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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>19.4. Resource Consumption</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="runtime-config-connection.html" title="19.3. Connections and Authentication" /><link rel="next" href="runtime-config-wal.html" title="19.5. Write Ahead Log" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">19.4. Resource Consumption</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="runtime-config-connection.html" title="19.3. Connections and Authentication">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="runtime-config.html" title="Chapter 19. Server Configuration">Up</a></td><th width="60%" align="center">Chapter 19. Server Configuration</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="runtime-config-wal.html" title="19.5. Write Ahead Log">Next</a></td></tr></table><hr /></div><div class="sect1" id="RUNTIME-CONFIG-RESOURCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">19.4. Resource Consumption <a href="#RUNTIME-CONFIG-RESOURCE" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-MEMORY">19.4.1. Memory</a></span></dt><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-DISK">19.4.2. Disk</a></span></dt><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-KERNEL">19.4.3. Kernel Resource Usage</a></span></dt><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER">19.4.4. Background Writer</a></span></dt><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-IO">19.4.5. I/O</a></span></dt><dt><span class="sect2"><a href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-WORKER-PROCESSES">19.4.6. Worker Processes</a></span></dt></dl></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-MEMORY"><div class="titlepage"><div><div><h3 class="title">19.4.1. Memory <a href="#RUNTIME-CONFIG-RESOURCE-MEMORY" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-SHARED-BUFFERS"><span class="term"><code class="varname">shared_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.1.1.3" class="indexterm"></a>
      </span> <a href="#GUC-SHARED-BUFFERS" class="id_link">#</a></dt><dd><p>
        Sets the amount of memory the database server uses for shared
        memory buffers.  The default is typically 128 megabytes
        (<code class="literal">128MB</code>), but might be less if your kernel settings will
        not support it (as determined during <span class="application">initdb</span>).
        This setting must be at least 128 kilobytes.  However,
        settings significantly higher than the minimum are usually needed
        for good performance.
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        (Non-default values of <code class="symbol">BLCKSZ</code> change the minimum
        value.)
        This parameter can only be set at server start.
       </p><p>
        If you have a dedicated database server with 1GB or more of RAM, a
        reasonable starting value for <code class="varname">shared_buffers</code> is 25%
        of the memory in your system.  There are some workloads where even
        larger settings for <code class="varname">shared_buffers</code> are effective, but
        because <span class="productname">PostgreSQL</span> also relies on the
        operating system cache, it is unlikely that an allocation of more than
        40% of RAM to <code class="varname">shared_buffers</code> will work better than a
        smaller amount.  Larger settings for <code class="varname">shared_buffers</code>
        usually require a corresponding increase in
        <code class="varname">max_wal_size</code>, in order to spread out the
        process of writing large quantities of new or changed data over a
        longer period of time.
       </p><p>
        On systems with less than 1GB of RAM, a smaller percentage of RAM is
        appropriate, so as to leave adequate space for the operating system.
       </p></dd><dt id="GUC-HUGE-PAGES"><span class="term"><code class="varname">huge_pages</code> (<code class="type">enum</code>)
      <a id="id-1.6.6.7.2.2.2.1.3" class="indexterm"></a>
      </span> <a href="#GUC-HUGE-PAGES" class="id_link">#</a></dt><dd><p>
        Controls whether huge pages are requested for the main shared memory
        area. Valid values are <code class="literal">try</code> (the default),
        <code class="literal">on</code>, and <code class="literal">off</code>.
        This parameter can only be set at server start. With
        <code class="varname">huge_pages</code> set to <code class="literal">try</code>, the
        server will try to request huge pages, but fall back to the default if
        that fails. With <code class="literal">on</code>, failure to request huge pages
        will prevent the server from starting up. With <code class="literal">off</code>,
        huge pages will not be requested. The actual state of huge pages is
        indicated by the server variable
        <a class="xref" href="runtime-config-preset.html#GUC-HUGE-PAGES-STATUS">huge_pages_status</a>.
       </p><p>
        At present, this setting is supported only on Linux and Windows. The
        setting is ignored on other systems when set to
        <code class="literal">try</code>.  On Linux, it is only supported when
        <code class="varname">shared_memory_type</code> is set to <code class="literal">mmap</code>
        (the default).
       </p><p>
        The use of huge pages results in smaller page tables and less CPU time
        spent on memory management, increasing performance. For more details about
        using huge pages on Linux, see <a class="xref" href="kernel-resources.html#LINUX-HUGE-PAGES" title="18.4.5. Linux Huge Pages">Section 18.4.5</a>.
       </p><p>
        Huge pages are known as large pages on Windows.  To use them, you need to
        assign the user right <span class="quote">“<span class="quote">Lock pages in memory</span>”</span> to the Windows user account
        that runs <span class="productname">PostgreSQL</span>.
        You can use Windows Group Policy tool (gpedit.msc) to assign the user right
        <span class="quote">“<span class="quote">Lock pages in memory</span>”</span>.
        To start the database server on the command prompt as a standalone process,
        not as a Windows service, the command prompt must be run as an administrator or
        User Access Control (UAC) must be disabled. When the UAC is enabled, the normal
        command prompt revokes the user right <span class="quote">“<span class="quote">Lock pages in memory</span>”</span> when started.
       </p><p>
        Note that this setting only affects the main shared memory area.
        Operating systems such as Linux, FreeBSD, and Illumos can also use
        huge pages (also known as <span class="quote">“<span class="quote">super</span>”</span> pages or
        <span class="quote">“<span class="quote">large</span>”</span> pages) automatically for normal memory
        allocation, without an explicit request from
        <span class="productname">PostgreSQL</span>. On Linux, this is called
        <span class="quote">“<span class="quote">transparent huge pages</span>”</span><a id="id-1.6.6.7.2.2.2.2.5.5" class="indexterm"></a> (THP). That feature has been known to
        cause performance degradation with
        <span class="productname">PostgreSQL</span> for some users on some Linux
        versions, so its use is currently discouraged (unlike explicit use of
        <code class="varname">huge_pages</code>).
       </p></dd><dt id="GUC-HUGE-PAGE-SIZE"><span class="term"><code class="varname">huge_page_size</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.3.1.3" class="indexterm"></a>
      </span> <a href="#GUC-HUGE-PAGE-SIZE" class="id_link">#</a></dt><dd><p>
        Controls the size of huge pages, when they are enabled with
        <a class="xref" href="runtime-config-resource.html#GUC-HUGE-PAGES">huge_pages</a>.
        The default is zero (<code class="literal">0</code>).
        When set to <code class="literal">0</code>, the default huge page size on the
        system will be used. This parameter can only be set at server start.
       </p><p>
        Some commonly available page sizes on modern 64 bit server architectures include:
        <code class="literal">2MB</code> and <code class="literal">1GB</code> (Intel and AMD), <code class="literal">16MB</code> and
        <code class="literal">16GB</code> (IBM POWER), and <code class="literal">64kB</code>, <code class="literal">2MB</code>,
        <code class="literal">32MB</code> and <code class="literal">1GB</code> (ARM). For more information
        about usage and support, see <a class="xref" href="kernel-resources.html#LINUX-HUGE-PAGES" title="18.4.5. Linux Huge Pages">Section 18.4.5</a>.
       </p><p>
        Non-default settings are currently supported only on Linux.
       </p></dd><dt id="GUC-TEMP-BUFFERS"><span class="term"><code class="varname">temp_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.4.1.3" class="indexterm"></a>
      </span> <a href="#GUC-TEMP-BUFFERS" class="id_link">#</a></dt><dd><p>
        Sets the maximum amount of memory used for temporary buffers within
        each database session.  These are session-local buffers used only
        for access to temporary tables.
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default is eight megabytes (<code class="literal">8MB</code>).
        (If <code class="symbol">BLCKSZ</code> is not 8kB, the default value scales
        proportionally to it.)
        This setting can be changed within individual
        sessions, but only before the first use of temporary tables
        within the session; subsequent attempts to change the value will
        have no effect on that session.
       </p><p>
        A session will allocate temporary buffers as needed up to the limit
        given by <code class="varname">temp_buffers</code>.  The cost of setting a large
        value in sessions that do not actually need many temporary
        buffers is only a buffer descriptor, or about 64 bytes, per
        increment in <code class="varname">temp_buffers</code>.  However if a buffer is
        actually used an additional 8192 bytes will be consumed for it
        (or in general, <code class="symbol">BLCKSZ</code> bytes).
       </p></dd><dt id="GUC-MAX-PREPARED-TRANSACTIONS"><span class="term"><code class="varname">max_prepared_transactions</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.5.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MAX-PREPARED-TRANSACTIONS" class="id_link">#</a></dt><dd><p>
        Sets the maximum number of transactions that can be in the
        <span class="quote">“<span class="quote">prepared</span>”</span> state simultaneously (see <a class="xref" href="sql-prepare-transaction.html" title="PREPARE TRANSACTION"><span class="refentrytitle">PREPARE TRANSACTION</span></a>).
        Setting this parameter to zero (which is the default)
        disables the prepared-transaction feature.
        This parameter can only be set at server start.
       </p><p>
        If you are not planning to use prepared transactions, this parameter
        should be set to zero to prevent accidental creation of prepared
        transactions.  If you are using prepared transactions, you will
        probably want <code class="varname">max_prepared_transactions</code> to be at
        least as large as <a class="xref" href="runtime-config-connection.html#GUC-MAX-CONNECTIONS">max_connections</a>, so that every
        session can have a prepared transaction pending.
       </p><p>
        When running a standby server, you must set this parameter to the
        same or higher value than on the primary server. Otherwise, queries
        will not be allowed in the standby server.
       </p></dd><dt id="GUC-WORK-MEM"><span class="term"><code class="varname">work_mem</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.6.1.3" class="indexterm"></a>
      </span> <a href="#GUC-WORK-MEM" class="id_link">#</a></dt><dd><p>
        Sets the base maximum amount of memory to be used by a query operation
        (such as a sort or hash table) before writing to temporary disk files.
        If this value is specified without units, it is taken as kilobytes.
        The default value is four megabytes (<code class="literal">4MB</code>).
        Note that a complex query might perform several sort and hash
        operations at the same time, with each operation generally being
        allowed to use as much memory as this value specifies before
        it starts
        to write data into temporary files.  Also, several running
        sessions could be doing such operations concurrently.
        Therefore, the total memory used could be many times the value
        of <code class="varname">work_mem</code>; it is necessary to keep this
        fact in mind when choosing the value.  Sort operations are used
        for <code class="literal">ORDER BY</code>, <code class="literal">DISTINCT</code>,
        and merge joins.
        Hash tables are used in hash joins, hash-based aggregation, memoize
        nodes and hash-based processing of <code class="literal">IN</code> subqueries.
       </p><p>
        Hash-based operations are generally more sensitive to memory
        availability than equivalent sort-based operations.  The
        memory limit for a hash table is computed by multiplying
        <code class="varname">work_mem</code> by
        <code class="varname">hash_mem_multiplier</code>.  This makes it
        possible for hash-based operations to use an amount of memory
        that exceeds the usual <code class="varname">work_mem</code> base
        amount.
       </p></dd><dt id="GUC-HASH-MEM-MULTIPLIER"><span class="term"><code class="varname">hash_mem_multiplier</code> (<code class="type">floating point</code>)
      <a id="id-1.6.6.7.2.2.7.1.3" class="indexterm"></a>
      </span> <a href="#GUC-HASH-MEM-MULTIPLIER" class="id_link">#</a></dt><dd><p>
        Used to compute the maximum amount of memory that hash-based
        operations can use.  The final limit is determined by
        multiplying <code class="varname">work_mem</code> by
        <code class="varname">hash_mem_multiplier</code>.  The default value is
        2.0, which makes hash-based operations use twice the usual
        <code class="varname">work_mem</code> base amount.
       </p><p>
        Consider increasing <code class="varname">hash_mem_multiplier</code> in
        environments where spilling by query operations is a regular
        occurrence, especially when simply increasing
        <code class="varname">work_mem</code> results in memory pressure (memory
        pressure typically takes the form of intermittent out of
        memory errors).  The default setting of 2.0 is often effective with
        mixed workloads.  Higher settings in the range of 2.0 - 8.0 or
        more may be effective in environments where
        <code class="varname">work_mem</code> has already been increased to 40MB
        or more.
       </p></dd><dt id="GUC-MAINTENANCE-WORK-MEM"><span class="term"><code class="varname">maintenance_work_mem</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.8.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MAINTENANCE-WORK-MEM" class="id_link">#</a></dt><dd><p>
        Specifies the maximum amount of memory to be used by maintenance
        operations, such as <code class="command">VACUUM</code>, <code class="command">CREATE
        INDEX</code>, and <code class="command">ALTER TABLE ADD FOREIGN KEY</code>.
        If this value is specified without units, it is taken as kilobytes.
        It defaults
        to 64 megabytes (<code class="literal">64MB</code>).  Since only one of these
        operations can be executed at a time by a database session, and
        an installation normally doesn't have many of them running
        concurrently, it's safe to set this value significantly larger
        than <code class="varname">work_mem</code>.  Larger settings might improve
        performance for vacuuming and for restoring database dumps.
       </p><p>
        Note that when autovacuum runs, up to
        <a class="xref" href="runtime-config-vacuum.html#GUC-AUTOVACUUM-MAX-WORKERS">autovacuum_max_workers</a> times this memory
        may be allocated, so be careful not to set the default value
        too high.  It may be useful to control for this by separately
        setting <a class="xref" href="runtime-config-resource.html#GUC-AUTOVACUUM-WORK-MEM">autovacuum_work_mem</a>.
       </p></dd><dt id="GUC-AUTOVACUUM-WORK-MEM"><span class="term"><code class="varname">autovacuum_work_mem</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.9.1.3" class="indexterm"></a>
      </span> <a href="#GUC-AUTOVACUUM-WORK-MEM" class="id_link">#</a></dt><dd><p>
        Specifies the maximum amount of memory to be used by each
        autovacuum worker process.
        If this value is specified without units, it is taken as kilobytes.
        It defaults to -1, indicating that
        the value of <a class="xref" href="runtime-config-resource.html#GUC-MAINTENANCE-WORK-MEM">maintenance_work_mem</a> should
        be used instead.  The setting has no effect on the behavior of
        <code class="command">VACUUM</code> when run in other contexts.
        This parameter can only be set in the
        <code class="filename">postgresql.conf</code> file or on the server command
        line.
       </p></dd><dt id="GUC-VACUUM-BUFFER-USAGE-LIMIT"><span class="term">
       <code class="varname">vacuum_buffer_usage_limit</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.2.2.10.1.3" class="indexterm"></a>
      </span> <a href="#GUC-VACUUM-BUFFER-USAGE-LIMIT" class="id_link">#</a></dt><dd><p>
        Specifies the size of the
        <a class="glossterm" href="glossary.html#GLOSSARY-BUFFER-ACCESS-STRATEGY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BUFFER-ACCESS-STRATEGY" title="Buffer Access Strategy">Buffer Access Strategy</a></em></a>
        used by the <code class="command">VACUUM</code> and <code class="command">ANALYZE</code>
        commands.  A setting of <code class="literal">0</code> will allow the operation
        to use any number of <code class="varname">shared_buffers</code>.  Otherwise
        valid sizes range from <code class="literal">128 kB</code> to
        <code class="literal">16 GB</code>.  If the specified size would exceed 1/8 the
        size of <code class="varname">shared_buffers</code>, the size is silently capped
        to that value.  The default value is <code class="literal">2MB</code>.  If
        this value is specified without units, it is taken as kilobytes.  This
        parameter can be set at any time.  It can be overridden for
        <a class="xref" href="sql-vacuum.html" title="VACUUM"><span class="refentrytitle">VACUUM</span></a> and <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a>
        when passing the <code class="option">BUFFER_USAGE_LIMIT</code> option.  Higher
        settings can allow <code class="command">VACUUM</code> and
        <code class="command">ANALYZE</code> to run more quickly, but having too large a
        setting may cause too many other useful pages to be evicted from
        shared buffers.
       </p></dd><dt id="GUC-LOGICAL-DECODING-WORK-MEM"><span class="term"><code class="varname">logical_decoding_work_mem</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.11.1.3" class="indexterm"></a>
      </span> <a href="#GUC-LOGICAL-DECODING-WORK-MEM" class="id_link">#</a></dt><dd><p>
        Specifies the maximum amount of memory to be used by logical decoding,
        before some of the decoded changes are written to local disk. This
        limits the amount of memory used by logical streaming replication
        connections. It defaults to 64 megabytes (<code class="literal">64MB</code>).
        Since each replication connection only uses a single buffer of this size,
        and an installation normally doesn't have many such connections
        concurrently (as limited by <code class="varname">max_wal_senders</code>), it's
        safe to set this value significantly higher than <code class="varname">work_mem</code>,
        reducing the amount of decoded changes written to disk.
       </p></dd><dt id="GUC-COMMIT-TIMESTAMP-BUFFERS"><span class="term"><code class="varname">commit_timestamp_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.12.1.3" class="indexterm"></a>
      </span> <a href="#GUC-COMMIT-TIMESTAMP-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of memory to use to cache the contents of
        <code class="literal">pg_commit_ts</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">0</code>, which requests
        <code class="varname">shared_buffers</code>/512 up to 1024 blocks,
        but not fewer than 16 blocks.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-MULTIXACT-MEMBER-BUFFERS"><span class="term"><code class="varname">multixact_member_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.13.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MULTIXACT-MEMBER-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_multixact/members</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">32</code>.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-MULTIXACT-OFFSET-BUFFERS"><span class="term"><code class="varname">multixact_offset_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.14.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MULTIXACT-OFFSET-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_multixact/offsets</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">16</code>.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-NOTIFY-BUFFERS"><span class="term"><code class="varname">notify_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.15.1.3" class="indexterm"></a>
      </span> <a href="#GUC-NOTIFY-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_notify</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">16</code>.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-SERIALIZABLE-BUFFERS"><span class="term"><code class="varname">serializable_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.16.1.3" class="indexterm"></a>
      </span> <a href="#GUC-SERIALIZABLE-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_serial</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">32</code>.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-SUBTRANSACTION-BUFFERS"><span class="term"><code class="varname">subtransaction_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.17.1.3" class="indexterm"></a>
      </span> <a href="#GUC-SUBTRANSACTION-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_subtrans</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">0</code>, which requests
        <code class="varname">shared_buffers</code>/512 up to 1024 blocks,
        but not fewer than 16 blocks.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-TRANSACTION-BUFFERS"><span class="term"><code class="varname">transaction_buffers</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.18.1.3" class="indexterm"></a>
      </span> <a href="#GUC-TRANSACTION-BUFFERS" class="id_link">#</a></dt><dd><p>
        Specifies the amount of shared memory to use to cache the contents
        of <code class="literal">pg_xact</code> (see
        <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 66.1. Contents of PGDATA">Table 66.1</a>).
        If this value is specified without units, it is taken as blocks,
        that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
        The default value is <code class="literal">0</code>, which requests
        <code class="varname">shared_buffers</code>/512 up to 1024 blocks,
        but not fewer than 16 blocks.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-MAX-STACK-DEPTH"><span class="term"><code class="varname">max_stack_depth</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.19.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MAX-STACK-DEPTH" class="id_link">#</a></dt><dd><p>
        Specifies the maximum safe depth of the server's execution stack.
        The ideal setting for this parameter is the actual stack size limit
        enforced by the kernel (as set by <code class="literal">ulimit -s</code> or local
        equivalent), less a safety margin of a megabyte or so.  The safety
        margin is needed because the stack depth is not checked in every
        routine in the server, but only in key potentially-recursive routines.
        If this value is specified without units, it is taken as kilobytes.
        The default setting is two megabytes (<code class="literal">2MB</code>), which
        is conservatively small and unlikely to risk crashes.  However,
        it might be too small to allow execution of complex functions.
        Only superusers and users with the appropriate <code class="literal">SET</code>
        privilege can change this setting.
       </p><p>
        Setting <code class="varname">max_stack_depth</code> higher than
        the actual kernel limit will mean that a runaway recursive function
        can crash an individual backend process.  On platforms where
        <span class="productname">PostgreSQL</span> can determine the kernel limit,
        the server will not allow this variable to be set to an unsafe
        value.  However, not all platforms provide the information,
        so caution is recommended in selecting a value.
       </p></dd><dt id="GUC-SHARED-MEMORY-TYPE"><span class="term"><code class="varname">shared_memory_type</code> (<code class="type">enum</code>)
      <a id="id-1.6.6.7.2.2.20.1.3" class="indexterm"></a>
      </span> <a href="#GUC-SHARED-MEMORY-TYPE" class="id_link">#</a></dt><dd><p>
        Specifies the shared memory implementation that the server
        should use for the main shared memory region that holds
        <span class="productname">PostgreSQL</span>'s shared buffers and other
        shared data.  Possible values are <code class="literal">mmap</code> (for
        anonymous shared memory allocated using <code class="function">mmap</code>),
        <code class="literal">sysv</code> (for System V shared memory allocated via
        <code class="function">shmget</code>) and <code class="literal">windows</code> (for Windows
        shared memory).  Not all values are supported on all platforms; the
        first supported option is the default for that platform.  The use of
        the <code class="literal">sysv</code> option, which is not the default on any
        platform, is generally discouraged because it typically requires
        non-default kernel settings to allow for large allocations (see <a class="xref" href="kernel-resources.html#SYSVIPC" title="18.4.1. Shared Memory and Semaphores">Section 18.4.1</a>).
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-DYNAMIC-SHARED-MEMORY-TYPE"><span class="term"><code class="varname">dynamic_shared_memory_type</code> (<code class="type">enum</code>)
      <a id="id-1.6.6.7.2.2.21.1.3" class="indexterm"></a>
      </span> <a href="#GUC-DYNAMIC-SHARED-MEMORY-TYPE" class="id_link">#</a></dt><dd><p>
        Specifies the dynamic shared memory implementation that the server
        should use.  Possible values are <code class="literal">posix</code> (for POSIX shared
        memory allocated using <code class="literal">shm_open</code>), <code class="literal">sysv</code>
        (for System V shared memory allocated via <code class="literal">shmget</code>),
        <code class="literal">windows</code> (for Windows shared memory),
        and <code class="literal">mmap</code> (to simulate shared memory using
        memory-mapped files stored in the data directory).
        Not all values are supported on all platforms; the first supported
        option is usually the default for that platform.  The use of the
        <code class="literal">mmap</code> option, which is not the default on any platform,
        is generally discouraged because the operating system may write
        modified pages back to disk repeatedly, increasing system I/O load;
        however, it may be useful for debugging, when the
        <code class="literal">pg_dynshmem</code> directory is stored on a RAM disk, or when
        other shared memory facilities are not available.
        This parameter can only be set at server start.
       </p></dd><dt id="GUC-MIN-DYNAMIC-SHARED-MEMORY"><span class="term"><code class="varname">min_dynamic_shared_memory</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.2.2.22.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MIN-DYNAMIC-SHARED-MEMORY" class="id_link">#</a></dt><dd><p>
        Specifies the amount of memory that should be allocated at server
        startup for use by parallel queries.  When this memory region is
        insufficient or exhausted by concurrent queries, new parallel queries
        try to allocate extra shared memory temporarily from the operating
        system using the method configured with
        <code class="varname">dynamic_shared_memory_type</code>, which may be slower due
        to memory management overheads.  Memory that is allocated at startup
        with <code class="varname">min_dynamic_shared_memory</code> is affected by
        the <code class="varname">huge_pages</code> setting on operating systems where
        that is supported, and may be more likely to benefit from larger pages
        on operating systems where that is managed automatically.
        The default value is <code class="literal">0</code> (none). This parameter can
        only be set at server start.
       </p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-DISK"><div class="titlepage"><div><div><h3 class="title">19.4.2. Disk <a href="#RUNTIME-CONFIG-RESOURCE-DISK" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-TEMP-FILE-LIMIT"><span class="term"><code class="varname">temp_file_limit</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.3.2.1.1.3" class="indexterm"></a>
      </span> <a href="#GUC-TEMP-FILE-LIMIT" class="id_link">#</a></dt><dd><p>
        Specifies the maximum amount of disk space that a process can use
        for temporary files, such as sort and hash temporary files, or the
        storage file for a held cursor.  A transaction attempting to exceed
        this limit will be canceled.
        If this value is specified without units, it is taken as kilobytes.
        <code class="literal">-1</code> (the default) means no limit.
        Only superusers and users with the appropriate <code class="literal">SET</code>
        privilege can change this setting.
       </p><p>
        This setting constrains the total space used at any instant by all
        temporary files used by a given <span class="productname">PostgreSQL</span> process.
        It should be noted that disk space used for explicit temporary
        tables, as opposed to temporary files used behind-the-scenes in query
        execution, does <span class="emphasis"><em>not</em></span> count against this limit.
       </p></dd><dt id="GUC-FILE-COPY-METHOD"><span class="term"><code class="varname">file_copy_method</code> (<code class="type">enum</code>)
      <a id="id-1.6.6.7.3.2.2.1.3" class="indexterm"></a>
      </span> <a href="#GUC-FILE-COPY-METHOD" class="id_link">#</a></dt><dd><p>
        Specifies the method used to copy files.
        Possible values are <code class="literal">COPY</code> (default) and
        <code class="literal">CLONE</code> (if operating support is available).
       </p><p>
       This parameter affects:
       </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
         <code class="literal"><code class="command">CREATE DATABASE ... STRATEGY=FILE_COPY</code></code>
        </p></li><li class="listitem"><p>
         <code class="command">ALTER DATABASE ... SET TABLESPACE ...</code>
        </p></li></ul></div><p>
        <code class="literal">CLONE</code> uses the <code class="function">copy_file_range()</code>
        (Linux, FreeBSD) or <code class="function">copyfile</code>
        (macOS) system calls, giving the kernel the opportunity to share disk
        blocks or push work down to lower layers on some file systems.
       </p></dd><dt id="GUC-FILE-EXTEND-METHOD"><span class="term"><code class="varname">file_extend_method</code> (<code class="type">enum</code>)
      <a id="id-1.6.6.7.3.2.3.1.3" class="indexterm"></a>
      </span> <a href="#GUC-FILE-EXTEND-METHOD" class="id_link">#</a></dt><dd><p>
        Specifies the method used to extend data files during bulk operations
        such as <code class="command">COPY</code>.  The first available option is used as
        the default, depending on the operating system:
        </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
           <code class="literal">posix_fallocate</code> (Unix) uses the standard POSIX
            interface for allocating disk space, but is missing on some systems.
            If it is present but the underlying file system doesn't support it,
            this option silently falls back to <code class="literal">write_zeros</code>.
            Current versions of BTRFS are known to disable compression when
            this option is used.
            This is the default on systems that have the function.
           </p></li><li class="listitem"><p>
           <code class="literal">write_zeros</code> extends files by writing out blocks
            of zero bytes.  This is the default on systems that don't have the
            function <code class="function">posix_fallocate</code>.
          </p></li></ul></div><p>
        The <code class="literal">write_zeros</code> method is always used when data
        files are extended by 8 blocks or fewer.
       </p></dd><dt id="GUC-MAX-NOTIFY-QUEUE-PAGES"><span class="term"><code class="varname">max_notify_queue_pages</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.3.2.4.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MAX-NOTIFY-QUEUE-PAGES" class="id_link">#</a></dt><dd><p>
        Specifies the maximum amount of allocated pages for
        <a class="xref" href="sql-notify.html" title="NOTIFY"><span class="refentrytitle">NOTIFY</span></a> / <a class="xref" href="sql-listen.html" title="LISTEN"><span class="refentrytitle">LISTEN</span></a> queue.
        The default value is 1048576. For 8 KB pages it allows to consume
        up to 8 GB of disk space.
        This parameter can only be set at server start.
       </p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-KERNEL"><div class="titlepage"><div><div><h3 class="title">19.4.3. Kernel Resource Usage <a href="#RUNTIME-CONFIG-RESOURCE-KERNEL" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-MAX-FILES-PER-PROCESS"><span class="term"><code class="varname">max_files_per_process</code> (<code class="type">integer</code>)
      <a id="id-1.6.6.7.4.2.1.1.3" class="indexterm"></a>
      </span> <a href="#GUC-MAX-FILES-PER-PROCESS" class="id_link">#</a></dt><dd><p>
        Sets the maximum number of open files each server subprocess is
        allowed to open simultaneously;  files already opened in the
        postmaster are not counted toward this limit. The default is one
        thousand files.
       </p><p>
        If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can actually support if many processes all try to open
        that many files. If you find yourself seeing <span class="quote">“<span class="quote">Too many open
        files</span>”</span> failures, try reducing this setting.
        This parameter can only be set at server start.
       </p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER"><div class="titlepage"><div><div><h3 class="title">19.4.4. Background Writer <a href="#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER" class="id_link">#</a></h3></div></div></div><p>
      There is a separate server
      process called the <em class="firstterm">background writer</em>, whose function
      is to issue writes of <span class="quote">“<span class="quote">dirty</span>”</span> (new or modified) shared
      buffers.  When the number of clean shared buffers appears to be
      insufficient, the background writer writes some dirty buffers to the
      file system and marks them as clean.  This reduces the likelihood
      that server processes handling user queries will be unable to find
      clean buffers and have to write dirty buffers themselves.
      However, the background writer does cause a net overall
      increase in I/O load, because while a repeatedly-dirtied page might
      otherwise be written only once per checkpoint interval, the
      background writer might write it several times as it is dirtied
      in the same interval.  The parameters discussed in this subsection
      can be used to tune the behavior for local needs.
     </p><div class="variablelist"><dl class="variablelist"><dt id="GUC-BGWRITER-DELAY"><span class="term"><code class="varname">bgwriter_delay</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.5.3.1.1.3" class="indexterm"></a>
       </span> <a href="#GUC-BGWRITER-DELAY" class="id_link">#</a></dt><dd><p>
         Specifies the delay between activity rounds for the
         background writer.  In each round the writer issues writes
         for some number of dirty buffers (controllable by the
         following parameters).  It then sleeps for
         the length of <code class="varname">bgwriter_delay</code>, and repeats.
         When there are no dirty buffers in the
         buffer pool, though, it goes into a longer sleep regardless of
         <code class="varname">bgwriter_delay</code>.
         If this value is specified without units, it is taken as milliseconds.
         The default value is 200
         milliseconds (<code class="literal">200ms</code>). Note that on some systems, the
         effective resolution of sleep delays is 10 milliseconds; setting
         <code class="varname">bgwriter_delay</code> to a value that is not a multiple of 10
         might have the same results as setting it to the next higher multiple
         of 10.  This parameter can only be set in the
         <code class="filename">postgresql.conf</code> file or on the server command line.
        </p></dd><dt id="GUC-BGWRITER-LRU-MAXPAGES"><span class="term"><code class="varname">bgwriter_lru_maxpages</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.5.3.2.1.3" class="indexterm"></a>
       </span> <a href="#GUC-BGWRITER-LRU-MAXPAGES" class="id_link">#</a></dt><dd><p>
         In each round, no more than this many buffers will be written
         by the background writer.  Setting this to zero disables
         background writing.  (Note that checkpoints, which are managed by
         a separate, dedicated auxiliary process, are unaffected.)
         The default value is 100 buffers.
         This parameter can only be set in the <code class="filename">postgresql.conf</code>
         file or on the server command line.
        </p></dd><dt id="GUC-BGWRITER-LRU-MULTIPLIER"><span class="term"><code class="varname">bgwriter_lru_multiplier</code> (<code class="type">floating point</code>)
       <a id="id-1.6.6.7.5.3.3.1.3" class="indexterm"></a>
       </span> <a href="#GUC-BGWRITER-LRU-MULTIPLIER" class="id_link">#</a></dt><dd><p>
         The number of dirty buffers written in each round is based on the
         number of new buffers that have been needed by server processes
         during recent rounds.  The average recent need is multiplied by
         <code class="varname">bgwriter_lru_multiplier</code> to arrive at an estimate of the
         number of buffers that will be needed during the next round.  Dirty
         buffers are written until there are that many clean, reusable buffers
         available.  (However, no more than <code class="varname">bgwriter_lru_maxpages</code>
         buffers will be written per round.)
         Thus, a setting of 1.0 represents a <span class="quote">“<span class="quote">just in time</span>”</span> policy
         of writing exactly the number of buffers predicted to be needed.
         Larger values provide some cushion against spikes in demand,
         while smaller values intentionally leave writes to be done by
         server processes.
         The default is 2.0.
         This parameter can only be set in the <code class="filename">postgresql.conf</code>
         file or on the server command line.
        </p></dd><dt id="GUC-BGWRITER-FLUSH-AFTER"><span class="term"><code class="varname">bgwriter_flush_after</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.5.3.4.1.3" class="indexterm"></a>
       </span> <a href="#GUC-BGWRITER-FLUSH-AFTER" class="id_link">#</a></dt><dd><p>
         Whenever more than this amount of data has
         been written by the background writer, attempt to force the OS to issue these
         writes to the underlying storage.  Doing so will limit the amount of
         dirty data in the kernel's page cache, reducing the likelihood of
         stalls when an <code class="function">fsync</code> is issued at the end of a checkpoint, or when
         the OS writes data back in larger batches in the background.  Often
         that will result in greatly reduced transaction latency, but there
         also are some cases, especially with workloads that are bigger than
         <a class="xref" href="runtime-config-resource.html#GUC-SHARED-BUFFERS">shared_buffers</a>, but smaller than the OS's page
         cache, where performance might degrade.  This setting may have no
         effect on some platforms.
         If this value is specified without units, it is taken as blocks,
         that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
         The valid range is between
         <code class="literal">0</code>, which disables forced writeback, and
         <code class="literal">2MB</code>.  The default is <code class="literal">512kB</code> on Linux,
         <code class="literal">0</code> elsewhere.  (If <code class="symbol">BLCKSZ</code> is not 8kB,
         the default and maximum values scale proportionally to it.)
         This parameter can only be set in the <code class="filename">postgresql.conf</code>
         file or on the server command line.
        </p></dd></dl></div><p>
      Smaller values of <code class="varname">bgwriter_lru_maxpages</code> and
      <code class="varname">bgwriter_lru_multiplier</code> reduce the extra I/O load
      caused by the background writer, but make it more likely that server
      processes will have to issue writes for themselves, delaying interactive
      queries.
     </p></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-IO"><div class="titlepage"><div><div><h3 class="title">19.4.5. I/O <a href="#RUNTIME-CONFIG-RESOURCE-IO" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-BACKEND-FLUSH-AFTER"><span class="term"><code class="varname">backend_flush_after</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.1.1.3" class="indexterm"></a>
       </span> <a href="#GUC-BACKEND-FLUSH-AFTER" class="id_link">#</a></dt><dd><p>
         Whenever more than this amount of data has
         been written by a single backend, attempt to force the OS to issue
         these writes to the underlying storage.  Doing so will limit the
         amount of dirty data in the kernel's page cache, reducing the
         likelihood of stalls when an <code class="function">fsync</code> is issued at the end of a
         checkpoint, or when the OS writes data back in larger batches in the
         background.  Often that will result in greatly reduced transaction
         latency, but there also are some cases, especially with workloads
         that are bigger than <a class="xref" href="runtime-config-resource.html#GUC-SHARED-BUFFERS">shared_buffers</a>, but smaller
         than the OS's page cache, where performance might degrade.  This
         setting may have no effect on some platforms.
         If this value is specified without units, it is taken as blocks,
         that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
         The valid range is
         between <code class="literal">0</code>, which disables forced writeback,
         and <code class="literal">2MB</code>.  The default is <code class="literal">0</code>, i.e., no
         forced writeback.  (If <code class="symbol">BLCKSZ</code> is not 8kB,
         the maximum value scales proportionally to it.)
        </p></dd><dt id="GUC-EFFECTIVE-IO-CONCURRENCY"><span class="term"><code class="varname">effective_io_concurrency</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.2.1.3" class="indexterm"></a>
       </span> <a href="#GUC-EFFECTIVE-IO-CONCURRENCY" class="id_link">#</a></dt><dd><p>
         Sets the number of concurrent storage I/O operations that
         <span class="productname">PostgreSQL</span> expects can be executed
         simultaneously.  Raising this value will increase the number of I/O
         operations that any individual <span class="productname">PostgreSQL</span>
         session attempts to initiate in parallel.  The allowed range is
         <code class="literal">1</code> to <code class="literal">1000</code>, or
         <code class="literal">0</code> to disable issuance of asynchronous I/O requests.
         The default is <code class="literal">16</code>.
        </p><p>
         Higher values will have the most impact on higher latency storage
         where queries otherwise experience noticeable I/O stalls and on
         devices with high IOPs. Unnecessarily high values may increase I/O
         latency for all queries on the system.
        </p><p>
         On systems with prefetch advice support,
         <code class="varname">effective_io_concurrency</code> also controls the
         prefetch distance.
        </p><p>
         This value can be overridden for tables in a particular tablespace by
         setting the tablespace parameter of the same name (see <a class="xref" href="sql-altertablespace.html" title="ALTER TABLESPACE"><span class="refentrytitle">ALTER TABLESPACE</span></a>).
        </p></dd><dt id="GUC-MAINTENANCE-IO-CONCURRENCY"><span class="term"><code class="varname">maintenance_io_concurrency</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.3.1.3" class="indexterm"></a>
       </span> <a href="#GUC-MAINTENANCE-IO-CONCURRENCY" class="id_link">#</a></dt><dd><p>
         Similar to <code class="varname">effective_io_concurrency</code>, but used
         for maintenance work that is done on behalf of many client sessions.
        </p><p>
         The default is <code class="literal">16</code>.  This value can be overridden
         for tables in a particular tablespace by setting the tablespace
         parameter of the same name (see <a class="xref" href="sql-altertablespace.html" title="ALTER TABLESPACE"><span class="refentrytitle">ALTER TABLESPACE</span></a>).
        </p></dd><dt id="GUC-IO-MAX-COMBINE-LIMIT"><span class="term"><code class="varname">io_max_combine_limit</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.4.1.3" class="indexterm"></a>
       </span> <a href="#GUC-IO-MAX-COMBINE-LIMIT" class="id_link">#</a></dt><dd><p>
         Controls the largest I/O size in operations that combine I/O, and silently
         limits the user-settable parameter <code class="varname">io_combine_limit</code>.
         This parameter can only be set at server start.
         If this value is specified without units, it is taken as blocks,
         that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
         The maximum possible size depends on the operating system and block
         size, but is typically 1MB on Unix and 128kB on Windows.
         The default is 128kB.
        </p></dd><dt id="GUC-IO-COMBINE-LIMIT"><span class="term"><code class="varname">io_combine_limit</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.5.1.3" class="indexterm"></a>
       </span> <a href="#GUC-IO-COMBINE-LIMIT" class="id_link">#</a></dt><dd><p>
         Controls the largest I/O size in operations that combine I/O.  If set
         higher than the <code class="varname">io_max_combine_limit</code> parameter, the
         lower value will silently be used instead, so both may need to be raised
         to increase the I/O size.
         If this value is specified without units, it is taken as blocks,
         that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
         The maximum possible size depends on the operating system and block
         size, but is typically 1MB on Unix and 128kB on Windows.
         The default is 128kB.
        </p></dd><dt id="GUC-IO-MAX-CONCURRENCY"><span class="term"><code class="varname">io_max_concurrency</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.6.1.3" class="indexterm"></a>
       </span> <a href="#GUC-IO-MAX-CONCURRENCY" class="id_link">#</a></dt><dd><p>
         Controls the maximum number of I/O operations that one process can
         execute simultaneously.
        </p><p>
         The default setting of <code class="literal">-1</code> selects a number based
         on <a class="xref" href="runtime-config-resource.html#GUC-SHARED-BUFFERS">shared_buffers</a> and the maximum number of
         processes (<a class="xref" href="runtime-config-connection.html#GUC-MAX-CONNECTIONS">max_connections</a>, <a class="xref" href="runtime-config-vacuum.html#GUC-AUTOVACUUM-WORKER-SLOTS">autovacuum_worker_slots</a>, <a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a> and <a class="xref" href="runtime-config-replication.html#GUC-MAX-WAL-SENDERS">max_wal_senders</a>), but not more than
         <code class="literal">64</code>.
        </p><p>
         This parameter can only be set at server start.
        </p></dd><dt id="GUC-IO-METHOD"><span class="term"><code class="varname">io_method</code> (<code class="type">enum</code>)
       <a id="id-1.6.6.7.6.2.7.1.3" class="indexterm"></a>
       </span> <a href="#GUC-IO-METHOD" class="id_link">#</a></dt><dd><p>
         Selects the method for executing asynchronous I/O.
         Possible values are:
         </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
            <code class="literal">worker</code> (execute asynchronous I/O using worker processes)
           </p></li><li class="listitem"><p>
            <code class="literal">io_uring</code> (execute asynchronous I/O using
            io_uring, requires a build with
            <a class="link" href="install-make.html#CONFIGURE-OPTION-WITH-LIBURING"><code class="option">--with-liburing</code></a> /
            <a class="link" href="install-meson.html#CONFIGURE-WITH-LIBURING-MESON"><code class="option">-Dliburing</code></a>)
           </p></li><li class="listitem"><p>
            <code class="literal">sync</code> (execute asynchronous-eligible I/O synchronously)
           </p></li></ul></div><p>
         The default is <code class="literal">worker</code>.
        </p><p>
         This parameter can only be set at server start.
        </p></dd><dt id="GUC-IO-WORKERS"><span class="term"><code class="varname">io_workers</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.6.2.8.1.3" class="indexterm"></a>
       </span> <a href="#GUC-IO-WORKERS" class="id_link">#</a></dt><dd><p>
         Selects the number of I/O worker processes to use. The default is
         3. This parameter can only be set in the
         <code class="filename">postgresql.conf</code> file or on the server command
         line.
        </p><p>
         Only has an effect if <a class="xref" href="runtime-config-resource.html#GUC-IO-METHOD">io_method</a> is set to
         <code class="literal">worker</code>.
        </p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-RESOURCE-WORKER-PROCESSES"><div class="titlepage"><div><div><h3 class="title">19.4.6. Worker Processes <a href="#RUNTIME-CONFIG-RESOURCE-WORKER-PROCESSES" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-MAX-WORKER-PROCESSES"><span class="term"><code class="varname">max_worker_processes</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.7.2.1.1.3" class="indexterm"></a>
       </span> <a href="#GUC-MAX-WORKER-PROCESSES" class="id_link">#</a></dt><dd><p>
         Sets the maximum number of background processes that the cluster
         can support.  This parameter can only be set at server start.  The
         default is 8.
        </p><p>
         When running a standby server, you must set this parameter to the
         same or higher value than on the primary server. Otherwise, queries
         will not be allowed in the standby server.
        </p><p>
         When changing this value, consider also adjusting
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS">max_parallel_workers</a>,
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-MAINTENANCE-WORKERS">max_parallel_maintenance_workers</a>, and
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS-PER-GATHER">max_parallel_workers_per_gather</a>.
        </p></dd><dt id="GUC-MAX-PARALLEL-WORKERS-PER-GATHER"><span class="term"><code class="varname">max_parallel_workers_per_gather</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.7.2.2.1.3" class="indexterm"></a>
       </span> <a href="#GUC-MAX-PARALLEL-WORKERS-PER-GATHER" class="id_link">#</a></dt><dd><p>
         Sets the maximum number of workers that can be started by a single
         <code class="literal">Gather</code> or <code class="literal">Gather Merge</code> node.
         Parallel workers are taken from the pool of processes established by
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a>, limited by
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS">max_parallel_workers</a>.  Note that the requested
         number of workers may not actually be available at run time.  If this
         occurs, the plan will run with fewer workers than expected, which may
         be inefficient.  The default value is 2.  Setting this value to 0
         disables parallel query execution.
        </p><p>
         Note that parallel queries may consume very substantially more
         resources than non-parallel queries, because each worker process is
         a completely separate process which has roughly the same impact on the
         system as an additional user session.  This should be taken into
         account when choosing a value for this setting, as well as when
         configuring other settings that control resource utilization, such
         as <a class="xref" href="runtime-config-resource.html#GUC-WORK-MEM">work_mem</a>.  Resource limits such as
         <code class="varname">work_mem</code> are applied individually to each worker,
         which means the total utilization may be much higher across all
         processes than it would normally be for any single process.
         For example, a parallel query using 4 workers may use up to 5 times
         as much CPU time, memory, I/O bandwidth, and so forth as a query which
         uses no workers at all.
        </p><p>
         For more information on parallel query, see
         <a class="xref" href="parallel-query.html" title="Chapter 15. Parallel Query">Chapter 15</a>.
        </p></dd><dt id="GUC-MAX-PARALLEL-MAINTENANCE-WORKERS"><span class="term"><code class="varname">max_parallel_maintenance_workers</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.7.2.3.1.3" class="indexterm"></a>
       </span> <a href="#GUC-MAX-PARALLEL-MAINTENANCE-WORKERS" class="id_link">#</a></dt><dd><p>
         Sets the maximum number of parallel workers that can be
         started by a single utility command.  Currently, the parallel
         utility commands that support the use of parallel workers are
         <code class="command">CREATE INDEX</code> when building a B-tree,
         GIN, or BRIN index,
         and <code class="command">VACUUM</code> without <code class="literal">FULL</code>
         option.  Parallel workers are taken from the pool of processes
         established by <a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a>, limited
         by <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS">max_parallel_workers</a>.  Note that the requested
         number of workers may not actually be available at run time.
         If this occurs, the utility operation will run with fewer
         workers than expected.  The default value is 2.  Setting this
         value to 0 disables the use of parallel workers by utility
         commands.
        </p><p>
         Note that parallel utility commands should not consume
         substantially more memory than equivalent non-parallel
         operations.  This strategy differs from that of parallel
         query, where resource limits generally apply per worker
         process.  Parallel utility commands treat the resource limit
         <code class="varname">maintenance_work_mem</code> as a limit to be applied to
         the entire utility command, regardless of the number of
         parallel worker processes.  However, parallel utility
         commands may still consume substantially more CPU resources
         and I/O bandwidth.
        </p></dd><dt id="GUC-MAX-PARALLEL-WORKERS"><span class="term"><code class="varname">max_parallel_workers</code> (<code class="type">integer</code>)
       <a id="id-1.6.6.7.7.2.4.1.3" class="indexterm"></a>
       </span> <a href="#GUC-MAX-PARALLEL-WORKERS" class="id_link">#</a></dt><dd><p>
         Sets the maximum number of workers that the cluster can support for
         parallel operations.  The default value is 8.  When increasing or
         decreasing this value, consider also adjusting
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-MAINTENANCE-WORKERS">max_parallel_maintenance_workers</a> and
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS-PER-GATHER">max_parallel_workers_per_gather</a>.
         Also, note that a setting for this value which is higher than
         <a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a> will have no effect,
         since parallel workers are taken from the pool of worker processes
         established by that setting.
        </p></dd><dt id="GUC-PARALLEL-LEADER-PARTICIPATION"><span class="term">
       <code class="varname">parallel_leader_participation</code> (<code class="type">boolean</code>)
       <a id="id-1.6.6.7.7.2.5.1.3" class="indexterm"></a>
       </span> <a href="#GUC-PARALLEL-LEADER-PARTICIPATION" class="id_link">#</a></dt><dd><p>
         Allows the leader process to execute the query plan under
         <code class="literal">Gather</code> and <code class="literal">Gather Merge</code> nodes
         instead of waiting for worker processes.  The default is
         <code class="literal">on</code>.  Setting this value to <code class="literal">off</code>
         reduces the likelihood that workers will become blocked because the
         leader is not reading tuples fast enough, but requires the leader
         process to wait for worker processes to start up before the first
         tuples can be produced.  The degree to which the leader can help or
         hinder performance depends on the plan type, number of workers and
         query duration.
        </p></dd></dl></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="runtime-config-connection.html" title="19.3. Connections and Authentication">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="runtime-config.html" title="Chapter 19. Server Configuration">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="runtime-config-wal.html" title="19.5. Write Ahead Log">Next</a></td></tr><tr><td width="40%" align="left" valign="top">19.3. Connections and Authentication </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 19.5. Write Ahead Log</td></tr></table></div></body></html>