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Replace checkpoint_segments with min_wal_size and max_wal_size.

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Instead of having a single knob (checkpoint_segments) that both triggers
checkpoints, and determines how many checkpoints to recycle, they are now
separate concerns. There is still an internal variable called
CheckpointSegments, which triggers checkpoints. But it no longer determines
how many segments to recycle at a checkpoint. That is now auto-tuned by
keeping a moving average of the distance between checkpoints (in bytes),
and trying to keep that many segments in reserve. The advantage of this is
that you can set max_wal_size very high, but the system won't actually
consume that much space if there isn't any need for it. The min_wal_size
sets a floor for that; you can effectively disable the auto-tuning behavior
by setting min_wal_size equal to max_wal_size.

The max_wal_size setting is now the actual target size of WAL at which a
new checkpoint is triggered, instead of the distance between checkpoints.
Previously, you could calculate the actual WAL usage with the formula
"(2 + checkpoint_completion_target) * checkpoint_segments + 1". With this
patch, you set the desired WAL usage with max_wal_size, and the system
calculates the appropriate CheckpointSegments with the reverse of that
formula. That's a lot more intuitive for administrators to set.

Reviewed by Amit Kapila and Venkata Balaji N.
This commit is contained in:
Heikki Linnakangas
2015-02-23 18:53:02 +02:00
parent 0fec000365
commit 88e9823026
9 changed files with 327 additions and 108 deletions
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30
src/backend/utils/misc/guc.c
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@ -685,6 +685,9 @@ typedef struct
#if XLOG_BLCKSZ < 1024 || XLOG_BLCKSZ > (1024*1024)
#error XLOG_BLCKSZ must be between 1KB and 1MB
#endif
#if XLOG_SEG_SIZE < (1024*1024) || XLOG_BLCKSZ > (1024*1024*1024)
#error XLOG_SEG_SIZE must be between 1MB and 1GB
#endif
static const char *memory_units_hint =
gettext_noop("Valid units for this parameter are \"kB\", \"MB\", \"GB\", and \"TB\".");
@ -706,6 +709,11 @@ static const unit_conversion memory_unit_conversion_table[] =
{ "MB", GUC_UNIT_XBLOCKS, 1024 / (XLOG_BLCKSZ / 1024) },
{ "kB", GUC_UNIT_XBLOCKS, -(XLOG_BLCKSZ / 1024) },
{ "TB", GUC_UNIT_XSEGS, (1024*1024*1024) / (XLOG_SEG_SIZE / 1024) },
{ "GB", GUC_UNIT_XSEGS, (1024*1024) / (XLOG_SEG_SIZE / 1024) },
{ "MB", GUC_UNIT_XSEGS, -(XLOG_SEG_SIZE / (1024 * 1024)) },
{ "kB", GUC_UNIT_XSEGS, -(XLOG_SEG_SIZE / 1024) },
{ "" } /* end of table marker */
};
@ -2146,15 +2154,27 @@ static struct config_int ConfigureNamesInt[] =
},
{
{"checkpoint_segments", PGC_SIGHUP, WAL_CHECKPOINTS,
gettext_noop("Sets the maximum distance in log segments between automatic WAL checkpoints."),
NULL
{"min_wal_size", PGC_SIGHUP, WAL_CHECKPOINTS,
gettext_noop("Sets the minimum size to shrink the WAL to."),
NULL,
GUC_UNIT_XSEGS
},
&CheckPointSegments,
3, 1, INT_MAX,
&min_wal_size,
5, 2, INT_MAX,
NULL, NULL, NULL
},
{
{"max_wal_size", PGC_SIGHUP, WAL_CHECKPOINTS,
gettext_noop("Sets the WAL size that triggers a checkpoint."),
NULL,
GUC_UNIT_XSEGS
},
&max_wal_size,
8, 2, INT_MAX,
NULL, assign_max_wal_size, NULL
},
{
{"checkpoint_timeout", PGC_SIGHUP, WAL_CHECKPOINTS,
gettext_noop("Sets the maximum time between automatic WAL checkpoints."),