Re: [HACKERS] Block level parallel vacuum

On Wed, Oct 9, 2019 at 6:13 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Sat, Oct 5, 2019 at 4:36 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > 3.
> > @@ -2888,6 +2888,8 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
> > (!wraparound ? VACOPT_SKIP_LOCKED : 0);
> > tab->at_params.index_cleanup = VACOPT_TERNARY_DEFAULT;
> > tab->at_params.truncate = VACOPT_TERNARY_DEFAULT;
> > + /* parallel lazy vacuum is not supported for autovacuum */
> > + tab->at_params.nworkers = -1;
> >
> > What is the reason for the same? Can we explain in the comments?
>
> I think it's just that we don't want to support parallel auto vacuum
> because it can consume more CPU resources in spite of background job,
> which might be an unexpected behavior of autovacuum.
>

I think the other reason is it can generate a lot of I/O which might
choke other operations. I think if we want we can provide Guc(s) to
control such behavior, but initially providing it via command should
be a good start so that users can knowingly use it in appropriate
cases. We can later extend it for autovacuum if required.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com