Add GUC full_page_writes to control writing full pages to WAL.

doc/src/sgml/runtime.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.335 2005/07/02 19:16:36 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.336 2005/07/05 23:18:09 momjian Exp $
 -->
 
 <chapter Id="runtime">
@@ -1660,7 +1660,9 @@ SET ENABLE_SEQSCAN TO OFF;
 
        <para>
         This option can only be set at server start or in the
-        <filename>postgresql.conf</filename> file.
+        <filename>postgresql.conf</filename> file.  If this option
+        is <literal>off</>, consider also turning off 
+        <varname>guc-full-page-writes</>.
        </para>
       </listitem>
      </varlistentry>
@@ -1687,6 +1689,37 @@ SET ENABLE_SEQSCAN TO OFF;
       </listitem>
      </varlistentry>
      
+     <varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
+      <indexterm>
+       <primary><varname>full_page_writes</> configuration parameter</primary>
+      </indexterm>
+      <term><varname>full_page_writes</varname> (<type>boolean</type>)</term>
+      <listitem>
+       <para>
+        A page write in process during an operating system crash might
+        be only partially written to disk, leading to an on-disk page
+        that contains a mix of old and new data. During recovery, the
+        row changes stored in WAL are not enough to completely restore
+        the page.
+       </para>
+
+       <para>
+        When this option is on, the <productname>PostgreSQL</> server
+        writes full pages to WAL when they first modified after a checkpoint
+        so full recovery is possible. Turning this option off might lead
+        to a corrupt system after an operating system crash because
+        uncorrected partial pages might contain inconsistent or corrupt
+        data. The risks are less but similar to <varname>fsync</>.
+       </para>
+
+       <para>
+        This option can only be set at server start or in the
+        <filename>postgresql.conf</filename> file.  The default is
+        <literal>on</>.
+       </para>
+      </listitem>
+     </varlistentry>
+     
      <varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
       <term><varname>wal_buffers</varname> (<type>integer</type>)</term>
       <indexterm>

src/backend/access/transam/xlog.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.206 2005/07/04 04:51:44 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.207 2005/07/05 23:18:09 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -103,6 +103,7 @@ int			XLOGbuffers = 8;
 char	   *XLogArchiveCommand = NULL;
 char	   *XLOG_sync_method = NULL;
 const char	XLOG_sync_method_default[] = DEFAULT_SYNC_METHOD_STR;
+bool		fullPageWrites = true;
 
 #ifdef WAL_DEBUG
 bool		XLOG_DEBUG = false;
@@ -594,7 +595,9 @@ begin:;
 				{
 					/* OK, put it in this slot */
 					dtbuf[i] = rdt->buffer;
-					if (XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
+					/* If fsync is off, no need to backup pages. */
+					if (fullPageWrites &&
+						XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
 					{
 						dtbuf_bkp[i] = true;
 						rdt->data = NULL;

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 1.272 2005/07/04 04:51:51 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.273 2005/07/05 23:18:10 momjian Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -83,6 +83,7 @@ extern DLLIMPORT bool check_function_bodies;
 extern int	CommitDelay;
 extern int	CommitSiblings;
 extern char *default_tablespace;
+extern bool	fullPageWrites;
 
 static const char *assign_log_destination(const char *value,
 					   bool doit, GucSource source);
@@ -482,6 +483,18 @@ static struct config_bool ConfigureNamesBool[] =
 		&zero_damaged_pages,
 		false, NULL, NULL
 	},
+	{
+		{"full_page_writes", PGC_SIGHUP, WAL_SETTINGS,
+			gettext_noop("Writes full pages to WAL when first modified after a checkpoint."),
+			gettext_noop("A page write in process during an operating system crash might be "
+						 "only partially written to disk.  During recovery, the row changes"
+						 "stored in WAL are not enough to recover.  This option writes "
+						 "pages when first modified after a checkpoint to WAL so full recovery "
+						 "is possible.")
+		},
+		&fullPageWrites,
+		true, NULL, NULL
+	},
 	{
 		{"silent_mode", PGC_POSTMASTER, LOGGING_WHEN,
 			gettext_noop("Runs the server silently."),

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

@@ -121,6 +121,7 @@
 #wal_sync_method = fsync	# the default varies across platforms:
 				# fsync, fdatasync, fsync_writethrough,
 				# open_sync, open_datasync
+#full_page_writes = on		# recover from partial page writes
 #wal_buffers = 8		# min 4, 8KB each
 #commit_delay = 0		# range 0-100000, in microseconds
 #commit_siblings = 5		# range 1-1000