From 10f6646847515b1ab02735c24b04abaf1996f65f Mon Sep 17 00:00:00 2001
From: Thomas Munro <tmunro@postgresql.org>
Date: Wed, 19 Mar 2025 11:40:56 +1300
Subject: Introduce io_max_combine_limit.

The existing io_combine_limit can be changed by users.  The new
io_max_combine_limit is fixed at server startup time, and functions as a
silent clamp on the user setting.  That in itself is probably quite
useful, but the primary motivation is:

aio_init.c allocates shared memory for all asynchronous IOs including
some per-block data, and we didn't want to waste memory you'd never used
by assuming they could be up to PG_IOV_MAX.  This commit already halves
the size of 'AioHandleIov' and 'AioHandleData'.  A follow-up commit can
now expand PG_IOV_MAX without affecting that.

Since our GUC system doesn't support dependencies or cross-checks
between GUCs, the user-settable one now assigns a "raw" value to
io_combine_limit_guc, and the lower of io_combine_limit_guc and
io_max_combine_limit is maintained in io_combine_limit.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
---
 doc/src/sgml/config.sgml | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

(limited to 'doc/src')

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 9e9c02cde83..2988865b116 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2625,6 +2625,24 @@ include_dir 'conf.d'
        </listitem>
       </varlistentry>
 
+      <varlistentry id="guc-io-max-combine-limit" xreflabel="io_max_combine_limit">
+       <term><varname>io_max_combine_limit</varname> (<type>integer</type>)
+       <indexterm>
+        <primary><varname>io_max_combine_limit</varname> configuration parameter</primary>
+       </indexterm>
+       </term>
+       <listitem>
+        <para>
+         Controls the largest I/O size in operations that combine I/O, and silently
+         limits the user-settable parameter <varname>io_combine_limit</varname>.
+         This parameter can only be set in
+         the <filename>postgresql.conf</filename> file or on the server
+         command line.
+         The default is 128kB.
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="guc-io-combine-limit" xreflabel="io_combine_limit">
        <term><varname>io_combine_limit</varname> (<type>integer</type>)
        <indexterm>
@@ -2633,7 +2651,10 @@ include_dir 'conf.d'
        </term>
        <listitem>
         <para>
-         Controls the largest I/O size in operations that combine I/O.
+         Controls the largest I/O size in operations that combine I/O.  If set
+         higher than the <varname>io_max_combine_limit</varname> parameter, the
+         lower value will silently be used instead, so both may need to be raised
+         to increase the I/O size.
          The default is 128kB.
         </para>
        </listitem>
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