Re: Disable page writes when fsync off, add GUC - Mailing list pgsql-patches

Bruce Momjian wrote:
> This also adds a full_page_writes GUC to turn off page writes to WAL.
> Some people might not want full_page_writes.

Fsync linkage removed, patch attached and applied.

--
  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    5 Jul 2005 23:15:33 -0000
***************
*** 1660,1666 ****

         <para>
          This option can only be set at server start or in the
!         <filename>postgresql.conf</filename> file.
         </para>
        </listitem>
       </varlistentry>
--- 1660,1668 ----

         <para>
          This option can only be set at server start or in the
!         <filename>postgresql.conf</filename> file.  If this option
!         is <literal>off</>, consider also turning off
!         <varname>guc-full-page-writes</>.
         </para>
        </listitem>
       </varlistentry>
***************
*** 1687,1692 ****
--- 1689,1725 ----
        </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>
Index: src/backend/access/transam/xlog.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v
retrieving revision 1.206
diff -c -c -r1.206 xlog.c
*** src/backend/access/transam/xlog.c    4 Jul 2005 04:51:44 -0000    1.206
--- src/backend/access/transam/xlog.c    5 Jul 2005 23:15:36 -0000
***************
*** 103,108 ****
--- 103,109 ----
  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;
***************
*** 594,600 ****
                  {
                      /* 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;
--- 595,603 ----
                  {
                      /* OK, put it in this slot */
                      dtbuf[i] = rdt->buffer;
!                     /* 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;
Index: src/backend/utils/misc/guc.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/misc/guc.c,v
retrieving revision 1.272
diff -c -c -r1.272 guc.c
*** src/backend/utils/misc/guc.c    4 Jul 2005 04:51:51 -0000    1.272
--- src/backend/utils/misc/guc.c    5 Jul 2005 23:15:39 -0000
***************
*** 83,88 ****
--- 83,89 ----
  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);
***************
*** 483,488 ****
--- 484,501 ----
          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."),
              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    5 Jul 2005 23:15:39 -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