Re: Is Recovery actually paused?

On Tue, Oct 20, 2020 at 1:22 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Tue, Oct 20, 2020 at 1:11 PM Simon Riggs <simon@2ndquadrant.com> wrote:
> >
> > On Mon, 19 Oct 2020 at 15:11, Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > > We have an interface to pause the WAL replay (pg_wal_replay_pause) and
> > > to know whether the WAL replay pause is requested
> > > (pg_is_wal_replay_paused).  But there is no way to know whether the
> > > recovery is actually paused or not.  Actually, the recovery process
> > > might process an extra WAL before pausing the recovery.  So does it
> > > make sense to provide a new interface to tell whether the recovery is
> > > actually paused or not?
> > >
> > > One solution could be that we convert the XLogCtlData->recoveryPause
> > > from bool to tri-state variable (0-> recovery not paused 1-> pause
> > > requested 2-> actually paused).
> > >
> > > Any opinion on this?
> >
> > Why would we want this? What problem are you trying to solve?
>
> The requirement is to know the last replayed WAL on the standby so
> unless we can guarantee that the recovery is actually paused we can
> never get the safe last_replay_lsn value.
>
> > If we do care, why not fix pg_is_wal_replay_paused() so it responds as you wish?
>
> Maybe we can also do that but pg_is_wal_replay_paused is an existing
> API and the behavior is to know whether the recovery paused is
> requested or not,  So I am not sure is it a good idea to change the
> behavior of the existing API?
>

Attached is the POC patch to show what I have in mind.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com