From: David Steele <david@pgmasters.net>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-committers@postgresql.org
Subject: Re: pgsql: Make WAL segment size configurable at initdb time.
Date: Fri, 5 Oct 2018 18:03:44 +0100
Message-ID: <fd0855d1-4bf5-65fc-9c0f-ff64af3061ab@pgmasters.net> (raw)
In-Reply-To: <20181005165447.zyzglieznybyi7wu@alap3.anarazel.de>
References: <E1duXBW-0003QB-4d@gemulon.postgresql.org>
<68ea97d6-2ed9-f339-e57d-ab3a33caf3b1@pgmasters.net>
<20181005165447.zyzglieznybyi7wu@alap3.anarazel.de>
Hi Andres,
On 10/5/18 5:54 PM, Andres Freund wrote:
> On 2018-09-20 11:48:08 -0400, David Steele wrote:
>
>> diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
>> index e1073ac6d3..3bfd172441 100644
>> --- a/doc/src/sgml/config.sgml
>> +++ b/doc/src/sgml/config.sgml
>> @@ -8440,10 +8440,8 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
>> </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>
>
> Why is this actually more correct? You mean because we have a conversion
> that does the mb conversion at display time?
In pre-11 versions of Postgres, you get this:
postgres=# select setting, unit from pg_settings where name =
'wal_segment_size';
setting | unit
---------+------
2048 | 8kB
But in v11 you get this:
select setting, unit from pg_settings where name = 'wal_segment_size';
setting | unit
----------+------
16777216 | B
So, while the WAL segment size used to be expressed in terms of 8K pages
it is now expressed in terms of absolute bytes. This seemed to me to be
a very deliberate change in the original commit so I guessed it was done
for clarity, but that the docs didn't get the message.
Regards,
--
-David
david@pgmasters.net
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To: pgsql-committers@postgresql.org
Cc: david@pgmasters.net, andres@anarazel.de
Subject: Re: pgsql: Make WAL segment size configurable at initdb time.
In-Reply-To: <fd0855d1-4bf5-65fc-9c0f-ff64af3061ab@pgmasters.net>
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