On 10/30/2014 08:51 PM, Robert Haas wrote:
> On Wed, Oct 29, 2014 at 2:36 PM, Tomas Vondra <tv@fuzzy.cz> wrote:
>>> I would tend not to worry too much about this case. I'm skeptical
>>> that there are a lot of people using large template databases. But
>>> if there are, or if some particular one of those people hits this
>>> problem, then they can raise checkpoint_segments to avoid it. The
>>> reverse problem, which you are encountering, cannot be fixed by
>>> adjusting settings.
>>
>> That however solves "only" the checkpoint, not the double amount of I/O
>> due to writing both the files and WAL, no? But maybe that's OK.
>
> I mean, it's not unimaginable that it's going to hurt somebody, but
> the current situation is pretty bad too. You don't have to be the
> world's foremost PostgreSQL performance expert to know that extra
> checkpoints are really bad for performance. Write volume is of course
> also a problem, but I bet there are a lot more people using small
> template databases (where the write volume isn't really an issue,
> because as Heikki points out the checkpoint wastes half a segment
> anyway, but the checkpoint may very well be a issue) than large ones
> (where either could be an issue).
Nitpick: I didn't say that a a checkpoint wastes half a segment. An xlog
switch does, but a checkpoint doesn't automatically cause an xlog switch.
But I agree with the sentiment in general.
- Heikki