Date: Tue, 5 Mar 2019 11:05:31 -0500
From: Bruce Momjian <bruce@momjian.us>
To: dick.wieland@wiline.com, pgsql-docs@lists.postgresql.org
Subject: Re: Error in 9.6 documentation? checkpoint_flush_after
Message-ID: <20190305160531.zwoosgpihb7tnonm@momjian.us>
References: <155141990108.1380.13791208099765451492@wrigleys.postgresql.org>
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On Fri, Mar  1, 2019 at 05:58:21AM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/9.6/runtime-config-wal.html
> Description:
> 
> https://www.postgresql.org/docs/9.6/runtime-config-wal.html
> 19.5.2
> 
> checkpoint_flush_after (integer)
> 
>     Whenever more than checkpoint_flush_after bytes have been written while
> performing a checkpoint, attempt to force the OS to issue these writes to
> the ...
> 
> bytes or pages? postgresql.conf says pages

You bring up a good point.  This problem affects these postgresql.conf
variables:

	bgwriter_flush_after
	backend_flush_after
	checkpoint_flush_after

and probably more.  The issue is that the value is in bytes, as opposed
to a value of time, like milliseconds.  You can specify the bytes in
terms of the number of pages, e.g., 2 = 16kB, or you can specify it
directly in bytes, e.g., 32kB.

The line in postgresql.conf is saying:

	#checkpoint_flush_after = 256kB         # measured in pages, 0 disables
	                                          -----------------

if you specify a number like:

	checkpoint_flush_after = 12

that is 12 * 8kB or 96kB.  If you specify an actual byte amount like
256kB, that is the value that is used.

Do you have any suggestions on how this could be made clearer?

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

+ As you are, so once was I.  As I am, so you will be. +
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