database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-23 14:01:44 +03:00
Code Activity

Consolidate docs for vacuum-related GUCs in new subsection

Browse Source 
GUCs related to vacuum's freezing behavior were documented in a
subsection of the Client Connection Defaults documentation. These GUCs
don't belong there, as they affect the freezing behavior of all vacuums
-- including autovacuums.

There wasn't a clear alternative location, so this commit makes a new
"Server Configuration" docs subsection, "Vacuuming", with a subsection
for "Freezing". It also moves the "Automatic Vacuuming" subsection and
the docs on GUCs controlling cost-based vacuum delay under the new
"Vacuuming" subsection.

The other vacuum-related GUCs under the "Resource Consumption"
subsection have been left in their current location, as they seem to fit
there.

The GUCs' documentation was largely lifted and shifted. The only
modification made was the addition of a few missing <literal> tags.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/flat/CAAKRu_aQUOaMYrcjNuXeSkJtaX9oRUzKP57bsYbC0gVVWS%2BcbA%40mail.gmail.com
This commit is contained in:
Melanie Plageman
2025-01-10 18:20:16 -05:00
parent 27a1f8d108
commit ca9c6a5680
1 changed files with 642 additions and 610 deletions
Download Patch File Download Diff File Expand all files Collapse all files

660
doc/src/sgml/config.sgml
View File

@ -2367,149 +2367,6 @@ include_dir 'conf.d'
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-vacuum-cost">
<title>Cost-based Vacuum Delay</title>
<para>
During the execution of <xref linkend="sql-vacuum"/>
and <xref linkend="sql-analyze"/>
commands, the system maintains an
internal counter that keeps track of the estimated cost of the
various I/O operations that are performed. When the accumulated
cost reaches a limit (specified by
<varname>vacuum_cost_limit</varname>), the process performing
the operation will sleep for a short period of time, as specified by
<varname>vacuum_cost_delay</varname>. Then it will reset the
counter and continue execution.
</para>
<para>
The intent of this feature is to allow administrators to reduce
the I/O impact of these commands on concurrent database
activity. There are many situations where it is not
important that maintenance commands like
<command>VACUUM</command> and <command>ANALYZE</command> finish
quickly; however, it is usually very important that these
commands do not significantly interfere with the ability of the
system to perform other database operations. Cost-based vacuum
delay provides a way for administrators to achieve this.
</para>
<para>
This feature is disabled by default for manually issued
<command>VACUUM</command> commands. To enable it, set the
<varname>vacuum_cost_delay</varname> variable to a nonzero
value.
</para>
<variablelist>
<varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
<term><varname>vacuum_cost_delay</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>vacuum_cost_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of time that the process will sleep
when the cost limit has been exceeded.
If this value is specified without units, it is taken as milliseconds.
The default value is zero, which disables the cost-based vacuum
delay feature. Positive values enable cost-based vacuuming.
</para>
<para>
When using cost-based vacuuming, appropriate values for
<varname>vacuum_cost_delay</varname> are usually quite small, perhaps
less than 1 millisecond. While <varname>vacuum_cost_delay</varname>
can be set to fractional-millisecond values, such delays may not be
measured accurately on older platforms. On such platforms,
increasing <command>VACUUM</command>'s throttled resource consumption
above what you get at 1ms will require changing the other vacuum cost
parameters. You should, nonetheless,
keep <varname>vacuum_cost_delay</varname> as small as your platform
will consistently measure; large delays are not helpful.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_hit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer found in the shared buffer
cache. It represents the cost to lock the buffer pool, lookup
the shared hash table and scan the content of the page. The
default value is one.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_miss</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer that has to be read from
disk. This represents the effort to lock the buffer pool,
lookup the shared hash table, read the desired block in from
the disk and scan its content. The default value is 2.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_dirty</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost charged when vacuum modifies a block that was
previously clean. It represents the extra I/O required to
flush the dirty block out to disk again. The default value is
20.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is the accumulated cost that will cause the vacuuming process to sleep
for <varname>vacuum_cost_delay</varname>. The default is 200.
</para>
</listitem>
</varlistentry>
</variablelist>
<note>
<para>
There are certain operations that hold critical locks and should
therefore complete as quickly as possible. Cost-based vacuum
delays do not occur during such operations. Therefore it is
possible that the cost accumulates far higher than the specified
limit. To avoid uselessly long delays in such cases, the actual
delay is calculated as <varname>vacuum_cost_delay</varname> *
<varname>accumulated_balance</varname> /
<varname>vacuum_cost_limit</varname> with a maximum of
<varname>vacuum_cost_delay</varname> * 4.
</para>
</note>
</sect2>
<sect2 id="runtime-config-resource-background-writer">
<title>Background Writer</title>
@ -8590,14 +8447,22 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</sect2>
</sect1>
<sect1 id="runtime-config-autovacuum">
<title>Automatic Vacuuming</title>
<sect1 id="runtime-config-vacuum">
<title>Vacuuming</title>
<indexterm>
<primary>autovacuum</primary>
<primary>vacuum</primary>
<secondary>configuration parameters</secondary>
</indexterm>
<para>
These parameters control vacuuming behavior. For more information on the
purpose and responsibilities of vacuum, see <xref linkend="routine-vacuuming"/>.
</para>
<sect2 id="runtime-config-autovacuum">
<title>Automatic Vacuuming</title>
<para>
These settings control the behavior of the <firstterm>autovacuum</firstterm>
feature. Refer to <xref linkend="autovacuum"/> for more information.
@ -8662,7 +8527,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
<para>
Specifies the maximum number of autovacuum processes (other than the
autovacuum launcher) that may be running at any one time. The default
is three. This parameter can only be set in the
is <literal>3</literal>. This parameter can only be set in the
<filename>postgresql.conf</filename> file or on the server command line.
</para>
<para>
@ -8768,7 +8633,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
Specifies a fraction of the table size to add to
<varname>autovacuum_vacuum_threshold</varname>
when deciding whether to trigger a <command>VACUUM</command>.
The default is 0.2 (20% of table size).
The default is <literal>0.2</literal> (20% of table size).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
@ -8789,7 +8654,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
Specifies a fraction of the table size to add to
<varname>autovacuum_vacuum_insert_threshold</varname>
when deciding whether to trigger a <command>VACUUM</command>.
The default is 0.2 (20% of table size).
The default is <literal>0.2</literal> (20% of table size).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
@ -8810,7 +8675,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
Specifies a fraction of the table size to add to
<varname>autovacuum_analyze_threshold</varname>
when deciding whether to trigger an <command>ANALYZE</command>.
The default is 0.1 (10% of table size).
The default is <literal>0.1</literal> (10% of table size).
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line;
but the setting can be overridden for individual tables by
@ -8909,8 +8774,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
<listitem>
<para>
Specifies the cost limit value that will be used in automatic
<command>VACUUM</command> operations. If -1 is specified (which is the
default), the regular
<command>VACUUM</command> operations. If <literal>-1</literal>
is specified (which is the default), the regular
<xref linkend="guc-vacuum-cost-limit"/> value will be used. Note that
the value is distributed proportionally among the running autovacuum
workers, if there is more than one, so that the sum of the limits for
@ -8924,6 +8789,335 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-vacuum-cost">
<title>Cost-based Vacuum Delay</title>
<para>
During the execution of <xref linkend="sql-vacuum"/>
and <xref linkend="sql-analyze"/>
commands, the system maintains an
internal counter that keeps track of the estimated cost of the
various I/O operations that are performed. When the accumulated
cost reaches a limit (specified by
<varname>vacuum_cost_limit</varname>), the process performing
the operation will sleep for a short period of time, as specified by
<varname>vacuum_cost_delay</varname>. Then it will reset the
counter and continue execution.
</para>
<para>
The intent of this feature is to allow administrators to reduce
the I/O impact of these commands on concurrent database
activity. There are many situations where it is not
important that maintenance commands like
<command>VACUUM</command> and <command>ANALYZE</command> finish
quickly; however, it is usually very important that these
commands do not significantly interfere with the ability of the
system to perform other database operations. Cost-based vacuum
delay provides a way for administrators to achieve this.
</para>
<para>
This feature is disabled by default for manually issued
<command>VACUUM</command> commands. To enable it, set the
<varname>vacuum_cost_delay</varname> variable to a nonzero
value.
</para>
<variablelist>
<varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
<term><varname>vacuum_cost_delay</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>vacuum_cost_delay</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of time that the process will sleep when the cost
limit has been exceeded. If this value is specified without
units, it is taken as milliseconds. The default value is
<literal>0</literal>, which disables the cost-based vacuum delay
feature. Positive values enable cost-based vacuuming.
</para>
<para>
When using cost-based vacuuming, appropriate values for
<varname>vacuum_cost_delay</varname> are usually quite small, perhaps
less than 1 millisecond. While <varname>vacuum_cost_delay</varname>
can be set to fractional-millisecond values, such delays may not be
measured accurately on older platforms. On such platforms,
increasing <command>VACUUM</command>'s throttled resource consumption
above what you get at 1ms will require changing the other vacuum cost
parameters. You should, nonetheless,
keep <varname>vacuum_cost_delay</varname> as small as your platform
will consistently measure; large delays are not helpful.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_hit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer found in the shared
buffer cache. It represents the cost to lock the buffer pool,
lookup the shared hash table and scan the content of the page.
The default value is <literal>1</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_miss</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer that has to be read from
disk. This represents the effort to lock the buffer pool,
lookup the shared hash table, read the desired block in from
the disk and scan its content. The default value is
<literal>2</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_dirty</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost charged when vacuum modifies a block that was
previously clean. It represents the extra I/O required to
flush the dirty block out to disk again. The default value is
<literal>20</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_limit</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is the accumulated cost that will cause the vacuuming
process to sleep for <varname>vacuum_cost_delay</varname>. The
default is <literal>200</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
<note>
<para>
There are certain operations that hold critical locks and should
therefore complete as quickly as possible. Cost-based vacuum
delays do not occur during such operations. Therefore it is
possible that the cost accumulates far higher than the specified
limit. To avoid uselessly long delays in such cases, the actual
delay is calculated as <varname>vacuum_cost_delay</varname> *
<varname>accumulated_balance</varname> /
<varname>vacuum_cost_limit</varname> with a maximum of
<varname>vacuum_cost_delay</varname> * 4.
</para>
</note>
</sect2>
<sect2 id="runtime-config-vacuum-freezing">
<title>Freezing</title>
<para>
To maintain correctness even after transaction IDs wrap around,
<productname>PostgreSQL</productname> marks rows that are sufficiently
old as <emphasis>frozen</emphasis>. These rows are visible to everyone;
other transactions do not need to examine their inserting XID to
determine visibility. <command>VACUUM</command> is responsible for
marking rows as frozen. The following settings control
<command>VACUUM</command>'s freezing behavior and should be tuned based
on the XID consumption rate of the system and data access patterns of the
dominant workloads. See <xref linkend="vacuum-for-wraparound"/> for more
information on transaction ID wraparound and tuning these parameters.
</para>
<variablelist>
<varlistentry id="guc-vacuum-freeze-table-age" xreflabel="vacuum_freeze_table_age">
<term><varname>vacuum_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million transactions. Although users can
set this value anywhere from zero to two billion, <command>VACUUM</command>
will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-freeze-max-age"/>, so that a
periodic manual <command>VACUUM</command> has a chance to run before an
anti-wraparound autovacuum is launched for the table. For more
information see
<xref linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
<term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in transactions) that
<command>VACUUM</command> should use to decide whether to
trigger freezing of pages that have an older XID.
The default is 50 million transactions. Although
users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-freeze-max-age"/>, so
that there is not an unreasonably short time between forced
autovacuums. For more information see <xref
linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-failsafe-age" xreflabel="vacuum_failsafe_age">
<term><varname>vacuum_failsafe_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_failsafe_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in transactions) that a table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield>
field can attain before <command>VACUUM</command> takes
extraordinary measures to avoid system-wide transaction ID
wraparound failure. This is <command>VACUUM</command>'s
strategy of last resort. The failsafe typically triggers
when an autovacuum to prevent transaction ID wraparound has
already been running for some time, though it's possible for
the failsafe to trigger during any <command>VACUUM</command>.
</para>
<para>
When the failsafe is triggered, any cost-based delay that is
in effect will no longer be applied, further non-essential
maintenance tasks (such as index vacuuming) are bypassed, and any
<glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
in use will be disabled resulting in <command>VACUUM</command> being
free to make use of all of
<glossterm linkend="glossary-shared-memory">shared buffers</glossterm>.
</para>
<para>
The default is 1.6 billion transactions. Although users can
set this value anywhere from zero to 2.1 billion,
<command>VACUUM</command> will silently adjust the effective
value to no less than 105% of <xref
linkend="guc-autovacuum-freeze-max-age"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-table-age" xreflabel="vacuum_multixact_freeze_table_age">
<term><varname>vacuum_multixact_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million multixacts.
Although users can set this value anywhere from zero to two billion,
<command>VACUUM</command> will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-multixact-freeze-max-age"/>, so that a
periodic manual <command>VACUUM</command> has a chance to run before an
anti-wraparound is launched for the table.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-min-age" xreflabel="vacuum_multixact_freeze_min_age">
<term><varname>vacuum_multixact_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in multixacts) that <command>VACUUM</command>
should use to decide whether to trigger freezing of pages with
an older multixact ID. The default is 5 million multixacts.
Although users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>,
so that there is not an unreasonably short time between forced
autovacuums.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-failsafe-age" xreflabel="vacuum_multixact_failsafe_age">
<term><varname>vacuum_multixact_failsafe_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_failsafe_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in multixacts) that a table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield>
field can attain before <command>VACUUM</command> takes
extraordinary measures to avoid system-wide multixact ID
wraparound failure. This is <command>VACUUM</command>'s
strategy of last resort. The failsafe typically triggers when
an autovacuum to prevent transaction ID wraparound has already
been running for some time, though it's possible for the
failsafe to trigger during any <command>VACUUM</command>.
</para>
<para>
When the failsafe is triggered, any cost-based delay that is
in effect will no longer be applied, and further non-essential
maintenance tasks (such as index vacuuming) are bypassed.
</para>
<para>
The default is 1.6 billion multixacts. Although users can set
this value anywhere from zero to 2.1 billion,
<command>VACUUM</command> will silently adjust the effective
value to no less than 105% of <xref
linkend="guc-autovacuum-multixact-freeze-max-age"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-client">
@ -9595,168 +9789,6 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-table-age" xreflabel="vacuum_freeze_table_age">
<term><varname>vacuum_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million transactions. Although users can
set this value anywhere from zero to two billion, <command>VACUUM</command>
will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-freeze-max-age"/>, so that a
periodic manual <command>VACUUM</command> has a chance to run before an
anti-wraparound autovacuum is launched for the table. For more
information see
<xref linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
<term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in transactions) that
<command>VACUUM</command> should use to decide whether to
trigger freezing of pages that have an older XID.
The default is 50 million transactions. Although
users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-freeze-max-age"/>, so
that there is not an unreasonably short time between forced
autovacuums. For more information see <xref
linkend="vacuum-for-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-failsafe-age" xreflabel="vacuum_failsafe_age">
<term><varname>vacuum_failsafe_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_failsafe_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in transactions) that a table's
<structname>pg_class</structname>.<structfield>relfrozenxid</structfield>
field can attain before <command>VACUUM</command> takes
extraordinary measures to avoid system-wide transaction ID
wraparound failure. This is <command>VACUUM</command>'s
strategy of last resort. The failsafe typically triggers
when an autovacuum to prevent transaction ID wraparound has
already been running for some time, though it's possible for
the failsafe to trigger during any <command>VACUUM</command>.
</para>
<para>
When the failsafe is triggered, any cost-based delay that is
in effect will no longer be applied, further non-essential
maintenance tasks (such as index vacuuming) are bypassed, and any
<glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
in use will be disabled resulting in <command>VACUUM</command> being
free to make use of all of
<glossterm linkend="glossary-shared-memory">shared buffers</glossterm>.
</para>
<para>
The default is 1.6 billion transactions. Although users can
set this value anywhere from zero to 2.1 billion,
<command>VACUUM</command> will silently adjust the effective
value to no less than 105% of <xref
linkend="guc-autovacuum-freeze-max-age"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-table-age" xreflabel="vacuum_multixact_freeze_table_age">
<term><varname>vacuum_multixact_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_table_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</command> performs an aggressive scan if the table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield> field has reached
the age specified by this setting. An aggressive scan differs from
a regular <command>VACUUM</command> in that it visits every page that might
contain unfrozen XIDs or MXIDs, not just those that might contain dead
tuples. The default is 150 million multixacts.
Although users can set this value anywhere from zero to two billion,
<command>VACUUM</command> will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-multixact-freeze-max-age"/>, so that a
periodic manual <command>VACUUM</command> has a chance to run before an
anti-wraparound is launched for the table.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-min-age" xreflabel="vacuum_multixact_freeze_min_age">
<term><varname>vacuum_multixact_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_min_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in multixacts) that <command>VACUUM</command>
should use to decide whether to trigger freezing of pages with
an older multixact ID. The default is 5 million multixacts.
Although users can set this value anywhere from zero to one billion,
<command>VACUUM</command> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>,
so that there is not an unreasonably short time between forced
autovacuums.
For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-failsafe-age" xreflabel="vacuum_multixact_failsafe_age">
<term><varname>vacuum_multixact_failsafe_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_failsafe_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in multixacts) that a table's
<structname>pg_class</structname>.<structfield>relminmxid</structfield>
field can attain before <command>VACUUM</command> takes
extraordinary measures to avoid system-wide multixact ID
wraparound failure. This is <command>VACUUM</command>'s
strategy of last resort. The failsafe typically triggers when
an autovacuum to prevent transaction ID wraparound has already
been running for some time, though it's possible for the
failsafe to trigger during any <command>VACUUM</command>.
</para>
<para>
When the failsafe is triggered, any cost-based delay that is
in effect will no longer be applied, and further non-essential
maintenance tasks (such as index vacuuming) are bypassed.
</para>
<para>
The default is 1.6 billion multixacts. Although users can set
this value anywhere from zero to 2.1 billion,
<command>VACUUM</command> will silently adjust the effective
value to no less than 105% of <xref
linkend="guc-autovacuum-multixact-freeze-max-age"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bytea-output" xreflabel="bytea_output">
<term><varname>bytea_output</varname> (<type>enum</type>)
<indexterm>