Date: Thu, 12 Nov 2020 12:37:30 -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?
Message-ID: <20201112173730.GF25584@momjian.us>
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On Thu, Nov 12, 2020 at 05:25:30PM +0000, Chris Wilson wrote:
> Hi Bruce,
> 
> Thanks, I absolutely agree that this documentation needs to explain properly
> how the bgwriter works. Your latest patch looks good, it significantly improves
> this section of the manual. I would just suggest changing "non-dirty" to
> "clean" in "When the number of non-dirty shared buffers appears to be
> insufficient", as this makes the language simpler and avoids introducing
> another new term (non-dirty, which means the same as clean).

OK, done.  I wasn't sure 'clean' would be assumed to be non-dirty, but you
are right the language is clearer with 'clean'.  (I was afraid 'clean'
would be assumed to be 'empty'.)

-- 
  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..a632cf98ba 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2146,8 +2146,11 @@ 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.
+      buffers.  When the number of clean shared buffers appears to be
+      insufficient, the background writer writes some dirty buffers 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 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

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