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Commit932a271

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Fix erroneous documentation of synchronous_commit = remote_write.
The docs claimed that this mode only waits for the standby to receive WALdata, but actually it waits for the data to be written out to the standby'sOS; which is a pretty significant difference because it removes the risk ofcrash of the walreceiver process.
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2 files changed

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‎doc/src/sgml/config.sgml

Lines changed: 28 additions & 20 deletions
Original file line numberDiff line numberDiff line change
@@ -1634,8 +1634,9 @@ SET ENABLE_SEQSCAN TO OFF;
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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</>
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indication to the client. Valid values are <literal>on</>, <literal>remote_write</>,
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<literal>local</>, and <literal>off</>. The default, and safe, value
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indication to the client. Valid values are <literal>on</>,
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<literal>remote_write</>, <literal>local</>, and <literal>off</>.
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The default, and safe, setting
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is <literal>on</>. When <literal>off</>, 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
@@ -1652,27 +1653,34 @@ SET ENABLE_SEQSCAN TO OFF;
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</para>
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<para>
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If <xref linkend="guc-synchronous-standby-names"> is set, this
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parameter also controls whether or not transaction commit will wait
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for the transaction's WAL records to be flushed to disk and replicated
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to the standby server. When <literal>remote_write</>, the commit wait will
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last until a reply from the current synchronous standby indicates
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it has received the commit record of the transaction to memory.
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Normally this causes no data loss at the time of failover. However,
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if both primary and standby crash, and the database cluster of
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the primary gets corrupted, recent committed transactions might
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be lost. When <literal>on</>, the commit wait will last until a reply
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from the current synchronous standby indicates it has flushed
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the commit record of the transaction to durable storage. This
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avoids any data loss unless the database cluster of both primary and
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standby gets corrupted simultaneously. If synchronous
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parameter also controls whether or not transaction commits will wait
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for the transaction's WAL records to be replicated to the standby
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server.
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When set to <literal>on</>, commits will wait until a reply
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from the current synchronous standby indicates it has received
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the commit record of the transaction and flushed it to disk. This
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ensures the transaction will not be lost unless both primary and
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standby suffer corruption of their database storage.
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When set to <literal>remote_write</>, commits will wait
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until a reply from the current synchronous standby indicates it has
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received the commit record of the transaction and written it out to
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the standby's operating system, but the data has not necessarily
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reached stable storage on the standby. This setting is sufficient to
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ensure data preservation even if the standby instance of
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<productname>PostgreSQL</> were to crash, but not if the standby
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suffers an operating-system-level crash.
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</para>
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<para>
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When synchronous
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replication is in use, it will normally be sensible either to wait
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for both local flush and replication of WAL records, or
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for both local flushto diskand replication of WAL records, or
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to allow the transaction to commit asynchronously. However, the
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special value <literal>local</> is available for transactions that
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setting <literal>local</> is available for transactions that
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wish to wait for local flush to disk, but not synchronous replication.
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If <varname>synchronous_standby_names</> is not set, <literal>on</>,
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<literal>remote_write</> and <literal>local</> provide the same
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synchronization level; transaction commit only waits for local flush.
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If <varname>synchronous_standby_names</> is not set, the settings
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<literal>on</>, <literal>remote_write</> and <literal>local</> all
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provide the same synchronization level: transaction commits only wait
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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

‎doc/src/sgml/high-availability.sgml

Lines changed: 10 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -989,9 +989,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
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<xref linkend="runtime-config-replication-master">.)
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This configuration will cause each commit to wait for
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confirmation that the standby has written the commit record to durable
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storage, even if that takes a very long time.
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storage.
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<varname>synchronous_commit</> can be set by individual
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users, so can be configured in the configuration file, for particular
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users, soitcan be configured in the configuration file, for particular
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users or databases, or dynamically by applications, in order to control
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the durability guarantee on a per-transaction basis.
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</para>
@@ -1015,10 +1015,14 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
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<para>
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Setting <varname>synchronous_commit</> to <literal>remote_write</> will
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cause each commit to wait for confirmation that the standby has received
1018-
the commit record to memory. This provides a lower level of durability
1019-
than <literal>on</> does. However, it's a practically useful setting
1020-
because it can decrease the response time for the transaction, and causes
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no data loss unless both the primary and the standby crashes and
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the commit record and written it out to its own operating system, but not
1019+
for the data to be flushed to disk on the standby. This
1020+
setting provides a weaker guarantee of durability than <literal>on</>
1021+
does: the standby could lose the data in the event of an operating system
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crash, though not a <productname>PostgreSQL</> crash.
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However, it's a useful setting in practice
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because it can decrease the response time for the transaction.
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Data loss could only occur if both the primary and the standby crash and
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the database of the primary gets corrupted at the same time.
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</para>
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