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Revert "Allow on-line enabling and disabling of data checksums"
This reverts the backend sides of commit 1fde38beaa.
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.
This commit is contained in:
Magnus Hagander
@ -19566,71 +19566,6 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
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</sect2>
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<sect2 id="functions-admin-checksum">
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<title>Data Checksum Functions</title>
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<para>
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The functions shown in <xref linkend="functions-checksums-table" /> can
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be used to enable or disable data checksums in a running cluster.
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See <xref linkend="checksums" /> for details.
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</para>
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<table id="functions-checksums-table">
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<title>Checksum <acronym>SQL</acronym> Functions</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Return Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>
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<indexterm>
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<primary>pg_enable_data_checksums</primary>
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</indexterm>
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<literal><function>pg_enable_data_checksums(<optional><parameter>cost_delay</parameter> <type>int</type>, <parameter>cost_limit</parameter> <type>int</type></optional>)</function></literal>
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</entry>
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<entry>
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void
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</entry>
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<entry>
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<para>
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Initiates data checksums for the cluster. This will switch the data checksums mode
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to <literal>in progress</literal> and start a background worker that will process
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all data in the database and enable checksums for it. When all data pages have had
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checksums enabled, the cluster will automatically switch to checksums
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<literal>on</literal>.
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</para>
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<para>
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If <parameter>cost_delay</parameter> and <parameter>cost_limit</parameter> are
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specified, the speed of the process is throttled using the same principles as
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<link linkend="runtime-config-resource-vacuum-cost">Cost-based Vacuum Delay</link>.
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</para>
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</entry>
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</row>
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<row>
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<entry>
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<indexterm>
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<primary>pg_disable_data_checksums</primary>
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</indexterm>
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<literal><function>pg_disable_data_checksums()</function></literal>
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</entry>
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<entry>
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void
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</entry>
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<entry>
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Disables data checksums for the cluster.
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect2>
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<sect2 id="functions-admin-dbobject">
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<title>Database Object Management Functions</title>
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@ -214,9 +214,9 @@ PostgreSQL documentation
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<para>
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Use checksums on data pages to help detect corruption by the
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I/O system that would otherwise be silent. Enabling checksums
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may incur a noticeable performance penalty. If set, checksums
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are calculated for all objects, in all databases. See
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<xref linkend="checksums" /> for details.
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may incur a noticeable performance penalty. This option can only
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be set during initialization, and cannot be changed later. If
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set, checksums are calculated for all objects, in all databases.
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</para>
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</listitem>
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</varlistentry>
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@ -284,7 +284,6 @@
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&pgtestfsync;
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&pgtesttiming;
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&pgupgrade;
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&pgVerifyChecksums;
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&pgwaldump;
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&postgres;
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&postmaster;
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@ -230,87 +230,6 @@
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</para>
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</sect1>
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<sect1 id="checksums">
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<title>Data checksums</title>
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<indexterm>
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<primary>checksums</primary>
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</indexterm>
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<para>
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Data pages are not checksum protected by default, but this can optionally be enabled for a cluster.
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When enabled, each data page will be assigned a checksum that is updated when the page is
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written and verified every time the page is read. Only data pages are protected by checksums,
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internal data structures and temporary files are not.
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</para>
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<para>
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Checksums are normally enabled when the cluster is initialized using
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<link linkend="app-initdb-data-checksums"><application>initdb</application></link>. They
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can also be enabled or disabled at runtime. In all cases, checksums are enabled or disabled
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at the full cluster level, and cannot be specified individually for databases or tables.
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</para>
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<para>
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The current state of checksums in the cluster can be verified by viewing the value
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of the read-only configuration variable <xref linkend="guc-data-checksums" /> by
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issuing the command <command>SHOW data_checksums</command>.
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</para>
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<para>
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When attempting to recover from corrupt data it may be necessary to bypass the checksum
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protection in order to recover data. To do this, temporarily set the configuration parameter
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<xref linkend="guc-ignore-checksum-failure" />.
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</para>
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<sect2 id="checksums-enable-disable">
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<title>On-line enabling of checksums</title>
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<para>
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Checksums can be enabled or disabled online, by calling the appropriate
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<link linkend="functions-admin-checksum">functions</link>.
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Disabling of checksums takes effect immediately when the function is called.
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</para>
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<para>
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Enabling checksums will put the cluster in <literal>inprogress</literal> mode.
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During this time, checksums will be written but not verified. In addition to
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this, a background worker process is started that enables checksums on all
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existing data in the cluster. Once this worker has completed processing all
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databases in the cluster, the checksum mode will automatically switch to
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<literal>on</literal>.
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</para>
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<para>
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The process will initially wait for all open transactions to finish before
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it starts, so that it can be certain that there are no tables that have been
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created inside a transaction that has not committed yet and thus would not
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be visible to the process enabling checksums. It will also, for each database,
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wait for all pre-existing temporary tables to get removed before it finishes.
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If long-lived temporary tables are used in the application it may be necessary
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to terminate these application connections to allow the process to complete.
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Information about open transactions and connections with temporary tables is
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written to log.
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</para>
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<para>
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If the cluster is stopped while in <literal>inprogress</literal> mode, for
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any reason, then this process must be restarted manually. To do this,
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re-execute the function <function>pg_enable_data_checksums()</function>
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once the cluster has been restarted. It is not possible to resume the work,
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the process has to start over and re-process the cluster.
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</para>
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<note>
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<para>
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Enabling checksums can cause significant I/O to the system, as most of the
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database pages will need to be rewritten, and will be written both to the
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data files and the WAL.
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</para>
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</note>
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</sect2>
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</sect1>
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<sect1 id="wal-intro">
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<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
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