Re: should we enable log_checkpoints out of the box?

On Wed, Nov 3, 2021 at 9:04 AM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Well, another issue is that if something unusual does happen, it appears
> very visibly if you are looking just for LOG messages, while if you have
> many checkpoint log messages, it might get lost.

That's a theoretical risk, but in practice the problem is actually
exactly the opposite of that, at least in my experience. If somebody
sends me a log file from a machine with log_checkpoints=on, the file
is typically hundreds of megabytes in size and in that file someplace
there are a handful of log lines generated by log_checkpoints mixed in
with millions of log lines about other things. So it's not that
log_checkpoints=on would keep you from noticing other things happening
on the machine. It's that other things happening on the machine would
keep you from noticing the log_checkpoints=on output.

In fact this problem is true for pretty much anything important that
shows up in the log: it's likely to be completely swamped by other
messages logged *at the exact same log level* that are not important
at all. The ERROR that could have alerted you to data corruption is
likely mixed in with millions of innocuous ERROR messages, for
example.

> If we want to log more
> by default, I think we are looking at several issues:
>
> * enabling log rotation and log file reuse in the default install
> * changing the labels of some of the normal-operation log messages
> * changing the way some of these log messages are controlled
> * perhaps using a ring buffer for common log messages

If we were talking about increasing the log volume by an amount that
was actually meaningful, we might need to think about these things,
but that's not what is being proposed. The only systems where this is
going to lead to a significant percentage increase in log volume are
the ones that are pretty much idle now. On real systems, this is going
to lead to a change that is less than 1%, and maybe less than 0.001%.
We don't have to rearchitect anything as a result of decisions that
have so little practical impact - especially considering that at least
some packagers are already putting log rotation in place
automatically, in a way consistent with distro policies.

--
Robert Haas
EDB: http://www.enterprisedb.com