Re: checkpoints after database start/immediate checkpoints

On Thu, Feb 4, 2016 at 4:42 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-02-03 09:57:00 -0500, Robert Haas wrote:
>> On Mon, Feb 1, 2016 at 7:43 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > I wonder if this essentially point at checkpoint_timeout being wrongly
>> > defined: Currently it means we'll try to finish a checkpoint
>> > (1-checkpoint_completion_target) * timeout before the next one - but
>> > perhaps it should instead be that we start checkpoint_timeout * _target
>> > before the next timeout? Afaics that'd work more graceful in the face of
>> > restarts and forced checkpoints.
>>
>> There's a certain appeal to that, but at the same time it seems pretty
>> wonky. Right now, you can say that a checkpoint is triggered when the
>> amount of WAL reaches X or the amount of time reaches Y, but with the
>> alternative definition it's a bit harder to explain what's going on
>> there.
>
> Hm, but can you, really? We *start* a checkpoint every
> checkpoint_timeout, but we only finish it after
> checkpoint_completion_target * timeout, or cct * segments. I find it
> pretty hard to explain that we have a gap of checkpoint_timeout, where
> nothing happens, after an immediate/manual checkpoint.
>
> Defining it as: We try to *finish* a checkpoint every checkpoint_timeout
> or checkpoint_segments/(max_wal_size/~3) actually seems simpler to
> me. Then we just need to add that we start a checkpoint
> checkpoint_completion_target before either of the above are reached.

Hmm, I could go for that.

--
Robert Haas
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