Create a new dedicated Postgres process, "wal writer", which exists to write

and fsync WAL at convenient intervals.  For the moment it just tries to
offload this work from backends, but soon it will be responsible for
guaranteeing a maximum delay before asynchronously-committed transactions
will be flushed to disk.

This is a portion of Simon Riggs' async-commit patch, committed to CVS
separately because a background WAL writer seems like it might be a good idea
independently of the async-commit feature.  I rebased walwriter.c on
bgwriter.c because it seemed like a more appropriate way of handling signals;
while the startup/shutdown logic in postmaster.c is more like autovac because
we want walwriter to quit before we start the shutdown checkpoint.

1 files changed, 24 insertions, 4 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index a3331bdef6e..0e49ba32178 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.132 2007/07/24 01:53:55 alvherre Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.133 2007/07/24 04:54:08 tgl Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -1413,7 +1413,7 @@ SET ENABLE_SEQSCAN TO OFF;
        </para>
       </listitem>
      </varlistentry>
-     
+
      <varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
       <term><varname>wal_buffers</varname> (<type>integer</type>)</term>
       <indexterm>
@@ -1438,7 +1438,27 @@ SET ENABLE_SEQSCAN TO OFF;
        </para>
       </listitem>
      </varlistentry>
-                
+
+     <varlistentry id="guc-wal-writer-delay" xreflabel="wal_writer_delay">
+      <term><varname>wal_writer_delay</varname> (<type>integer</type>)</term>
+      <indexterm>
+       <primary><varname>wal_writer_delay</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        Specifies the delay between activity rounds for the WAL writer.
+        In each round the writer will flush WAL to disk. It then sleeps for
+        <varname>wal_writer_delay</> milliseconds, and repeats.  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 not a multiple
+        of 10 might have the same results as setting it to the next higher
+        multiple of 10. This parameter can only be set in the
+        <filename>postgresql.conf</> file or on the server command line.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-commit-delay" xreflabel="commit_delay">
       <term><varname>commit_delay</varname> (<type>integer</type>)</term>
       <indexterm>
@@ -1521,7 +1541,7 @@ SET ENABLE_SEQSCAN TO OFF;
       </indexterm>
       <listitem>
        <para>
-        Specifies the target length of checkpoints, as a fraction of 
+        Specifies the target length of checkpoints, as a fraction of
         the checkpoint interval. The default is 0.5.
 
         This parameter can only be set in the <filename>postgresql.conf</>