Re: postgresql latency & bgwriter not doing its job

On Wed, Aug 27, 2014 at 6:05 AM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>> [...] What's your evidence the pacing doesn't work? Afaik it's the fsync
>> that causes the problem, not the the writes themselves.
>
>
> Hmmm. My (poor) understanding is that fsync would work fine if everything
> was already written beforehand:-) that is it has nothing to do but assess
> that all is already written. If there is remaining write work, it starts
> doing it "now" with the disastrous effects I'm complaining about.
>
> When I say "pacing does not work", I mean that things where not written out
> to disk by the OS, it does not mean that pg did not ask for it.
>
> However it does not make much sense for an OS scheduler to wait several
> minutes with tens of thousands of pages to write and do nothing about it...
> So I'm wondering.

Maybe what's needed, is to slightly tweak checkpoint logic to give the
kernel some time to flush buffers.

Correct me if I'm wrong, but the checkpointer does the sync right
after the reads. Of course there will be about 30s-worth of
accumulated writes (it's the default amount of time the kernel holds
on to dirty buffers).

Perhaps it should be delayed a small time, say 30s, to let the kernel
do the writing on its own.