Re: min_safe_lsn column in pg_replication_slots view

On Tue, Jun 30, 2020 at 7:53 PM Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>
> On 2020/06/30 17:07, Fujii Masao wrote:
> >
> >
> > On 2020/06/26 13:45, Amit Kapila wrote:
> >>
> >> Can we consider an option to "Remove min_safe_lsn; document how a user
> >> can monitor the distance"? We have a function to get current WAL
> >> insert location and other things required are available either via
> >> view or as guc variable values. The reason I am thinking of this
> >> option is that it might be better to get some more feedback on what is
> >> the most appropriate value to display. However, I am okay if we can
> >> reach a consensus on one of the above options.
> >
> > Yes, that's an idea. But it might not be easy to calculate that distance
> > manually by subtracting max_slot_wal_keep_size from the current LSN.
> > Because we've not supported -(pg_lsn, numeric) operator yet. I'm
> > proposing that operator, but it's for v14.
>
> Sorry this is not true. That distance can be calculated without those operators.
> For example,
>
> SELECT restart_lsn - pg_current_wal_lsn() + (SELECT setting::numeric * 1024 * 1024 FROM pg_settings WHERE name = 'max_slot_wal_keep_size') distance FROM pg_replication_slots;
>
> If the calculated distance is small or negative value, which means that
> we may lose some required WAL files. So in this case it's worth considering
> to increase max_slot_wal_keep_size.
>
> I still think it's better and more helpful to display something like
> that distance in pg_replication_slots rather than making each user
> calculate it...
>

Okay, but do we think it is better to display this in
pg_replication_slots or some new view like pg_stat_*_slots as being
discussed in [1]? It should not happen that we later decide to move
this or similar stats to that view.

[1] - https://www.postgresql.org/message-id/CA%2Bfd4k5_pPAYRTDrO2PbtTOe0eHQpBvuqmCr8ic39uTNmR49Eg%40mail.gmail.com

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