Revert "Allow on-line enabling and disabling of data checksums"

This reverts the backend sides of commit 1fde38beaa0c3e66c340efc7cc0dc272d6254bb0.
I have, at least for now, left the pg_verify_checksums tool in place, as
this tool can be very valuable without the rest of the patch as well,
and since it's a read-only tool that only runs when the cluster is down
it should be a lot safer.

4 files changed, 3 insertions, 150 deletions

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 3bf21477adb..53a40ddeecf 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -19566,71 +19566,6 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
 
   </sect2>
 
-  <sect2 id="functions-admin-checksum">
-   <title>Data Checksum Functions</title>
-
-   <para>
-    The functions shown in <xref linkend="functions-checksums-table" /> can
-    be used to enable or disable data checksums in a running cluster.
-    See <xref linkend="checksums" /> for details.
-   </para>
-
-   <table id="functions-checksums-table">
-    <title>Checksum <acronym>SQL</acronym> Functions</title>
-    <tgroup cols="3">
-     <thead>
-      <row>
-       <entry>Function</entry>
-       <entry>Return Type</entry>
-       <entry>Description</entry>
-      </row>
-     </thead>
-     <tbody>
-      <row>
-       <entry>
-        <indexterm>
-         <primary>pg_enable_data_checksums</primary>
-        </indexterm>
-        <literal><function>pg_enable_data_checksums(<optional><parameter>cost_delay</parameter> <type>int</type>, <parameter>cost_limit</parameter> <type>int</type></optional>)</function></literal>
-       </entry>
-       <entry>
-        void
-       </entry>
-       <entry>
-        <para>
-         Initiates data checksums for the cluster. This will switch the data checksums mode
-         to <literal>in progress</literal> and start a background worker that will process
-         all data in the database and enable checksums for it. When all data pages have had
-         checksums enabled, the cluster will automatically switch to checksums
-         <literal>on</literal>.
-        </para>
-        <para>
-         If <parameter>cost_delay</parameter> and <parameter>cost_limit</parameter> are
-         specified, the speed of the process is throttled using the same principles as
-         <link linkend="runtime-config-resource-vacuum-cost">Cost-based Vacuum Delay</link>.
-        </para>
-       </entry>
-      </row>
-      <row>
-       <entry>
-        <indexterm>
-         <primary>pg_disable_data_checksums</primary>
-        </indexterm>
-        <literal><function>pg_disable_data_checksums()</function></literal>
-       </entry>
-       <entry>
-        void
-       </entry>
-       <entry>
-        Disables data checksums for the cluster.
-       </entry>
-      </row>
-     </tbody>
-    </tgroup>
-   </table>
-
-  </sect2>
-
   <sect2 id="functions-admin-dbobject">
    <title>Database Object Management Functions</title>
 
diff --git a/doc/src/sgml/ref/initdb.sgml b/doc/src/sgml/ref/initdb.sgml
index 10a8a86a030..feefd9a41e0 100644
--- a/doc/src/sgml/ref/initdb.sgml
+++ b/doc/src/sgml/ref/initdb.sgml
@@ -214,9 +214,9 @@ PostgreSQL documentation
        <para>
         Use checksums on data pages to help detect corruption by the
         I/O system that would otherwise be silent. Enabling checksums
-        may incur a noticeable performance penalty. If set, checksums
-        are calculated for all objects, in all databases. See
-        <xref linkend="checksums" /> for details.
+        may incur a noticeable performance penalty. This option can only
+        be set during initialization, and cannot be changed later. If
+        set, checksums are calculated for all objects, in all databases.
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/reference.sgml b/doc/src/sgml/reference.sgml
index 78c214f1b08..ef2270c4673 100644
--- a/doc/src/sgml/reference.sgml
+++ b/doc/src/sgml/reference.sgml
@@ -284,7 +284,6 @@
    &pgtestfsync;
    &pgtesttiming;
    &pgupgrade;
-   &pgVerifyChecksums;
    &pgwaldump;
    &postgres;
    &postmaster;
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index 6249cb41361..f4bc2d4161e 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -230,87 +230,6 @@
   </para>
  </sect1>
 
- <sect1 id="checksums">
-  <title>Data checksums</title>
-  <indexterm>
-   <primary>checksums</primary>
-  </indexterm>
-
-  <para>
-   Data pages are not checksum protected by default, but this can optionally be enabled for a cluster.
-   When enabled, each data page will be assigned a checksum that is updated when the page is
-   written and verified every time the page is read. Only data pages are protected by checksums,
-   internal data structures and temporary files are not.
-  </para>
-
-  <para>
-   Checksums are normally enabled when the cluster is initialized using
-   <link linkend="app-initdb-data-checksums"><application>initdb</application></link>. They
-   can also be enabled or disabled at runtime. In all cases, checksums are enabled or disabled
-   at the full cluster level, and cannot be specified individually for databases or tables.
-  </para>
-
-  <para>
-   The current state of checksums in the cluster can be verified by viewing the value
-   of the read-only configuration variable <xref linkend="guc-data-checksums" /> by
-   issuing the command <command>SHOW data_checksums</command>.
-  </para>
-
-  <para>
-   When attempting to recover from corrupt data it may be necessary to bypass the checksum
-   protection in order to recover data. To do this, temporarily set the configuration parameter
-   <xref linkend="guc-ignore-checksum-failure" />.
-  </para>
-
-  <sect2 id="checksums-enable-disable">
-   <title>On-line enabling of checksums</title>
-
-   <para>
-    Checksums can be enabled or disabled online, by calling the appropriate
-    <link linkend="functions-admin-checksum">functions</link>.
-    Disabling of checksums takes effect immediately when the function is called.
-   </para>
-
-   <para>
-    Enabling checksums will put the cluster in <literal>inprogress</literal> mode.
-    During this time, checksums will be written but not verified. In addition to
-    this, a background worker process is started that enables checksums on all
-    existing data in the cluster. Once this worker has completed processing all
-    databases in the cluster, the checksum mode will automatically switch to
-    <literal>on</literal>.
-   </para>
-
-   <para>
-    The process will initially wait for all open transactions to finish before
-    it starts, so that it can be certain that there are no tables that have been
-    created inside a transaction that has not committed yet and thus would not
-    be visible to the process enabling checksums. It will also, for each database,
-    wait for all pre-existing temporary tables to get removed before it finishes.
-    If long-lived temporary tables are used in the application it may be necessary
-    to terminate these application connections to allow the process to complete.
-    Information about open transactions and connections with temporary tables is
-    written to log.
-   </para>
-
-   <para>
-    If the cluster is stopped while in <literal>inprogress</literal> mode, for
-    any reason, then this process must be restarted manually. To do this,
-    re-execute the function <function>pg_enable_data_checksums()</function>
-    once the cluster has been restarted. It is not possible to resume the work,
-    the process has to start over and re-process the cluster.
-   </para>
-
-   <note>
-    <para>
-     Enabling checksums can cause significant I/O to the system, as most of the
-     database pages will need to be rewritten, and will be written both to the
-     data files and the WAL.
-    </para>
-   </note>
-
-  </sect2>
- </sect1>
-
   <sect1 id="wal-intro">
    <title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>