PostgreSQL 9.3 initdb add -S only do fsync used with pg_upgrade when server set fsync from off to on

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.

initdb -S选项目前仅仅被pg_upgrade用于fsync数据库集群目录, 因为pg_upgrade使用了fsync=off来加速升级过程.

然后在数据库集群起来之前会修改为fsync=on, 这个过程之间必须要将kernel或文件系统cache中的脏块写入持久化存储中.

所以就有了initdb -S这个选项, 用来做fsync的事情.

initdb src:

+   /* If we only need to fsync, just to it and exit */
+   if (sync_only)
+   {
+       setup_pgdata();
+       perform_fsync();
+       return 0;
+   }

contrib/pg_upgrade/server.c

+    * Turn off durability requirements to improve object creation speed, and
+    * we only modify the new cluster, so only use it there.  If there is a
+    * crash, the new cluster has to be recreated anyway.  fsync=off is a big
+    * win on ext4.

+            (cluster == &new_cluster) ?
+               " -c synchronous_commit=off -c fsync=off -c full_page_writes=off" : "",

contrib/pg_upgrade/pg_upgrade.c

+   prep_status("Sync data directory to disk");
+   exec_prog(UTILITY_LOG_FILE, NULL, true,
+             "\"%s/initdb\" --sync-only \"%s\"", new_cluster.bindir,
+             new_cluster.pgdata);
+   check_ok();

doc/src/sgml/config.sgml

+        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.

doc/src/sgml/ref/initdb.sgml

+        Safely write all database files to disk and exit.  This does not
+        perform any of the normal <application>initdb</> operations.

[参考]
1. http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=630cd14426dc1daf85163ad417f3a224eb4ac7b0

2. man initdb

       -S, --sync-only
           Safely write all database files to disk and exit. This does not perform any of the normal initdb
           operations.