Re: pgbench -i spends all its time doing CHECKPOINT

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > question is if we do a big transaction that needs 10 log segments, do we
> > force an early CHECKPOINT to clear out the WAL segments or do we just
> > wait for the proper interval?
> 
> A checkpoint is forced after every CHECKPOINT_SEGMENTS log segments,
> regardless of longevity of transactions.  See
> http://developer.postgresql.org/docs/postgres/wal-configuration.html
> 
> Since segments before the checkpoint-before-last are deleted or recycled
> after each checkpoint, the maximum number of back segments would
> normally be 2 * CHECKPOINT_SEGMENTS.  We also pre-create WAL_FILES
> future log segments.  Counting the current segment gives a total of
> WAL_FILES + 2 * CHECKPOINT_SEGMENTS + 1 log segments.
> 
> AFAICS, the only way to force the current code into creating more than
> WAL_FILES + 2 * CHECKPOINT_SEGMENTS + 1 log segments is to be generating
> WAL entries at such a high rate that more than WAL_FILES log segments
> are filled before a triggered checkpoint can be completed.
> 

Very interesting.  Thanks.  Is there a reason someone would manually run
the CHECKPOINT command?

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