Re: Improvement of checkpoint IO scheduler for stable transaction responses

Greg Smith escribió:
> On 7/18/13 11:04 AM, Robert Haas wrote:
> >On a system where fsync is sometimes very very slow, that
> >might result in the checkpoint overrunning its time budget - but SO
> >WHAT?
>
> Checkpoints provide a boundary on recovery time. That is their only
> purpose. You can always do better by postponing them, but you've
> now changed the agreement with the user about how long recovery
> might take.
>
> And if you don't respect the checkpoint boundary, what you can't do
> is then claim better execution performance than something that did.
> It's always possible to improvement throughput by postponing I/O.
> SO WHAT? If that's OK, you don't need complicated logic to do that.
> Increase checkpoint_timeout. The system with checkpoint_timeout at
> 6 minutes will always outperform one where it's 5.

I think the idea is to have a system in which most of the time the
recovery time will be that for checkpoint_timeout=5, but in those
(hopefully rare) cases where checkpoints take a bit longer, the recovery
time will be that for checkpoint_timeout=6.

In any case, if the system crashes past minute 5 after the previous
checkpoint (the worst possible timing), the current checkpoint will have
already started, so recovery will take slightly less time because some
flush work had already been done.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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