Re: postgresql latency & bgwriter not doing its job

On Thu, Aug 28, 2014 at 3:27 AM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
> Hello Aidan,
>
>
>> 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?
>
>
> I tried that by setting:
> vm.dirty_expire_centisecs = 100
> vm.dirty_writeback_centisecs = 100
>
> So it should start writing returned buffers at most 2s after they are
> returned, if I understood the doc correctly, instead of at most 35s.
>
> The result is that with a 5000s 25tps pretty small load (the system can do
> 300tps with the default configuration), I lost 2091 (1.7%) of transactions,
> that is they were beyond the 200ms schedule limit. Another change is that
> overall the lost transactions are more spread than without this setting,
> although there still is stretches of unresponsiveness.
>
> So although the situation is significantly better, it is still far from good
> with the much reduced value I tried.

What do the checkpoint logs look like now? (especially interested in sync times)