Provide better guidance for adjusting shared_buffers.

This change was previously committed to HEAD, but the consensus seems to be
in favor of back-patching it.  I'm only backpatching as far as 8.3.X, however,
because it's not clear to me to what degree this advice applies to older
branches, and in any case our first advice to anyone attempting to tune those
versions is likely to be "upgrade".

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d1edcd12bbc1ad4d7d1ffc2254da1de3dd0d3bb0..460fabe7e3aff4e3e5e06b07f6cb5e299322c37e 100644 (file)
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.164.2.8 2010/03/17 18:04:25 mha Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.164.2.9 2010/04/19 00:00:22 rhaas Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -766,9 +766,32 @@ SET ENABLE_SEQSCAN TO OFF;
         kilobytes times <xref linkend="guc-max-connections">.  (Non-default
         values of <symbol>BLCKSZ</symbol> change the minimum.)  However,
         settings significantly higher than the minimum are usually needed
-        for good performance.  Several tens of megabytes are recommended
-        for production installations.  This parameter can only be set at
-        server start.
+        for good performance.  This parameter can only be set at server start.
+       </para>
+
+       <para>
+        If you have a dedicated database server with 1GB or more of RAM, a
+        reasonable starting value for <varname>shared_buffers</varname> is 25%
+        of the memory in your system.  There are some workloads where even
+        large settings for <varname>shared_buffers</varname> are effective, but
+        because <productname>PostgreSQL</productname> also relies on the
+        operating system cache, it is unlikely that an allocation of more than
+        40% of RAM to <varname>shared_buffers</varname> will work better than a
+        smaller amount.  Larger settings for <varname>shared_buffers</varname>
+        usually require a corresponding increase in
+        <varname>checkpoint_segments</varname>, in order to spread out the
+        process of writing large quantities of new or changed data over a
+        longer period of time.
+       </para>
+
+       <para>
+        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.
+        Also, on Windows, large values for <varname>shared_buffers</varname>
+        aren't as effective.  You may find better results keeping the setting
+        relatively low and using the operating system cache more instead.  The
+        useful range for <varname>shared_buffers</varname> on Windows systems
+        is generally from 64MB to 512MB.
        </para>
 
        <para>