #include "commands/async.h"
 #include "libpq/auth.h"
 #include "libpq/pqcomm.h"
@@ -33,23 +34,17 @@
 #include "tcop/tcopprot.h"
 
 
-/* XXX should be in a header file */
+/* XXX these should be in other modules' header files */
 extern bool Log_connections;
-
-extern int CheckPointSegments;
 extern int CheckPointTimeout;
-extern int XLOGbuffers;
-extern int XLOGfiles;
-extern int XLOG_DEBUG;
 extern int CommitDelay;
 extern int CommitSiblings;
-
 extern bool FixBTree;
 
 #ifdef ENABLE_SYSLOG
 extern char *Syslog_facility;
 extern char *Syslog_ident;
-bool check_facility(const char *facility);
+static bool check_facility(const char *facility);
 #endif
 
 /*
@@ -138,7 +133,8 @@ struct config_string
 	GucContext  context;
 	char      **variable;
 	const char *default_val;
-	bool       (*parse_hook)(const char *);
+	bool       (*parse_hook)(const char *proposed);
+	void       (*assign_hook)(const char *newval);
 };
 
 
@@ -330,25 +326,29 @@ static struct config_string
 ConfigureNamesString[] =
 {
 	{"krb_server_keyfile",        PGC_POSTMASTER,       &pg_krb_server_keyfile,
-	 PG_KRB_SRVTAB, NULL},
-
-	{"unix_socket_group",         PGC_POSTMASTER,       &Unix_socket_group,
-	 "", NULL},
+	 PG_KRB_SRVTAB, NULL, NULL},
 
 #ifdef ENABLE_SYSLOG
 	{"syslog_facility",           PGC_POSTMASTER,	    &Syslog_facility,
-	"LOCAL0", check_facility},
+	"LOCAL0", check_facility, NULL},
 	{"syslog_ident",              PGC_POSTMASTER,	    &Syslog_ident,
-	"postgres", NULL},
+	"postgres", NULL, NULL},
 #endif
 
+	{"unix_socket_group",         PGC_POSTMASTER,       &Unix_socket_group,
+	 "", NULL, NULL},
+
 	{"unix_socket_directory",	  PGC_POSTMASTER,       &UnixSocketDir,
-	 "", NULL},
+	 "", NULL, NULL},
 
 	{"virtual_host",			  PGC_POSTMASTER,		&VirtualHost,
-	 "", NULL},
+	 "", NULL, NULL},
 
-	{NULL, 0, NULL, NULL, NULL}
+	{"wal_sync_method",			  PGC_SIGHUP,			&XLOG_sync_method,
+	 XLOG_sync_method_default,
+	 check_xlog_sync_method, assign_xlog_sync_method},
+
+	{NULL, 0, NULL, NULL, NULL, NULL}
 };
 
 /******** end of options list ********/
@@ -723,7 +723,10 @@ set_config_option(const char * name, const char * value, GucContext
 						elog(elevel, "out of memory");
 						return false;
 					}
-					free(*conf->variable);
+					if (conf->assign_hook)
+						(conf->assign_hook)(str);
+					if (*conf->variable)
+						free(*conf->variable);
 					*conf->variable = str;
 				}
 			}
@@ -737,7 +740,10 @@ set_config_option(const char * name, const char * value, GucContext
 					elog(elevel, "out of memory");
 					return false;
 				}
-				free(*conf->variable);
+				if (conf->assign_hook)
+					(conf->assign_hook)(str);
+				if (*conf->variable)
+					free(*conf->variable);
 				*conf->variable = str;
 			}
 			break;
@@ -855,7 +861,7 @@ ParseLongOption(const char * string, char ** name, char ** value)
 
 
 #ifdef ENABLE_SYSLOG
-bool
+static bool
 check_facility(const char *facility)
 {
 	if (strcasecmp(facility,"LOCAL0") == 0) return true;

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

@@ -107,6 +107,8 @@
 #
 #wal_buffers = 8 # min 4
 #wal_files = 0 # range 0-64
+#wal_sync_method = fsync # fsync or fdatasync or open_sync or open_datasync
+# Note: default wal_sync_method varies across platforms
 #wal_debug = 0 # range 0-16
 #commit_delay = 0 # range 0-100000
 #commit_siblings = 5 # range 1-1000

src/include/access/xlog.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: xlog.h,v 1.20 2001/03/13 20:32:37 tgl Exp $
+ * $Id: xlog.h,v 1.21 2001/03/16 05:44:33 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -176,6 +176,15 @@ extern	StartUpID	ThisStartUpID;	/* current SUI */
 extern	bool		InRecovery;
 extern	XLogRecPtr	MyLastRecPtr;
 
+/* these variables are GUC parameters related to XLOG */
+extern int CheckPointSegments;
+extern int XLOGbuffers;
+extern int XLOGfiles;
+extern int XLOG_DEBUG;
+extern char *XLOG_sync_method;
+extern const char XLOG_sync_method_default[];
+
+
 extern XLogRecPtr XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata);
 extern void XLogFlush(XLogRecPtr RecPtr);
 
@@ -202,4 +211,7 @@ extern void GetRedoRecPtr(void);
  */
 extern XLogRecPtr GetUndoRecPtr(void);
 
+extern bool check_xlog_sync_method(const char *method);
+extern void assign_xlog_sync_method(const char *method);
+
 #endif	 /* XLOG_H */