Re: postgresql latency & bgwriter not doing its job

On 2014-08-27 10:32:19 -0400, Aidan Van Dyk wrote:
> On Wed, Aug 27, 2014 at 3:32 AM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>
> >
> > Hello Andres,
> >
> > [...]
> >>
> >> I think you're misunderstanding how spread checkpoints work.
> >>
> >
> > Yep, definitely:-) On the other hand I though I was seeking something
> > "simple", namely correct latency under small load, that I would expect out
> > of the box.
> >
> > What you describe is reasonable, and is more or less what I was hoping
> > for, although I thought that bgwriter was involved from the start and
> > checkpoint would only do what is needed in the end. My mistake.
> >
> >
> If all you want is to avoid the write storms when fsyncs start happening on
> slow storage, can you not just adjust the kernel vm.dirty* tunables to
> start making the kernel write out dirty buffers much sooner instead of
> letting them accumulate until fsyncs force them out all at once?

Well. For one that's a os specific global tunable requiring root to be
adjustable. For another we actually do want some buffering: If a backend
writes out a buffer's data itself (happens very frequently) it *should*
get buffered... So I don't think a process independent tunable is going
to do the trick.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services