Re: [COMMITTERS] pgsql: Mark pg_start_backup and pg_stop_backup as parallel-restricted.

On Thu, Jun 25, 2020 at 4:56 PM David Steele <david(at)pgmasters(dot)net> wrote:

> On 6/25/20 8:43 AM, Magnus Hagander wrote:
> > On Thu, Jun 25, 2020 at 2:11 PM David Steele <david(at)pgmasters(dot)net
> > <mailto:david(at)pgmasters(dot)net>> wrote:
> > On 6/24/20 6:27 PM, Tom Lane wrote:
> > >
> > > I was able to force it like this:
> > >
> > > regression=# set force_parallel_mode TO 'on';
> > > SET
> >
> > Ah, yes, that works. Now at least I can test it.
> >
> > > It doesn't seem terribly likely that anybody would be using
> > > force_parallel_mode = on in production, but perhaps there's some
> > > reasonable combination of the other parallelization planning GUCs
> > > that can make this plan look attractive.
> >
> > I'll also ask the user if they have this GUC enabled.
>
> The user confirmed they are running with force_parallel_mode=on so that
> probably explains why we have never seen this in the field before.
>

That's good.

> Maybe have pgbackrest issue an explicit SET force_parallel_mode=off when
> > it runs against a 9.6?
>
> According to the documentation the way to disable parallelism is:
>
> set max_parallel_workers_per_gather = 0
>

Interesting. I had somehow gotten into my head that force_parallel_mode
would override that. But it doesn't.

So we added that to session initialization in pgBackRest:
>
>
> https://github.com/pgbackrest/pgbackrest/commit/ea04ec7b3f4c6cf25c1b827c25c6a3c5896159a8

Personally I would've done it *just* for 9.6 and not for 10+, but that's
mostly semantic :) But if you do it for 9.6 then *eventually* you will be
able to retire it.

I'm worried that (as Tom said) the planner might find another reason to
> choose a parallel query.
>
> I'm looking at this as more than a fix for 9.6. I can't see any reason
> for the backup control session to run queries in parallel and possibly
> use more resources, so parallelism is disabled for all versions that
> support it.
>

Right. But since the parameters are flagged as parallel restricted in 10+...

Or are you saying you're worried about other things, completely unrelated
to start/stop backup, that the session might run?

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>