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Remove vacuum_defer_cleanup_age

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vacuum_defer_cleanup_age was introduced before hot_standby_feedback and
replication slots existed. It is hard to use reasonably - commonly it will
either be set too low (not preventing recovery conflicts, while still causing
some bloat), or too high (causing a lot of bloat). The alternatives do not
have that issue.

That on its own might not be sufficient reason to remove
vacuum_defer_cleanup_age, but it also complicates computation of xid
horizons. See e.g. the bug fixed in be504a3e97. It also is untested.

This commit removes TransactionIdRetreatSafely(), as there are no users
anymore. There might be potential future users, hence noting that here.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20230317230930.nhsgk3qfk7f4axls@awork3.anarazel.de
This commit is contained in:
Andres Freund
2023-04-24 12:20:52 -07:00
parent 441ee1677e
commit 1118cd37eb
8 changed files with 15 additions and 170 deletions
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35
doc/src/sgml/config.sgml
View File

@@ -4614,41 +4614,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
<term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_defer_cleanup_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of transactions by which <command>VACUUM</command> and
<link linkend="storage-hot"><acronym>HOT</acronym> updates</link>
will defer cleanup of dead row versions. The
default is zero transactions, meaning that dead row versions can be
removed as soon as possible, that is, as soon as they are no longer
visible to any open transaction. You may wish to set this to a
non-zero value on a primary server that is supporting hot standby
servers, as described in <xref linkend="hot-standby"/>. This allows
more time for queries on the standby to complete without incurring
conflicts due to early cleanup of rows. However, since the value
is measured in terms of number of write transactions occurring on the
primary server, it is difficult to predict just how much additional
grace time will be made available to standby queries.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
<para>
You should also consider setting <varname>hot_standby_feedback</varname>
on standby server(s) as an alternative to using this parameter.
</para>
<para>
This does not prevent cleanup of dead rows which have reached the age
specified by <varname>old_snapshot_threshold</varname>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>

28
doc/src/sgml/high-availability.sgml
View File

@@ -944,12 +944,11 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
retained by replication slots.
</para>
<para>
Similarly, <xref linkend="guc-hot-standby-feedback"/>
and <xref linkend="guc-vacuum-defer-cleanup-age"/> provide protection against
relevant rows being removed by vacuum, but the former provides no
protection during any time period when the standby is not connected,
and the latter often needs to be set to a high value to provide adequate
protection. Replication slots overcome these disadvantages.
Similarly, <xref linkend="guc-hot-standby-feedback"/> on its own, without
also using a replication slot, provides protection against relevant rows
being removed by vacuum, but provides no protection during any time period
when the standby is not connected. Replication slots overcome these
disadvantages.
</para>
<sect3 id="streaming-replication-slots-manipulation">
<title>Querying and Manipulating Replication Slots</title>
@@ -1910,17 +1909,6 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
by newly-arrived streaming WAL entries after reconnection.
</para>
<para>
Another option is to increase <xref linkend="guc-vacuum-defer-cleanup-age"/>
on the primary server, so that dead rows will not be cleaned up as quickly
as they normally would be. This will allow more time for queries to
execute before they are canceled on the standby, without having to set
a high <varname>max_standby_streaming_delay</varname>. However it is
difficult to guarantee any specific execution-time window with this
approach, since <varname>vacuum_defer_cleanup_age</varname> is measured in
transactions executed on the primary server.
</para>
<para>
The number of query cancels and the reason for them can be viewed using
the <structname>pg_stat_database_conflicts</structname> system view on the standby
@@ -2257,8 +2245,7 @@ HINT: You can then restart the server after making the necessary configuration
</para>
<para>
On the primary, parameters <xref linkend="guc-wal-level"/> and
<xref linkend="guc-vacuum-defer-cleanup-age"/> can be used.
On the primary, the <xref linkend="guc-wal-level"/> parameter can be used.
<xref linkend="guc-max-standby-archive-delay"/> and
<xref linkend="guc-max-standby-streaming-delay"/> have no effect if set on
the primary.
@@ -2268,9 +2255,6 @@ HINT: You can then restart the server after making the necessary configuration
On the standby, parameters <xref linkend="guc-hot-standby"/>,
<xref linkend="guc-max-standby-archive-delay"/> and
<xref linkend="guc-max-standby-streaming-delay"/> can be used.
<xref linkend="guc-vacuum-defer-cleanup-age"/> has no effect
as long as the server remains in standby mode, though it will
become relevant if the standby becomes primary.
</para>
</sect2>