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

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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.
This commit is contained in:
Tom Lane
2011-01-22 20:31:24 -05:00
parent 518b1e96c0
commit 0f73aae13d
4 changed files with 61 additions and 9 deletions
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41
src/backend/access/transam/xlog.c
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@ -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 */