Re: Expose checkpoint start/finish times into SQL.

"Joshua D. Drake" <jd@commandprompt.com> writes:
> Heikki Linnakangas <heikki@enterprisedb.com> wrote:
>> Why is that useful?

> For knowing how long checkpoints are taking. If they are taking too
> long you may need to adjust your bgwriter settings, and it is a
> serious drag to parse postgresql logs for this info.

1. To do anything useful along those lines, you would need to look at a
lot of checkpoints over time, which is what log_checkpoints is good for.
This patch only tells you about the latest, which isn't very useful
for making any good decisions about parameters.

2. If I read the patch correctly, half of the time what you'd be seeing
is the start time of the currently-active checkpoint and the completion
time of the prior checkpoint.  I don't know what those numbers are good
for at all.

3. As of PG 8.3, the bgwriter tries very hard to make the elapsed time
of a checkpoint be just about checkpoint_timeout *
checkpoint_completion_target, regardless of load factors.  So unless
your settings are completely broken, measuring the actual time isn't
going to tell you much.

In short: Heikki's question is on point.

            regards, tom lane