doc: improve description of synchronous_commit modes
Previously it wasn't clear exactly what each of the synchronous_commit modes accomplished. This clarifies that, and adds a table describing it. Only backpatched through 9.6 since 9.5 doesn't have all the options. Reported-by: kghost0@gmail.com Discussion: https://postgr.es/m/159741195522.14321.13812604195366728976@wrigleys.postgresql.org Backpatch-through: 9.6
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Bruce Momjian
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a97e85f2be
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@ -2703,14 +2703,26 @@ include_dir 'conf.d'
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</term>
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<listitem>
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<para>
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Specifies whether transaction commit will wait for WAL records
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to be written to disk before the command returns a <quote>success</quote>
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indication to the client. Valid values are <literal>on</literal>,
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<literal>remote_apply</literal>, <literal>remote_write</literal>, <literal>local</literal>,
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and <literal>off</literal>. The default, and safe, setting
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is <literal>on</literal>. When <literal>off</literal>, there can be a delay between
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when success is reported to the client and when the transaction is
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really guaranteed to be safe against a server crash. (The maximum
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Specifies how much WAL processing must complete before
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the database server returns a <quote>success</quote>
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indication to the client. Valid values are
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<literal>remote_apply</literal>, <literal>on</literal>
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(the default), <literal>remote_write</literal>,
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<literal>local</literal>, and <literal>off</literal>.
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</para>
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<para>
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If <varname>synchronous_standby_names</varname> is empty,
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the only meaningful settings are <literal>on</literal> and
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<literal>off</literal>; <literal>remote_apply</literal>,
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<literal>remote_write</literal> and <literal>local</literal>
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all provide the same local synchronization level
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as <literal>on</literal>. The local behavior of all
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non-<literal>off</literal> modes is to wait for local flush of WAL
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to disk. In <literal>off</literal> mode, there is no waiting,
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so there can be a delay between when success is reported to the
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client and when the transaction is later guaranteed to be safe
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against a server crash. (The maximum
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delay is three times <xref linkend="guc-wal-writer-delay"/>.) Unlike
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<xref linkend="guc-fsync"/>, setting this parameter to <literal>off</literal>
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does not create any risk of database inconsistency: an operating
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@ -2722,38 +2734,40 @@ include_dir 'conf.d'
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exact certainty about the durability of a transaction. For more
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discussion see <xref linkend="wal-async-commit"/>.
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</para>
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<para>
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If <xref linkend="guc-synchronous-standby-names"/> is non-empty, this
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parameter also controls whether or not transaction commits will wait
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for their WAL records to be replicated to the standby server(s).
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When set to <literal>on</literal>, commits will wait until replies
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If <xref linkend="guc-synchronous-standby-names"/> is non-empty,
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<varname>synchronous_commit</varname> also controls whether
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transaction commits will wait for their WAL records to be
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processed on the standby server(s).
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</para>
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<para>
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When set to <literal>remote_apply</literal>, commits will wait
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until replies from the current synchronous standby(s) indicate they
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have received the commit record of the transaction and applied
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it, so that it has become visible to queries on the standby(s),
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and also written to durable storage on the standbys. This will
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cause much larger commit delays than previous settings since
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it waits for WAL replay. When set to <literal>on</literal>,
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commits wait until replies
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from the current synchronous standby(s) indicate they have received
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the commit record of the transaction and flushed it to disk. This
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the commit record of the transaction and flushed it to durable storage. This
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ensures the transaction will not be lost unless both the primary and
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all synchronous standbys suffer corruption of their database storage.
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When set to <literal>remote_apply</literal>, commits will wait until replies
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from the current synchronous standby(s) indicate they have received the
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commit record of the transaction and applied it, so that it has become
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visible to queries on the standby(s).
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When set to <literal>remote_write</literal>, commits will wait until replies
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from the current synchronous standby(s) indicate they have
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received the commit record of the transaction and written it out to
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their operating system. This setting is sufficient to
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ensure data preservation even if a standby instance of
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<productname>PostgreSQL</productname> were to crash, but not if the standby
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suffers an operating-system-level crash, since the data has not
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received the commit record of the transaction and written it to
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their file systems. This setting ensures data preservation if a standby instance of
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<productname>PostgreSQL</productname> crashes, but not if the standby
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suffers an operating-system-level crash because the data has not
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necessarily reached durable storage on the standby.
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Finally, the setting <literal>local</literal> causes commits to wait for
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local flush to disk, but not for replication. This is not usually
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The setting <literal>local</literal> causes commits to wait for
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local flush to disk, but not for replication. This is usually not
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desirable when synchronous replication is in use, but is provided for
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completeness.
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</para>
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<para>
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If <varname>synchronous_standby_names</varname> is empty, the settings
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<literal>on</literal>, <literal>remote_apply</literal>, <literal>remote_write</literal>
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and <literal>local</literal> all provide the same synchronization level:
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transaction commits only wait for local flush to disk.
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</para>
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<para>
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This parameter can be changed at any time; the behavior for any
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one transaction is determined by the setting in effect when it
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@ -2763,6 +2777,76 @@ include_dir 'conf.d'
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asynchronously when the default is the opposite, issue <command>SET
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LOCAL synchronous_commit TO OFF</command> within the transaction.
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</para>
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<para>
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<xref linkend="synchronous-commit-matrix"/> summarizes the
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capabilities of the <varname>synchronous_commit</varname> settings.
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</para>
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<table id="synchronous-commit-matrix">
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<title>synchronous_commit Modes</title>
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<tgroup cols="5">
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<colspec colname="col1" colwidth="1.1*"/>
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<colspec colname="col2" colwidth="1*"/>
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<colspec colname="col3" colwidth="1*"/>
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<colspec colname="col4" colwidth="1*"/>
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<colspec colname="col5" colwidth="1*"/>
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<thead>
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<row>
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<entry>synchronous_commit setting</entry>
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<entry>local durable commit</entry>
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<entry>standby durable commit after PG crash</entry>
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<entry>standby durable commit after OS crash</entry>
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<entry>standby query consistency</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>remote_apply</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>on</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>remote_write</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>local</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>off</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center"></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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</listitem>
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</varlistentry>
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@ -72,7 +72,8 @@ typedef enum
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SYNCHRONOUS_COMMIT_REMOTE_WRITE, /* wait for local flush and remote
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* write */
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SYNCHRONOUS_COMMIT_REMOTE_FLUSH, /* wait for local and remote flush */
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SYNCHRONOUS_COMMIT_REMOTE_APPLY /* wait for local flush and remote apply */
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SYNCHRONOUS_COMMIT_REMOTE_APPLY /* wait for local and remote flush
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and remote apply */
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} SyncCommitLevel;
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/* Define the default setting for synchronous_commit */
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