Re: min_safe_lsn column in pg_replication_slots view

On Fri, Jun 19, 2020 at 6:32 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> At Thu, 18 Jun 2020 18:18:37 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > On Thu, Jun 18, 2020 at 11:52 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > >
> > > At Wed, 17 Jun 2020 21:37:55 +0900, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote in
> > > > On 2020/06/15 16:35, Kyotaro Horiguchi wrote:
> > > > Isn't it better to use 1 as the second argument of the above,
> > > > in order to address the issue that I reported upthread?
> > > > Otherwise, the WAL file name that pg_walfile_name(min_safe_lsn)
> > > > returns
> > > > would be confusing.
> > >
> > > Mmm. pg_walfile_name seems too specialize to
> > > pg_stop_backup(). (pg_walfile_name_offset() returns wrong result for
> > > segment boundaries.) I'm not willing to do that only to follow such
> > > suspicious(?) specification, but surely it would practically be better
> > > doing that. Please find the attached first patch.
> > >
> >
> > It is a little unclear to me how this or any proposed patch will solve
> > the original problem reported by Fujii-San? Basically, the problem
> > arises because we don't have an interlock between when the checkpoint
> > removes the WAL segment and the view tries to acquire the same. Am, I
> > missing something?
>
> I'm not sure, but I don't get the point of blocking WAL segment
> removal until the view is completed.
>

I am not suggesting to do that.

> The said columns of the view are
> just for monitoring, which needs an information snapshot seemingly
> taken at a certain time. And InvalidateObsoleteReplicationSlots kills
> walsenders using lastRemovedSegNo of a different time. The two are
> independent each other.
>
> Also the patch changes min_safe_lsn to show an LSN at segment boundary
> + 1.
>

But aren't we doing last_removed_seg+1 even without the patch? See code below

- {
- XLogRecPtr min_safe_lsn;
-
- XLogSegNoOffsetToRecPtr(last_removed_seg + 1, 0,
- wal_segment_size, min_safe_lsn);

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