Disable page writes when fsync off, add GUC

This patch disables page writes to WAL when fsync is off, because with
no fsync guarantee, the page write recovery isn't useful.

This also adds a full_page_writes GUC to turn off page writes to WAL.
Some people might not want full_page_writes, but still might want fsync.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073
Index: doc/src/sgml/runtime.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v
retrieving revision 1.335
diff -c -c -r1.335 runtime.sgml
*** doc/src/sgml/runtime.sgml    2 Jul 2005 19:16:36 -0000    1.335
--- doc/src/sgml/runtime.sgml    4 Jul 2005 03:58:34 -0000
***************
*** 1687,1692 ****
--- 1687,1723 ----
        </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 recover from this
+         situation.
+        </para>
+
+        <para>
+         When this option is on, the <productname>PostgreSQL</> server
+         writes full pages when first modified after a checkpoint to WAL
+         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>
Index: src/backend/access/transam/xlog.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v
retrieving revision 1.205
diff -c -c -r1.205 xlog.c
*** src/backend/access/transam/xlog.c    30 Jun 2005 00:00:50 -0000    1.205
--- src/backend/access/transam/xlog.c    4 Jul 2005 03:58:38 -0000
***************
*** 97,102 ****
--- 97,103 ----
  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;
***************
*** 593,599 ****
                  {
                      /* OK, put it in this slot */
                      dtbuf[i] = rdt->buffer;
!                     if (XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                      {
                          dtbuf_bkp[i] = true;
                          rdt->data = NULL;
--- 594,602 ----
                  {
                      /* OK, put it in this slot */
                      dtbuf[i] = rdt->buffer;
!                     /* If fsync is off, no need to backup pages. */
!                     if (enableFsync && fullPageWrites &&
!                         XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                      {
                          dtbuf_bkp[i] = true;
                          rdt->data = NULL;
Index: src/backend/utils/misc/guc.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/misc/guc.c,v
retrieving revision 1.271
diff -c -c -r1.271 guc.c
*** src/backend/utils/misc/guc.c    28 Jun 2005 05:09:02 -0000    1.271
--- src/backend/utils/misc/guc.c    4 Jul 2005 03:58:46 -0000
***************
*** 82,87 ****
--- 82,88 ----
  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);
***************
*** 482,487 ****
--- 483,500 ----
          false, NULL, NULL
      },
      {
+         {"full_page_writes", PGC_SIGHUP, WAL_SETTINGS,
+             gettext_noop("Fully writes pages 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."),
              gettext_noop("If this parameter is set, the server will automatically run in the "
Index: src/backend/utils/misc/postgresql.conf.sample
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/misc/postgresql.conf.sample,v
retrieving revision 1.151
diff -c -c -r1.151 postgresql.conf.sample
*** src/backend/utils/misc/postgresql.conf.sample    2 Jul 2005 18:46:45 -0000    1.151
--- src/backend/utils/misc/postgresql.conf.sample    4 Jul 2005 03:58:46 -0000
***************
*** 121,126 ****
--- 121,127 ----
  #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