Re: raised checkpoint limit & manual checkpoint

On Sat, Sep 24, 2016 at 12:12 PM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>
> Hello,
>
> The checkpoint time limit has just been raised to one day after a discussion
> started by Andres Freund:
>
> https://www.postgresql.org/message-id/20160202001320.GP8743%40awork2.anarazel.de
>
> I would have gone further up, say one week or even one month, but I think
> that this new limit is an improvement over the previous 1 hour maximum.
>
> Now ISTM that there is a possible use case which arises with this new
> setting and is not well addressed by postgresql:
>
> Let us say that an application has periods of high and low usage, say over a
> day, so that I want to avoid a checkpoint from 8 to 20, but I'm okay after
> that. I could raise the size and time limits so that they do not occur
> during these hours and plan to do a manual CHECKPOINT once a day when I see
> fit.
>
> Now the problem I see is that CHECKPOINT means "do a CHECKPOINT right now as
> fast as possible", i.e. there is no throttling whatsoever, which leads to
> heavy IO and may result in a very unresponsive database.
>
> I would suggest that a good complementary feature would be to allow a manual
> checkpoint to run over a period of time, say something like:
>
> CHECKPOINT OVER '10 hours';
>
> That would target to complete after this period (whether it succeeds or not
> is another issue) instead of going as fast as possible, thus avoiding
> some performance degradation.
>

Isn't it somewhat overlaps with existing parameter
checkpoint_completion_target? You can use
checkpoint_completion_target to throttle the checkpoints. The option
you are suggesting seems to be more straight forward, but how will
user decide the time he wants Checkpoint to take.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com