Re: pgsql: Make WAL segment size configurable at initdb time.

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