Re: Redesigning checkpoint_segments

On 06/26/2015 02:08 PM, Heikki Linnakangas wrote:
> I'm not sure what to do about this. With the attached patch, you get the
> same leisurely pacing with restartpoints as you get with checkpoints,
> but you exceed max_wal_size during recovery, by the amount determined by
> checkpoint_completion_target. Alternatively, we could try to perform
> restartpoints faster then checkpoints, but then you'll get nasty
> checkpoint I/O storms in recovery.

Ok, committed this patch. IMHO it's definitely better than the old
behaviour.

> A bigger change would be to write a WAL record at the beginning of a
> checkpoint. It wouldn't do anything else, but it would be a hint to
> recovery that there's going to be a checkpoint record later whose
> redo-pointer will point to that record. We could then start the
> restartpoint at that record already, before seeing the checkpoint record
> itself.
>
> I think the attached is better than nothing, but I'll take a look at
> that beginning-of-checkpoint idea. It might be too big a change to do at
> this point, but I'd really like to fix this properly for 9.5, since
> we've changed with the way checkpoints are scheduled anyway.

This would've been a much more complicated patch, so I dropped that
idea, for 9.5 anyway. Maybe later, but it's not urgent.

- Heikki