Re: Improvement of checkpoint IO scheduler for stable transaction responses

On Sun, Jul 14, 2013 at 3:13 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> Accordingly, the current behavior--no delay--is already the best possible
> throughput. If you apply a write timing change and it seems to increase
> TPS, that's almost certainly because it executed less checkpoint writes.
> It's not a fair comparison. You have to adjust any delaying to still hit
> the same end point on the checkpoint schedule. That's what my later
> submissions did, and under that sort of controlled condition most of the
> improvements went away.

This is all valid logic, but I don't think it's makes the patch a bad
idea. What KONDO Mitsumasa is proposing (or proposed at one point,
upthread), is that when an fsync takes a long time, we should wait
before issuing the next fsync, and the delay should be proportional to
how long the previous fsync took. On a system that's behaving well,
where fsyncs are always fast, that's going to make very little
difference. On a system where fsync is sometimes very very slow, that
might result in the checkpoint overrunning its time budget - but SO
WHAT?

I mean, yes, we want checkpoints to complete in the time specified,
but if the I/O system is completely flogged, I suspect most people
would prefer to overrun the checkpoint's time budget rather than have
all foreground activity grind to a halt until the checkpoint finishes.
As I'm pretty sure you've pointed out in the past, when this
situation develops, the checkpoint may be doomed to overrun whether we
like it or not. We should view this as an emergency pressure release
valve; if we think not everyone will want it, then make it a GUC.

--
Robert Haas
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