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Date: Wed, 11 Nov 2020 18:24:17 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Chris Wilson <chris+google@qwirx.com>
Cc: Thomas Munro <thomas.munro@gmail.com>,
	Pg Docs <pgsql-docs@lists.postgresql.org>
Subject: Re: What does "[backends] should seldom or never need to wait for a
 write to occur" mean?
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On Wed, Nov 11, 2020 at 11:29:09AM +0000, Chris Wilson wrote:
> I still believe that my original proposed change, to "This reduces the chances
> that a backend needing an empty buffer must [itself] write a dirty one back to
> disk before evicting it" (with one extra word added), resolves the ambiguity
> and also more clearly and directly focuses it on what the bgwriter does and
> why, making it better documentation. It might be incorrect if my understanding
> is incorrect - is it?

You make some very good points.  Here is an updated patch.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee


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diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index f043433e31..294017c86e 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2146,9 +2146,12 @@ include_dir 'conf.d'
       There is a separate server
       process called the <firstterm>background writer</firstterm>, whose function
       is to issue writes of <quote>dirty</quote> (new or modified) shared
-      buffers.  It writes shared buffers so server processes handling
-      user queries seldom or never need to wait for a write to occur.
-      However, the background writer does cause a net overall
+      buffers.  When the percentage of dirty shared buffers is high, the
+      background writer writes some of them to the file system and marks
+      them as clean.  This reduces the likelihood that server processes
+      handling user queries will be unable to find clean buffers and have
+      to write dirty buffers to the file system themselves.  However,
+      the background writer does cause a net overall
       increase in I/O load, because while a repeatedly-dirtied page might
       otherwise be written only once per checkpoint interval, the
       background writer might write it several times as it is dirtied

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