Re: checkpoints after database start/immediate checkpoints

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.

- Andres