diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/config.sgml | 83 | ||||
| -rw-r--r-- | doc/src/sgml/monitoring.sgml | 86 | ||||
| -rw-r--r-- | doc/src/sgml/wal.sgml | 17 |
3 files changed, 184 insertions, 2 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index ea5cf3a2dc0..26628f3e6d3 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -3565,6 +3565,89 @@ include_dir 'conf.d' </variablelist> </sect2> + <sect2 id="runtime-config-wal-recovery"> + + <title>Recovery</title> + + <indexterm> + <primary>configuration</primary> + <secondary>of recovery</secondary> + <tertiary>general settings</tertiary> + </indexterm> + + <para> + This section describes the settings that apply to recovery in general, + affecting crash recovery, streaming replication and archive-based + replication. + </para> + + + <variablelist> + <varlistentry id="guc-recovery-prefetch" xreflabel="recovery_prefetch"> + <term><varname>recovery_prefetch</varname> (<type>boolean</type>) + <indexterm> + <primary><varname>recovery_prefetch</varname> configuration parameter</primary> + </indexterm> + </term> + <listitem> + <para> + Whether to try to prefetch blocks that are referenced in the WAL that + are not yet in the buffer pool, during recovery. Prefetching blocks + that will soon be needed can reduce I/O wait times in some workloads. + See also the <xref linkend="guc-wal-decode-buffer-size"/> and + <xref linkend="guc-maintenance-io-concurrency"/> settings, which limit + prefetching activity. + This setting is disabled by default. + </para> + <para> + This feature currently depends on an effective + <function>posix_fadvise</function> function, which some + operating systems lack. + </para> + </listitem> + </varlistentry> + + <varlistentry id="guc-recovery-prefetch-fpw" xreflabel="recovery_prefetch_fpw"> + <term><varname>recovery_prefetch_fpw</varname> (<type>boolean</type>) + <indexterm> + <primary><varname>recovery_prefetch_fpw</varname> configuration parameter</primary> + </indexterm> + </term> + <listitem> + <para> + Whether to prefetch blocks that were logged with full page images, + during recovery. Often this doesn't help, since such blocks will not + be read the first time they are needed and might remain in the buffer + pool after that. However, on file systems with a block size larger + than + <productname>PostgreSQL</productname>'s, prefetching can avoid a + costly read-before-write when a blocks are later written. + The default is off. + </para> + </listitem> + </varlistentry> + + <varlistentry id="guc-wal-decode-buffer-size" xreflabel="wal_decode_buffer_size"> + <term><varname>wal_decode_buffer_size</varname> (<type>integer</type>) + <indexterm> + <primary><varname>wal_decode_buffer_size</varname> configuration parameter</primary> + </indexterm> + </term> + <listitem> + <para> + A limit on how far ahead the server can look in the WAL, to find + blocks to prefetch. Setting it too high might be counterproductive, + if it means that data falls out of the + kernel cache before it is needed. If this value is specified without + units, it is taken as bytes. + The default is 512kB. + </para> + </listitem> + </varlistentry> + + </variablelist> + </sect2> + <sect2 id="runtime-config-wal-archive-recovery"> <title>Archive Recovery</title> diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml index da16c461f08..f637fe04152 100644 --- a/doc/src/sgml/monitoring.sgml +++ b/doc/src/sgml/monitoring.sgml @@ -338,6 +338,13 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser </row> <row> + <entry><structname>pg_stat_prefetch_recovery</structname><indexterm><primary>pg_stat_prefetch_recovery</primary></indexterm></entry> + <entry>Only one row, showing statistics about blocks prefetched during recovery. + See <xref linkend="pg-stat-prefetch-recovery-view"/> for details. + </entry> + </row> + + <row> <entry><structname>pg_stat_subscription</structname><indexterm><primary>pg_stat_subscription</primary></indexterm></entry> <entry>At least one row per subscription, showing information about the subscription workers. @@ -2917,6 +2924,78 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i copy of the subscribed tables. </para> + <table id="pg-stat-prefetch-recovery-view" xreflabel="pg_stat_prefetch_recovery"> + <title><structname>pg_stat_prefetch_recovery</structname> View</title> + <tgroup cols="3"> + <thead> + <row> + <entry>Column</entry> + <entry>Type</entry> + <entry>Description</entry> + </row> + </thead> + + <tbody> + <row> + <entry><structfield>prefetch</structfield></entry> + <entry><type>bigint</type></entry> + <entry>Number of blocks prefetched because they were not in the buffer pool</entry> + </row> + <row> + <entry><structfield>skip_hit</structfield></entry> + <entry><type>bigint</type></entry> + <entry>Number of blocks not prefetched because they were already in the buffer pool</entry> + </row> + <row> + <entry><structfield>skip_new</structfield></entry> + <entry><type>bigint</type></entry> + <entry>Number of blocks not prefetched because they were new (usually relation extension)</entry> + </row> + <row> + <entry><structfield>skip_fpw</structfield></entry> + <entry><type>bigint</type></entry> + <entry>Number of blocks not prefetched because a full page image was included in the WAL and <xref linkend="guc-recovery-prefetch-fpw"/> was set to <literal>off</literal></entry> + </row> + <row> + <entry><structfield>skip_seq</structfield></entry> + <entry><type>bigint</type></entry> + <entry>Number of blocks not prefetched because of repeated access</entry> + </row> + <row> + <entry><structfield>distance</structfield></entry> + <entry><type>integer</type></entry> + <entry>How far ahead of recovery the prefetcher is currently reading, in bytes</entry> + </row> + <row> + <entry><structfield>queue_depth</structfield></entry> + <entry><type>integer</type></entry> + <entry>How many prefetches have been initiated but are not yet known to have completed</entry> + </row> + <row> + <entry><structfield>avg_distance</structfield></entry> + <entry><type>float4</type></entry> + <entry>How far ahead of recovery the prefetcher is on average, while recovery is not idle</entry> + </row> + <row> + <entry><structfield>avg_queue_depth</structfield></entry> + <entry><type>float4</type></entry> + <entry>Average number of prefetches in flight while recovery is not idle</entry> + </row> + </tbody> + </tgroup> + </table> + + <para> + The <structname>pg_stat_prefetch_recovery</structname> view will contain only + one row. It is filled with nulls if recovery is not running or WAL + prefetching is not enabled. See <xref linkend="guc-recovery-prefetch"/> + for more information. The counters in this view are reset whenever the + <xref linkend="guc-recovery-prefetch"/>, + <xref linkend="guc-recovery-prefetch-fpw"/> or + <xref linkend="guc-maintenance-io-concurrency"/> setting is changed and + the server configuration is reloaded. + </para> + <table id="pg-stat-subscription" xreflabel="pg_stat_subscription"> <title><structname>pg_stat_subscription</structname> View</title> <tgroup cols="1"> @@ -5049,8 +5128,11 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i all the counters shown in the <structname>pg_stat_bgwriter</structname> view, <literal>archiver</literal> to reset all the counters shown in - the <structname>pg_stat_archiver</structname> view or <literal>wal</literal> - to reset all the counters shown in the <structname>pg_stat_wal</structname> view. + the <structname>pg_stat_archiver</structname> view, + <literal>wal</literal> to reset all the counters shown in the + <structname>pg_stat_wal</structname> view or + <literal>prefetch_recovery</literal> to reset all the counters shown + in the <structname>pg_stat_prefetch_recovery</structname> view. </para> <para> This function is restricted to superusers by default, but other users diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml index 7d48f427109..0f13c43095e 100644 --- a/doc/src/sgml/wal.sgml +++ b/doc/src/sgml/wal.sgml @@ -803,6 +803,23 @@ counted as <literal>wal_write</literal> and <literal>wal_sync</literal> in <structname>pg_stat_wal</structname>, respectively. </para> + + <para> + The <xref linkend="guc-recovery-prefetch"/> parameter can + be used to improve I/O performance during recovery by instructing + <productname>PostgreSQL</productname> to initiate reads + of disk blocks that will soon be needed but are not currently in + <productname>PostgreSQL</productname>'s buffer pool. + The <xref linkend="guc-maintenance-io-concurrency"/> and + <xref linkend="guc-wal-decode-buffer-size"/> settings limit prefetching + concurrency and distance, respectively. The + prefetching mechanism is most likely to be effective on systems + with <varname>full_page_writes</varname> set to + <varname>off</varname> (where that is safe), and where the working + set is larger than RAM. By default, prefetching in recovery is enabled + on operating systems that have <function>posix_fadvise</function> + support. + </para> </sect1> <sect1 id="wal-internals"> |