Date: Thu, 20 Sep 2018 18:23:54 -0700
From: Andres Freund <andres@anarazel.de>
To: David Steele <david@pgmasters.net>
Cc: Michael Paquier <michael@paquier.xyz>, pgsql-committers@postgresql.org
Subject: Re: pgsql: Make WAL segment size configurable at initdb time.
Message-ID: <20180921012354.mvkql65radrpv4gx@alap3.anarazel.de>
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 <20180920235932.GD1338@paquier.xyz>
 <4b5568f5-72b7-5355-3450-973e11447569@pgmasters.net>
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On 2018-09-20 21:14:51 -0400, David Steele wrote:
> On 9/20/18 7:59 PM, Michael Paquier wrote:
> > On Thu, Sep 20, 2018 at 11:48:08AM -0400, David Steele wrote:
> >>        </term>
> >>        <listitem>
> >>         <para>
> >> -        Reports the number of blocks (pages) in a WAL segment file.
> >> -        The total size of a WAL segment file in bytes is equal to
> >> -        <varname>wal_segment_size</varname> multiplied by <varname>wal_block_size</varname>;
> >> -        by default this is 16MB.  See <xref linkend="wal-configuration"/> for
> >> +        Reports the size of write ahead log segments.
> >> +        The default value is 16MB. See <xref linkend="wal-configuration"/> for
> >>          more information.
> >>         </para>
> >>        </listitem>
> > 
> > The GUC wal_segment_size reports as well 16MB in v10 or older versions,
> > so shouldn't the mention to wal_block_size be removed even further down?
> 
> Oddly, the GUC reports 16GB on `show wal_segment_size` but you get 2048
> when querying pg_settings.  I thought that was just the normal behavior
> for older versions -- and it might be a problem to change it.

16*M*B, right?  If so, that's normal - pg_settings just reports the
values in the underlying unit - which is XLOG_BLCKSZ, compile-time
defaulting to 8KB. 8192 * 2048 = 16MB.  That's the same in various other
settings.

Greetings,

Andres Freund