Make vacuum_defer_cleanup_age be PGC_SIGHUP level, since it's not sensible

to have different values in different processes of the primary server.
Also put it into the "Streaming Replication" GUC category; it doesn't belong
in "Standby Servers" because you use it on the master not the standby.
In passing also correct guc.c's idea of wal_keep_segments' category.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.289 2010/07/03 20:43:57 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.290 2010/07/03 21:23:58 tgl Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -1914,6 +1914,31 @@ SET ENABLE_SEQSCAN TO OFF;
        </para>
        </listitem>
       </varlistentry>
+
+     <varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
+      <term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)</term>
+      <indexterm>
+       <primary><varname>vacuum_defer_cleanup_age</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        Specifies the number of transactions by which <command>VACUUM</> and
+        <acronym>HOT</> updates will defer cleanup of dead row versions. The
+        default is zero transactions, meaning that dead row versions can be
+        removed as soon as possible, that is, as soon as they are no longer
+        visible to any open transaction.  You may wish to set this to a
+	non-zero value on a primary server that is supporting hot standby
+        servers, as described in <xref linkend="hot-standby">.  This allows
+        more time for queries on the standby to complete without incurring
+        conflicts due to early cleanup of rows.  However, since the value
+        is measured in terms of number of write transactions occurring on the
+        primary server, it is difficult to predict just how much additional
+        grace time will be made available to standby queries.
+        This parameter can only be set in the <filename>postgresql.conf</>
+        file or on the server command line.
+       </para>
+      </listitem>
+     </varlistentry>
      </variablelist>
     </sect2>
 
@@ -2004,29 +2029,6 @@ SET ENABLE_SEQSCAN TO OFF;
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
-      <term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)</term>
-      <indexterm>
-       <primary><varname>vacuum_defer_cleanup_age</> configuration parameter</primary>
-      </indexterm>
-      <listitem>
-       <para>
-        Specifies the number of transactions by which <command>VACUUM</> and
-        <acronym>HOT</> updates will defer cleanup of dead row versions. The
-        default is 0 transactions, meaning that dead row versions will be
-        removed as soon as possible. You may wish to set this to a non-zero
-        value when planning or maintaining a Hot Standby connection, as
-        described in <xref linkend="hot-standby">.  The recommended value is
-        <literal>0</> unless you have clear reason to increase it. The purpose
-        of the parameter is to allow the user to specify an approximate time
-        delay before cleanup occurs. However, it should be noted that there is
-        no direct link with any specific time delay and so the results will be
-        application and installation specific, as well as variable over time,
-        depending upon the transaction rate (of writes only).
-       </para>
-      </listitem>
-     </varlistentry>
-
      </variablelist>
     </sect2>
    </sect1>

src/backend/storage/ipc/procarray.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.70 2010/05/14 07:11:49 sriggs Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.71 2010/07/03 21:23:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1117,7 +1117,15 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
 	LWLockRelease(ProcArrayLock);
 
 	/*
-	 * Compute the cutoff XID, being careful not to generate a "permanent" XID
+	 * Compute the cutoff XID, being careful not to generate a "permanent" XID.
+	 *
+	 * vacuum_defer_cleanup_age provides some additional "slop" for the
+	 * benefit of hot standby queries on slave servers.  This is quick and
+	 * dirty, and perhaps not all that useful unless the master has a
+	 * predictable transaction rate, but it's what we've got.  Note that
+	 * we are assuming vacuum_defer_cleanup_age isn't large enough to cause
+	 * wraparound --- so guc.c should limit it to no more than the xidStopLimit
+	 * threshold in varsup.c.
 	 */
 	result -= vacuum_defer_cleanup_age;
 	if (!TransactionIdIsNormal(result))

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.558 2010/07/03 20:43:58 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.559 2010/07/03 21:23:58 tgl Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -1631,8 +1631,8 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"vacuum_defer_cleanup_age", PGC_USERSET, WAL_STANDBY_SERVERS,
-			gettext_noop("Age by which VACUUM and HOT cleanup should be deferred, if any."),
+		{"vacuum_defer_cleanup_age", PGC_SIGHUP, WAL_REPLICATION,
+			gettext_noop("Number of transactions by which VACUUM and HOT cleanup should be deferred, if any."),
 			NULL
 		},
 		&vacuum_defer_cleanup_age,
@@ -1675,7 +1675,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"wal_keep_segments", PGC_SIGHUP, WAL_CHECKPOINTS,
+		{"wal_keep_segments", PGC_SIGHUP, WAL_REPLICATION,
 			gettext_noop("Sets the number of WAL files held for standby servers."),
 			NULL
 		},

src/backend/utils/misc/postgresql.conf.sample

@@ -188,6 +188,7 @@
 #max_wal_senders = 0		# max number of walsender processes
 #wal_sender_delay = 200ms	# walsender cycle time, 1-10000 milliseconds
 #wal_keep_segments = 0		# in logfile segments, 16MB each; 0 disables
+#vacuum_defer_cleanup_age = 0	# number of xacts by which cleanup is delayed
 
 # - Standby Servers -
 
@@ -198,7 +199,6 @@
 #max_standby_streaming_delay = 30s	# max delay before canceling queries
 					# when reading streaming WAL;
 					# -1 allows indefinite delay
-#vacuum_defer_cleanup_age = 0		# number of transactions by which cleanup is deferred
 
 
 #------------------------------------------------------------------------------