作者
digoal
日期
2021-01-18
标签
PostgreSQL , checksum
背景
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=cf621d9d84db1e6edaff8ffa26bad93fdce5f830
```
Add documentation chapter about checksums
Data checksums did not have a longer discussion in the docs,
this adds a short section with an overview.
Extracted from the larger patch for on-line enabling of checksums, which
has many more authors and reviewers.
Author: Daniel Gustafsson
Reviewed-By: Magnus Hagander, Michael Banck (and others through the big patch)
Discussion: https://postgr.es/m/5ff49fa4.1c69fb81.658f3.04ac@mx.google.com
```
开启checksum后, 在需要数据库恢复时, 如果遇到恢复corrupt的block异常会导致恢复失败, 遇到这种情况, 可以设置参数跳过checksum异常的block, 恢复出部分数据.
+ <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 a later time as an offline
+ operation. Data 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-offline-enable-disable">
+ <title>Off-line Enabling of Checksums</title>
+
+ <para>
+ The <link linkend="app-pgchecksums"><application>pg_checksums</application></link>
+ application can be used to enable or disable data checksums, as well as
+ verify checksums, on an offline cluster.
+ </para>
+
+ </sect2>
+ </sect1>
相关参数:
https://www.postgresql.org/docs/devel/runtime-config-developer.html
ignore_checksum_failure
Detection of a checksum failure during a read normally causes PostgreSQL to report an error, aborting the current transaction. Setting ignore_checksum_failure to on causes the system to ignore the failure (but still report a warning), and continue processing. This behavior may cause crashes, propagate or hide corruption, or other serious problems. However, it may allow you to get past the error and retrieve undamaged tuples that might still be present in the table if the block header is still sane. If the header is corrupt an error will be reported even if this option is enabled. The default setting is off, and it can only be changed by a superuser.
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