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, 9 Nov 2018 21:45:18 -0500
Message-ID: <f572c66f-1840-b789-04e2-b767f4765c3a@pgmasters.net> (raw)
In-Reply-To: <fd0855d1-4bf5-65fc-9c0f-ff64af3061ab@pgmasters.net>
References: <E1duXBW-0003QB-4d@gemulon.postgresql.org>
<68ea97d6-2ed9-f339-e57d-ab3a33caf3b1@pgmasters.net>
<20181005165447.zyzglieznybyi7wu@alap3.anarazel.de>
<fd0855d1-4bf5-65fc-9c0f-ff64af3061ab@pgmasters.net>
On 10/5/18 1:03 PM, David Steele wrote:
> 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.
Thoughts on this?
I know it's minor in the grand scheme of things but it caused me some
confusion when I was updating pgBackRest for v11 and I imagine it might
do the same for others.
--
-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: <f572c66f-1840-b789-04e2-b767f4765c3a@pgmasters.net>
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