Thread: Checkpointing question
During heavy writes times we get the checkpoint too often error, what's the real knock down effect of checkpointing too often? The documents don’t really say what is wrong with checkpointing too often, does it cause block, io contention, etc, etc? From my understanding it's just IO contention, but I wanted to make sure.
14.4.6. Increase checkpoint_segments
Temporarily increasing the checkpoint_segments configuration variable can also make large data loads faster. This is because loading a large amount of data into PostgreSQL will cause checkpoints to occur more often than the normal checkpoint frequency (specified by the checkpoint_timeout configuration variable). Whenever a checkpoint occurs, all dirty pages must be flushed to disk. By increasing checkpoint_segments temporarily during bulk data loads, the number of checkpoints that are required can be reduced.
- John
[ 2011-02-15 09:34:30.549 GMT ] :4d404b6e.47ef LOG: checkpoint starting: xlog
[ 2011-02-15 09:34:43.656 GMT ] :4d404b6e.47ef LOG: checkpoint complete: wrote 36135 buffers (0.4%); 0 transaction log file(s) added, 0 removed, 12 recycled; write=13.101 s, sync=0.000 s, total=13.107 s
[ 2011-02-15 09:34:57.090 GMT ] :4d404b6e.47ef LOG: checkpoints are occurring too frequently (27 seconds apart)
[ 2011-02-15 09:34:57.090 GMT ] :4d404b6e.47ef HINT: Consider increasing the configuration parameter "checkpoint_segments".
[ 2011-02-15 09:34:57.090 GMT ] :4d404b6e.47ef LOG: checkpoint starting: xlog
[ 2011-02-15 09:35:11.492 GMT ] :4d404b6e.47ef LOG: checkpoint complete: wrote 54634 buffers (0.7%); 0 transaction log file(s) added, 0 removed, 30 recycled; write=14.290 s, sync=0.000 s, total=14.401 s
[ 2011-02-15 09:35:25.496 GMT ] :4d404b6e.47ef LOG: checkpoints are occurring too frequently (28 seconds apart)
[ 2011-02-15 09:35:25.496 GMT ] :4d404b6e.47ef HINT: Consider increasing the configuration parameter "checkpoint_segments".
[ 2011-02-15 09:35:25.496 GMT ] :4d404b6e.47ef LOG: checkpoint starting: xlog
[ 2011-02-15 09:35:39.688 GMT ] :4d404b6e.47ef LOG: checkpoint complete: wrote 39352 buffers (0.5%); 0 transaction log file(s) added, 0 removed, 30 recycled; write=14.185 s, sync=0.000 s, total=14.192 s
[ 2011-02-15 09:35:53.417 GMT ] :4d404b6e.47ef LOG: checkpoints are occurring too frequently (28 seconds apart)
[ 2011-02-15 09:35:53.417 GMT ] :4d404b6e.47ef HINT: Consider increasing the configuration parameter "checkpoint_segments".
[ 2011-02-15 09:35:53.417 GMT ] :4d404b6e.47ef LOG: checkpoint starting: xlog
[ 2011-02-15 09:36:09.059 GMT ] :4d404b6e.47ef LOG: checkpoint complete: wrote 48803 buffers (0.6%); 0 transaction log file(s) added, 0 removed, 30 recycled; write=15.408 s, sync=0.000 s, total=15.641 s
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"Strange, John W" <john.w.strange@jpmchase.com> wrote: > During heavy writes times we get the checkpoint too often error, > what's the real knock down effect of checkpointing too often? The main concern is that it may cause an increase in disk writes, possibly to the point of causing blocking while waiting for the disk. Generally bigger checkpoint_segments settings improve performance, especially as shared_buffers is increased. There are some counter-examples, particularly during bulk loads, which haven't really been explained: http://archives.postgresql.org/pgsql-hackers/2010-04/msg00848.php That makes this an area where careful testing of your real workload with different settings can be important, at least if you're trying to wring that last ounce of performance out of your server.. -Kevin