Re: The suggestion of reducing autovacuum_vacuum_cost_delay should be documented

On Tue, May 12, 2020 at 03:38:16PM -0700, David G. Johnston wrote:
> On Tue, May 12, 2020 at 2:41 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Tue, May 12, 2020 at 05:40:14PM -0400, Bruce Momjian wrote:
> > On Tue, May 12, 2020 at 10:26:27AM +0000, PG Doc comments form wrote:
> > > The following documentation comment has been logged on the website:
> > >
> > > Page: https://www.postgresql.org/docs/11/runtime-config-autovacuum.html
> > > Description:
> > >
> > > The `autovacuum_vacuum_cost_delay` setting changes to 2ms in PostgreSQL
> 12
> > > but in the old Postgresql version, the default setting is still 20ms. I
> > > would suggest adding a suggestion in the old document
> > > to lower the autovacuum_vacuum_cost_delay as:
> > >
> > > > The default value of autovacuum_vacuum_cost_delay is reduced to 2ms
> in
> > > PostgreSQL 12. Reducing the autovacuum_vacuum_cost_delay will make the
> > > autovacuum more aggressive and might reduce the vacuum cost for
> > > write-intensive workload on big table.
> >
> > Uh, we usually don't suggest new defaults in back branches.
>
> Basically, what I am saying is that if you want this, it would be a new
> behavior that would need general discussion.
>
>
>
> The proposal is to document in versions 9.4 to 11 that the recommended value
> for the setting is 2ms while for reasons of continuity the default in these
> versions is 20ms.
>
> I don't really see any harm in it.  Its not like the choice to reduce the value
> was made because of new features introduced in 12 - it was a re-evaluation of a
> 15 year old default.

Well, we really need to have some general discussion about whether
changing defaults in major releases should trigger a mention to change
the defaults in back branches. This is something that would have to be
discussed on the hackers list.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +