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Bring some clarity to the defaults for the xxx_flush_after parameters.

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Instead of confusingly stating platform-dependent defaults for these
parameters in the comments in postgresql.conf.sample (with the main
entry being a lie on Linux), teach initdb to install the correct
platform-dependent value in postgresql.conf, similarly to the way
we handle other platform-dependent defaults.  This won't do anything
for existing 9.6 installations, but since it's effectively only a
documentation improvement, that seems OK.

Since this requires initdb to have access to the default values,
move the #define's for those to pg_config_manual.h; the original
placement in bufmgr.h is unworkable because that file can't be
included by frontend programs.

Adjust the default value for wal_writer_flush_after so that it is 1MB
regardless of XLOG_BLCKSZ, conforming to what is stated in both the
SGML docs and postgresql.conf.  (We could alternatively make it scale
with XLOG_BLCKSZ, but I'm not sure I see the point.)

Copy-edit related SGML documentation.

Fabien Coelho and Tom Lane, per a gripe from Tomas Vondra.

Discussion: <30ebc6e3-8358-09cf-44a8-578252938424@2ndquadrant.com>
This commit is contained in:
Tom Lane
2016-11-25 18:36:10 -05:00
parent ab77a5a456
commit dbdfd114f3
7 changed files with 66 additions and 41 deletions
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33
doc/src/sgml/config.sgml
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@ -1903,10 +1903,10 @@ include_dir 'conf.d'
<xref linkend="guc-shared-buffers">, but smaller than the OS's page
cache, where performance might degrade. This setting may have no
effect on some platforms. The valid range is between
<literal>0</literal>, which disables controlled writeback, and
<literal>0</literal>, which disables forced writeback, and
<literal>2MB</literal>. The default is <literal>512kB</> on Linux,
<literal>0</> elsewhere. (Non-default values of
<symbol>BLCKSZ</symbol> change the default and maximum.)
<literal>0</> elsewhere. (If <symbol>BLCKSZ</symbol> is not 8kB,
the default and maximum values scale proportionally to it.)
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
@ -2055,10 +2055,10 @@ include_dir 'conf.d'
that are bigger than <xref linkend="guc-shared-buffers">, but smaller
than the OS's page cache, where performance might degrade. This
setting may have no effect on some platforms. The valid range is
between <literal>0</literal>, which disables controlled writeback,
and <literal>2MB</literal>. The default is <literal>0</> (i.e. no
flush control). (Non-default values of <symbol>BLCKSZ</symbol>
change the maximum.)
between <literal>0</literal>, which disables forced writeback,
and <literal>2MB</literal>. The default is <literal>0</>, i.e., no
forced writeback. (If <symbol>BLCKSZ</symbol> is not 8kB,
the maximum value scales proportionally to it.)
</para>
</listitem>
</varlistentry>
@ -2518,10 +2518,11 @@ include_dir 'conf.d'
<para>
Specifies how often the WAL writer flushes WAL. After flushing WAL it
sleeps for <varname>wal_writer_delay</> milliseconds, unless woken up
by an asynchronously committing transaction. In case the last flush
by an asynchronously committing transaction. If the last flush
happened less than <varname>wal_writer_delay</> milliseconds ago and
less than <varname>wal_writer_flush_after</> bytes of WAL have been
produced since, WAL is only written to the OS, not flushed to disk.
produced since, then WAL is only written to the operating system, not
flushed to disk.
The default value is 200 milliseconds (<literal>200ms</>). Note that
on many systems, the effective resolution of sleep delays is 10
milliseconds; setting <varname>wal_writer_delay</> to a value that is
@ -2540,12 +2541,12 @@ include_dir 'conf.d'
</term>
<listitem>
<para>
Specifies how often the WAL writer flushes WAL. In case the last flush
Specifies how often the WAL writer flushes WAL. If the last flush
happened less than <varname>wal_writer_delay</> milliseconds ago and
less than <varname>wal_writer_flush_after</> bytes of WAL have been
produced since, WAL is only written to the OS, not flushed to disk.
If <varname>wal_writer_flush_after</> is set to <literal>0</> WAL is
flushed every time the WAL writer has written WAL. The default is
produced since, then WAL is only written to the operating system, not
flushed to disk. If <varname>wal_writer_flush_after</> is set
to <literal>0</> then WAL data is flushed immediately. The default is
<literal>1MB</literal>. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
@ -2665,10 +2666,10 @@ include_dir 'conf.d'
that are bigger than <xref linkend="guc-shared-buffers">, but smaller
than the OS's page cache, where performance might degrade. This
setting may have no effect on some platforms. The valid range is
between <literal>0</literal>, which disables controlled writeback,
between <literal>0</literal>, which disables forced writeback,
and <literal>2MB</literal>. The default is <literal>256kB</> on
Linux, <literal>0</> elsewhere. (Non-default values of
<symbol>BLCKSZ</symbol> change the default and maximum.)
Linux, <literal>0</> elsewhere. (If <symbol>BLCKSZ</symbol> is not
8kB, the default and maximum values scale proportionally to it.)
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>