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.
regards, tom lane
