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Introduce io_max_combine_limit.

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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
This commit is contained in:
Thomas Munro
2025-03-19 11:40:56 +13:00
parent 17d8bba6da
commit 10f6646847
8 changed files with 72 additions and 15 deletions
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23
doc/src/sgml/config.sgml
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@ -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>