Date: Tue, 5 Mar 2019 09:11:14 -0800
From: Andres Freund <andres@anarazel.de>
To: Bruce Momjian <bruce@momjian.us>
Cc: dick.wieland@wiline.com, pgsql-docs@lists.postgresql.org
Subject: Re: Error in 9.6 documentation? checkpoint_flush_after
Message-ID: <20190305171114.vxvpjfnfusidyzfi@alap3.anarazel.de>
References: <155141990108.1380.13791208099765451492@wrigleys.postgresql.org>
 <20190305160531.zwoosgpihb7tnonm@momjian.us>
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Hi,

On 2019-03-05 11:05:31 -0500, Bruce Momjian wrote:
> 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

It's bytes, rounded to pages. Describing it as pages is too complicated
because we'd need to reference the block size for it to make sense etc.
What's the problem with describing it in bytes?


> 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.

I'm not following? Why is bytes vs time a problem? We have similar
base-unit issues in plenty time based GUCs?

Greetings,

Andres Freund