From 0f73aae13def660371c34c8feda6e684e6366bdb Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 22 Jan 2011 20:31:24 -0500
Subject: [PATCH] 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 | 25 ++++++++---
src/backend/access/transam/xlog.c | 41 ++++++++++++++++++-
src/backend/utils/misc/guc.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
4 files changed, 61 insertions(+), 9 deletions(-)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 8e2a2c5d736..570c7c3b7de 100644
--- a/doc/src/sgml/config.sgml
+++ b/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>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 5b6a2302b19..85b2dcae070 100644
--- a/src/backend/access/transam/xlog.c
+++ b/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 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index ffff6015596..73b9f1b01ca 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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
},
{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index f436b834680..6c6f9a9a0d8 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/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
--
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