Allow the wal_buffers setting to be auto-tuned to a reasonable value.

If wal_buffers is initially set to -1 (which is now the default), it's
replaced by 1/32nd of shared_buffers, with a minimum of 8 (the old default)
and a maximum of the XLOG segment size.  The allowed range for manual
settings is still from 4 up to whatever will fit in shared memory.

Greg Smith, with implementation correction by me.

doc/src/sgml/config.sgml

@@ -1638,12 +1638,25 @@ SET ENABLE_SEQSCAN TO OFF;
       </indexterm>
       <listitem>
        <para>
-        The amount of memory used in shared memory for WAL data.  The
-        default is 64 kilobytes (<literal>64kB</>).  The setting need only
-        be large enough to hold the amount of WAL data generated by one
-        typical transaction, since the data is written out to disk at
-        every transaction commit.  This parameter can only be set at server
-        start.
+        The amount of shared memory used for WAL data that has not yet been
+        written to disk.  The default setting of -1 selects a size equal to
+        1/32nd (about 3%) of <xref linkend="guc-shared-buffers">, but not less
+        than <literal>64kB</literal> nor more than the size of one WAL
+        segment, typically <literal>16MB</literal>.  This value can be set
+        manually if the automatic choice is too large or too small,
+        but any positive value less than <literal>32kB</literal> will be
+        treated as <literal>32kB</literal>.
+        This parameter can only be set at server start.
+       </para>
+
+       <para>
+        The contents of the WAL buffers are written out to disk at every
+        transaction commit, so extremely large values are unlikely to
+        provide a significant benefit.  However, setting this value to at
+        least a few megabytes can improve write performance on a busy
+        server where many clients are committing at once.  The auto-tuning
+        selected by the default setting of -1 should give reasonable
+        results in most cases.
        </para>
 
        <para>

src/backend/access/transam/xlog.c

@@ -69,7 +69,7 @@
 /* User-settable parameters */
 int			CheckPointSegments = 3;
 int			wal_keep_segments = 0;
-int			XLOGbuffers = 8;
+int			XLOGbuffers = -1;
 int			XLogArchiveTimeout = 0;
 bool		XLogArchiveMode = false;
 char	   *XLogArchiveCommand = NULL;
@@ -4777,6 +4777,41 @@ GetSystemIdentifier(void)
 	return ControlFile->system_identifier;
 }
 
+/*
+ * Auto-tune the number of XLOG buffers.
+ *
+ * If the user-set value of wal_buffers is -1, we auto-tune to about 3% of
+ * shared_buffers, with a maximum of one XLOG segment and a minimum of 8
+ * blocks (8 was the default value prior to PostgreSQL 9.1, when auto-tuning
+ * was added).  We also clamp manually-set values to at least 4 blocks; prior
+ * to PostgreSQL 9.1, a minimum of 4 was enforced by guc.c, but since that
+ * is no longer possible, we just silently treat such values as a request for
+ * the minimum.
+ */
+static void
+XLOGTuneNumBuffers(void)
+{
+	int			xbuffers = XLOGbuffers;
+	char		buf[32];
+
+	if (xbuffers == -1)
+	{
+		xbuffers = NBuffers / 32;
+		if (xbuffers > XLOG_SEG_SIZE / XLOG_BLCKSZ)
+			xbuffers = XLOG_SEG_SIZE / XLOG_BLCKSZ;
+		if (xbuffers < 8)
+			xbuffers = 8;
+	}
+	else if (xbuffers < 4)
+		xbuffers = 4;
+
+	if (xbuffers != XLOGbuffers)
+	{
+		snprintf(buf, sizeof(buf), "%d", xbuffers);
+		SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
+	}
+}
+
 /*
  * Initialization of shared memory for XLOG
  */
@@ -4785,6 +4820,10 @@ XLOGShmemSize(void)
 {
 	Size		size;
 
+	/* Figure out how many XLOG buffers we need. */
+	XLOGTuneNumBuffers();
+	Assert(XLOGbuffers > 0);
+
 	/* XLogCtl */
 	size = sizeof(XLogCtlData);
 	/* xlblocks array */

src/backend/utils/misc/guc.c

@@ -1765,7 +1765,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_XBLOCKS
 		},
 		&XLOGbuffers,
-		8, 4, INT_MAX, NULL, NULL
+		-1, -1, INT_MAX, NULL, NULL
 	},
 
 	{

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

@@ -162,7 +162,7 @@
 					#   fsync_writethrough
 					#   open_sync
 #full_page_writes = on			# recover from partial page writes
-#wal_buffers = 64kB			# min 32kB
+#wal_buffers = -1			# min 32kB, -1 sets based on shared_buffers
 					# (change requires restart)
 #wal_writer_delay = 200ms		# 1-10000 milliseconds