Enable IO concurrency on all systems

Previously effective_io_concurrency and maintenance_io_concurrency could not
be set above 0 on machines without fadvise support. AIO enables IO concurrency
without such support, via io_method=worker.

Currently only subsystems using the read stream API will take advantage of
this. Other users of maintenance_io_concurrency (like recovery prefetching)
which leverage OS advice directly will not benefit from this change. In those
cases, maintenance_io_concurrency will have no effect on I/O behavior.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CAAKRu_atGgZePo=_g6T3cNtfMf0QxpvoUh5OUqa_cnPdhLd=gw@mail.gmail.com

3 files changed, 10 insertions, 10 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index f55f38cb85b..0d02e21a1ab 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2585,8 +2585,7 @@ include_dir 'conf.d'
          session attempts to initiate in parallel.  The allowed range is
          <literal>1</literal> to <literal>1000</literal>, or
          <literal>0</literal> to disable issuance of asynchronous I/O requests.
-         The default is <literal>16</literal> on supported systems, otherwise
-         <literal>0</literal>.
+         The default is <literal>16</literal>.
         </para>
 
         <para>
@@ -2597,8 +2596,9 @@ include_dir 'conf.d'
         </para>
 
         <para>
-         On systems without prefetch advice support, attempting to configure
-         any value other than <literal>0</literal> will error out.
+         On systems with prefetch advice support,
+         <varname>effective_io_concurrency</varname> also controls the
+         prefetch distance.
         </para>
 
         <para>
@@ -2621,10 +2621,10 @@ include_dir 'conf.d'
          for maintenance work that is done on behalf of many client sessions.
         </para>
         <para>
-         The default is <literal>16</literal> on supported systems, otherwise
-         <literal>0</literal>.  This value can be overridden for tables in a
-         particular tablespace by setting the tablespace parameter of the same
-         name (see <xref linkend="sql-altertablespace"/>).
+         The default is <literal>16</literal>.  This value can be overridden
+         for tables in a particular tablespace by setting the tablespace
+         parameter of the same name (see <xref
+         linkend="sql-altertablespace"/>).
         </para>
        </listitem>
       </varlistentry>
diff --git a/doc/src/sgml/ref/alter_tablespace.sgml b/doc/src/sgml/ref/alter_tablespace.sgml
index 6ec863400d1..d0e08089ddb 100644
--- a/doc/src/sgml/ref/alter_tablespace.sgml
+++ b/doc/src/sgml/ref/alter_tablespace.sgml
@@ -88,7 +88,7 @@ ALTER TABLESPACE <replaceable>name</replaceable> RESET ( <replaceable class="par
       and <varname>maintenance_io_concurrency</varname>.
       Setting these values for a particular tablespace will override the
       planner's usual estimate of the cost of reading pages from tables in
-      that tablespace, and the executor's prefetching behavior, as established
+      that tablespace, and how many concurrent I/Os are issued, as established
       by the configuration parameters of the
       same name (see <xref linkend="guc-seq-page-cost"/>,
       <xref linkend="guc-random-page-cost"/>,
diff --git a/doc/src/sgml/ref/create_tablespace.sgml b/doc/src/sgml/ref/create_tablespace.sgml
index 9d5ab025261..b77e774c53f 100644
--- a/doc/src/sgml/ref/create_tablespace.sgml
+++ b/doc/src/sgml/ref/create_tablespace.sgml
@@ -110,7 +110,7 @@ CREATE TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
         and <varname>maintenance_io_concurrency</varname>.
         Setting these values for a particular tablespace will override the
         planner's usual estimate of the cost of reading pages from tables in
-        that tablespace, and the executor's prefetching behavior, as established
+        that tablespace, and how many concurrent I/Os are issued, as established
         by the configuration parameters of the
         same name (see <xref linkend="guc-seq-page-cost"/>,
         <xref linkend="guc-random-page-cost"/>,