Adjust documentation for syncfs().

Commit 8c16ad3 created a new appendix for syncfs(), which is
excessive for such a small amount of content.  This commit moves
the description of the caveats to be aware of when using syncfs()
back to the documentation for recovery_init_sync_method.  The
documentation for the other utilities with syncfs() support now
directs readers to recovery_init_sync_method for information about
these caveats.

Reported-by: Peter Eisentraut, Robert Haas
Suggested-by: Robert Haas
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/42804669-7063-1320-ed37-3226d5f1067d%40eisentraut.org
Discussion: https://postgr.es/m/CA%2BTgmobUiqKr%2BZMCLc5Qap-sXBnjfGUU%2BZBmzYEjUuWyjsGr1g%40mail.gmail.com

Author: nathan-bossart

doc/src/sgml/config.sgml

@@ -10870,9 +10870,15 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
         On Linux, <literal>syncfs</literal> may be used instead, to ask the
         operating system to synchronize the file systems that contain the
         data directory, the WAL files and each tablespace (but not any other
-        file systems that may be reachable through symbolic links).  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        file systems that may be reachable through symbolic links).  This may
+        be a lot faster than the <literal>fsync</literal> setting, because it
+        doesn't need to open each file one by one.  On the other hand, it may
+        be slower if a file system is shared by other applications that
+        modify a lot of files, since those files will also be written to disk.
+        Furthermore, on versions of Linux before 5.8, I/O errors encountered
+        while writing data to disk may not be reported to
+        <productname>PostgreSQL</productname>, and relevant error messages may
+        appear only in kernel logs.
        </para>
        <para>
         This parameter can only be set in the

doc/src/sgml/filelist.sgml

@@ -182,7 +182,6 @@
 <!ENTITY acronyms   SYSTEM "acronyms.sgml">
 <!ENTITY glossary   SYSTEM "glossary.sgml">
 <!ENTITY color      SYSTEM "color.sgml">
-<!ENTITY syncfs     SYSTEM "syncfs.sgml">
 
 <!ENTITY features-supported   SYSTEM "features-supported.sgml">
 <!ENTITY features-unsupported SYSTEM "features-unsupported.sgml">

doc/src/sgml/postgres.sgml

@@ -289,7 +289,6 @@ break is not needed in a wider output rendering.
   &acronyms;
   &glossary;
   &color;
-  &syncfs;
   &obsolete;
 
  </part>

doc/src/sgml/ref/initdb.sgml

@@ -394,8 +394,8 @@ PostgreSQL documentation
         On Linux, <literal>syncfs</literal> may be used instead to ask the
         operating system to synchronize the whole file systems that contain the
         data directory, the WAL files, and each tablespace.  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        <xref linkend="guc-recovery-init-sync-method"/> for information about
+        the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.

doc/src/sgml/ref/pg_basebackup.sgml

@@ -642,8 +642,8 @@ PostgreSQL documentation
         backup directory.  When the plain format is used,
         <command>pg_basebackup</command> will also synchronize the file systems
         that contain the WAL files and each tablespace.  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        <xref linkend="guc-recovery-init-sync-method"/> for information about
+        the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.

doc/src/sgml/ref/pg_checksums.sgml

@@ -152,8 +152,8 @@ PostgreSQL documentation
         On Linux, <literal>syncfs</literal> may be used instead to ask the
         operating system to synchronize the whole file systems that contain the
         data directory, the WAL files, and each tablespace.  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        <xref linkend="guc-recovery-init-sync-method"/> for information about
+        the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.

doc/src/sgml/ref/pg_combinebackup.sgml

@@ -176,8 +176,8 @@ PostgreSQL documentation
         backup directory.  When the plain format is used,
         <command>pg_combinebackup</command> will also synchronize the file systems
         that contain the WAL files and each tablespace.  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        <xref linkend="guc-recovery-init-sync-method"/> for information about
+        the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.

doc/src/sgml/ref/pg_dump.sgml

@@ -1319,8 +1319,9 @@ PostgreSQL documentation
        <para>
         On Linux, <literal>syncfs</literal> may be used instead to ask the
         operating system to synchronize the whole file system that contains the
-        archive directory.  See <xref linkend="syncfs"/> for more information
-        about using <function>syncfs()</function>.
+        archive directory.  See <xref linkend="guc-recovery-init-sync-method"/>
+        for information about the caveats to be aware of when using
+        <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used or

doc/src/sgml/ref/pg_rewind.sgml

@@ -297,8 +297,8 @@ PostgreSQL documentation
         On Linux, <literal>syncfs</literal> may be used instead to ask the
         operating system to synchronize the whole file systems that contain the
         data directory, the WAL files, and each tablespace.  See
-        <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        <xref linkend="guc-recovery-init-sync-method"/> for information about
+        the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.

doc/src/sgml/ref/pgupgrade.sgml

@@ -204,8 +204,8 @@ PostgreSQL documentation
         On Linux, <literal>syncfs</literal> may be used instead to ask the
         operating system to synchronize the whole file systems that contain the
         upgraded cluster's data directory, its WAL files, and each tablespace.
-        See <xref linkend="syncfs"/> for more information about using
-        <function>syncfs()</function>.
+        See <xref linkend="guc-recovery-init-sync-method"/> for information
+        about the caveats to be aware of when using <literal>syncfs</literal>.
        </para>
        <para>
         This option has no effect when <option>--no-sync</option> is used.