Re: WAL insert delay settings

Hi,

On 2019-02-19 13:28:00 -0500, Robert Haas wrote:
> On Fri, Feb 15, 2019 at 1:42 PM Andres Freund  wrote:
> > I think it'd not be insane to add two things:
> > - WAL write rate limiting, independent of the vacuum stuff. It'd also be
> >   used by lots of other bulk commands (CREATE INDEX, ALTER TABLE
> >   rewrites, ...)
> > - Account for WAL writes in the current vacuum costing logic, by
> >   accounting for it using a new cost parameter
> >
> > Then VACUUM would be throttled by the *minimum* of the two, which seems
> > to make plenty sense to me, given the usecases.
> 
> Or maybe we should just blow up the current vacuum cost delay stuff
> and replace it with something that is easier to tune.  For example, we
> could just have one parameter that sets the maximum read rate in kB/s
> and another that sets the maximum dirty-page rate in kB/s.  Whichever
> limit is tighter binds.  If we also have the thing that is the topic
> of this thread, that's a third possible upper limit.

> I really don't see much point in doubling down on the current vacuum
> cost delay logic.  The overall idea is good, but the specific way that
> you have to set the parameters is pretty inscrutable, and I think we
> should just fix it so that it can be, uh, scruted.

I agree that that's something worthwhile to do, but given that the
proposal in this thread is *NOT* just about VACUUM, I don't see why it'd
be useful to tie a general WAL rate limiting to rewriting cost limiting
of vacuum.  It seems better to write the WAL rate limiting logic with an
eye towards structuring it in a way that'd potentially allow reusing
some of the code for a better VACUUM cost limiting.

I still don't *AT ALL* buy Stephen and Tomas' argument that it'd be
confusing that when both VACUUM and WAL cost limiting are active, the
lower limit takes effect.

Greetings,

Andres Freund