Date: Tue, 12 May 2020 18:45:20 -0400
From: Bruce Momjian <bruce@momjian.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: yigongh@gmail.com, pgsql-docs@lists.postgresql.org
Subject: Re: The suggestion of reducing autovacuum_vacuum_cost_delay should
 be documented
Message-ID: <20200512224520.GE4666@momjian.us>
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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@momjian.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@momjian.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 +