Add initdb --sync-only option to sync the data directory to durable

storage.

Have pg_upgrade use it, and enable server options fsync=off and
full_page_writes=off.

Document that users turning fsync from off to on should run initdb
--sync-only.

[ Previous commit was incorrectly applied as a git merge. ]

doc/src/sgml/config.sgml

@@ -1696,6 +1696,15 @@ include 'filename'
         turning off <varname>fsync</varname>.
        </para>
 
+       <para>
+        For reliable recovery when changing <varname>fsync</varname>
+        off to on, it is necessary to force all modified buffers in the
+        kernel to durable storage.  This can be done while the cluster
+        is shutdown or while fsync is on by running <command>initdb
+        --sync-only</command>, running <command>sync</>, unmounting the
+        file system, or rebooting the server.
+       </para>
+
        <para>
         In many situations, turning off <xref linkend="guc-synchronous-commit">
         for noncritical transactions can provide much of the potential

doc/src/sgml/ref/initdb.sgml

@@ -244,6 +244,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>-S</option></term>
+      <term><option>--sync-only</option></term>
+      <listitem>
+       <para>
+        Safely write all database files to disk and exit.  This does not
+        perform any of the normal <application>initdb</> operations.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-T <replaceable>CFG</></option></term>
       <term><option>--text-search-config=<replaceable>CFG</></option></term>