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Commit0f73aae

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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'sreplaced 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 manualsettings is still from 4 up to whatever will fit in shared memory.Greg Smith, with implementation correction by me.
1 parent518b1e9 commit0f73aae

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4 files changed

+61
-9
lines changed

4 files changed

+61
-9
lines changed

‎doc/src/sgml/config.sgml‎

Lines changed: 19 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1638,12 +1638,25 @@ SET ENABLE_SEQSCAN TO OFF;
16381638
</indexterm>
16391639
<listitem>
16401640
<para>
1641-
The amount of memory used in shared memory for WAL data. The
1642-
default is 64 kilobytes (<literal>64kB</>). The setting need only
1643-
be large enough to hold the amount of WAL data generated by one
1644-
typical transaction, since the data is written out to disk at
1645-
every transaction commit. This parameter can only be set at server
1646-
start.
1641+
The amount of shared memory used for WAL data that has not yet been
1642+
written to disk. The default setting of -1 selects a size equal to
1643+
1/32nd (about 3%) of <xref linkend="guc-shared-buffers">, but not less
1644+
than <literal>64kB</literal> nor more than the size of one WAL
1645+
segment, typically <literal>16MB</literal>. This value can be set
1646+
manually if the automatic choice is too large or too small,
1647+
but any positive value less than <literal>32kB</literal> will be
1648+
treated as <literal>32kB</literal>.
1649+
This parameter can only be set at server start.
1650+
</para>
1651+
1652+
<para>
1653+
The contents of the WAL buffers are written out to disk at every
1654+
transaction commit, so extremely large values are unlikely to
1655+
provide a significant benefit. However, setting this value to at
1656+
least a few megabytes can improve write performance on a busy
1657+
server where many clients are committing at once. The auto-tuning
1658+
selected by the default setting of -1 should give reasonable
1659+
results in most cases.
16471660
</para>
16481661

16491662
<para>

‎src/backend/access/transam/xlog.c‎

Lines changed: 40 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -69,7 +69,7 @@
6969
/* User-settable parameters */
7070
intCheckPointSegments=3;
7171
intwal_keep_segments=0;
72-
intXLOGbuffers=8;
72+
intXLOGbuffers=-1;
7373
intXLogArchiveTimeout=0;
7474
boolXLogArchiveMode= false;
7575
char*XLogArchiveCommand=NULL;
@@ -4777,6 +4777,41 @@ GetSystemIdentifier(void)
47774777
returnControlFile->system_identifier;
47784778
}
47794779

4780+
/*
4781+
* Auto-tune the number of XLOG buffers.
4782+
*
4783+
* If the user-set value of wal_buffers is -1, we auto-tune to about 3% of
4784+
* shared_buffers, with a maximum of one XLOG segment and a minimum of 8
4785+
* blocks (8 was the default value prior to PostgreSQL 9.1, when auto-tuning
4786+
* was added). We also clamp manually-set values to at least 4 blocks; prior
4787+
* to PostgreSQL 9.1, a minimum of 4 was enforced by guc.c, but since that
4788+
* is no longer possible, we just silently treat such values as a request for
4789+
* the minimum.
4790+
*/
4791+
staticvoid
4792+
XLOGTuneNumBuffers(void)
4793+
{
4794+
intxbuffers=XLOGbuffers;
4795+
charbuf[32];
4796+
4797+
if (xbuffers==-1)
4798+
{
4799+
xbuffers=NBuffers /32;
4800+
if (xbuffers>XLOG_SEG_SIZE /XLOG_BLCKSZ)
4801+
xbuffers=XLOG_SEG_SIZE /XLOG_BLCKSZ;
4802+
if (xbuffers<8)
4803+
xbuffers=8;
4804+
}
4805+
elseif (xbuffers<4)
4806+
xbuffers=4;
4807+
4808+
if (xbuffers!=XLOGbuffers)
4809+
{
4810+
snprintf(buf,sizeof(buf),"%d",xbuffers);
4811+
SetConfigOption("wal_buffers",buf,PGC_POSTMASTER,PGC_S_OVERRIDE);
4812+
}
4813+
}
4814+
47804815
/*
47814816
* Initialization of shared memory for XLOG
47824817
*/
@@ -4785,6 +4820,10 @@ XLOGShmemSize(void)
47854820
{
47864821
Sizesize;
47874822

4823+
/* Figure out how many XLOG buffers we need. */
4824+
XLOGTuneNumBuffers();
4825+
Assert(XLOGbuffers>0);
4826+
47884827
/* XLogCtl */
47894828
size=sizeof(XLogCtlData);
47904829
/* xlblocks array */

‎src/backend/utils/misc/guc.c‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1765,7 +1765,7 @@ static struct config_int ConfigureNamesInt[] =
17651765
GUC_UNIT_XBLOCKS
17661766
},
17671767
&XLOGbuffers,
1768-
8,4,INT_MAX,NULL,NULL
1768+
-1,-1,INT_MAX,NULL,NULL
17691769
},
17701770

17711771
{

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

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -162,7 +162,7 @@
162162
# fsync_writethrough
163163
# open_sync
164164
#full_page_writes = on# recover from partial page writes
165-
#wal_buffers =64kB# min 32kB
165+
#wal_buffers =-1# min 32kB, -1 sets based on shared_buffers
166166
# (change requires restart)
167167
#wal_writer_delay = 200ms# 1-10000 milliseconds
168168

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