Re: Is Recovery actually paused? - Mailing list pgsql-hackers

On Sun, Jan 17, 2021 at 5:19 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Sat, Jan 16, 2021 at 8:59 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Jan 13, 2021 at 9:20 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > >
> > > On Wed, Jan 13, 2021 at 3:35 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > > >
> > > > On Wed, Jan 13, 2021 at 3:27 PM Yugo NAGATA <nagata@sraoss.co.jp> wrote:
> > > > >
> > > > > On Thu, 10 Dec 2020 11:25:23 +0530
> > > > > Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > > > >
> > > > > > > > However, I wonder users don't expect pg_is_wal_replay_paused to wait.
> > > > > > > > Especially, if max_standby_streaming_delay is -1, this will be blocked forever,
> > > > > > > > although this setting may not be usual. In addition, some users may set
> > > > > > > > recovery_min_apply_delay for a large.  If such users call pg_is_wal_replay_paused,
> > > > > > > > it could wait for a long time.
> > > > > > > >
> > > > > > > > At least, I think we need some descriptions on document to explain
> > > > > > > > pg_is_wal_replay_paused could wait while a time.
> > > > > > >
> > > > > > > Ok
> > > > > >
> > > > > > Fixed this, added some comments in .sgml as well as in function header
> > > > >
> > > > > Thank you for fixing this.
> > > > >
> > > > > Also, is it better to fix the description of pg_wal_replay_pause from
> > > > > "Pauses recovery." to "Request to pause recovery." in according with
> > > > > pg_is_wal_replay_paused?
> > > >
> > > > Okay
> > > >
> > > > >
> > > > > > > > Also, how about adding a new boolean argument to pg_is_wal_replay_paused to
> > > > > > > > control whether this waits for recovery to get paused or not? By setting its
> > > > > > > > default value to true or false, users can use the old format for calling this
> > > > > > > > and the backward compatibility can be maintained.
> > > > > > >
> > > > > > > So basically, if the wait_recovery_pause flag is false then we will
> > > > > > > immediately return true if the pause is requested?  I agree that it is
> > > > > > > good to have an API to know whether the recovery pause is requested or
> > > > > > > not but I am not sure is it good idea to make this API serve both the
> > > > > > > purpose?  Anyone else have any thoughts on this?
> > > > > > >
> > > > >
> > > > > I think the current pg_is_wal_replay_paused() already has another purpose;
> > > > > this waits recovery to actually get paused. If we want to limit this API's
> > > > > purpose only to return the pause state, it seems better to fix this to return
> > > > > the actual state at the cost of lacking the backward compatibility. If we want
> > > > > to know whether pause is requested, we may add a new API like
> > > > > pg_is_wal_replay_paluse_requeseted(). Also, if we want to wait recovery to actually
> > > > > get paused, we may add an option to pg_wal_replay_pause() for this purpose.
> > > > >
> > > > > However, this might be a bikeshedding. If anyone don't care that
> > > > > pg_is_wal_replay_paused() can make user wait for a long time, I don't care either.
> > > >
> > > > I don't think that it will be blocked ever, because
> > > > pg_wal_replay_pause is sending the WakeupRecovery() which means the
> > > > recovery process will not be stuck on waiting for the WAL.
> > > >
> > > > > > > > As another comment, while pg_is_wal_replay_paused is blocking, I can not cancel
> > > > > > > > the query. I think CHECK_FOR_INTERRUPTS() is necessary in the waiting loop.
> > > > >
> > > > > How about this fix? I think users may want to cancel pg_is_wal_replay_paused() during
> > > > > this is blocking.
> > > >
> > > > Yeah, we can do this.  I will send the updated patch after putting
> > > > some more thought into these comments.  Thanks again for the feedback.
> > > >
> > >
> > > Please find the updated patch.
> >
> > I've looked at the patch. Here are review comments:
> >
> > +       /* Recovery pause state */
> > +       RecoveryPauseState              recoveryPause;
> >
> > Now that the value can have tri-state, how about renaming it to
> > recoveryPauseState?
>
> This makes sense to me.
>
> > ---
> >  bool
> >  RecoveryIsPaused(void)
> > +{
> > +       bool    recoveryPause;
> > +
> > +       SpinLockAcquire(&XLogCtl->info_lck);
> > +       recoveryPause = (XLogCtl->recoveryPause == RECOVERY_PAUSED) ?
> > true : false;
> > +       SpinLockRelease(&XLogCtl->info_lck);
> > +
> > +       return recoveryPause;
> > +}
> > +
> > +bool
> > +RecoveryPauseRequested(void)
> >  {
> >         bool            recoveryPause;
> >
> >         SpinLockAcquire(&XLogCtl->info_lck);
> > -       recoveryPause = XLogCtl->recoveryPause;
> > +       recoveryPause = (XLogCtl->recoveryPause !=
> > RECOVERY_IN_PROGRESS) ? true : false;
> >         SpinLockRelease(&XLogCtl->info_lck);
> >
> >         return recoveryPause;
> >  }
> >
> > We can write like recoveryPause = (XLogCtl->recoveryPause == RECOVERY_PAUSED);
>
> In RecoveryPauseRequested, we just want to know whether the pause is
> requested or not, even if the pause requested and not yet pause then
> also we want to return true. So how
> recoveryPause = (XLogCtl->recoveryPause == RECOVERY_PAUSED) will work?

Sorry for the late response.

What I wanted to say is that the ternary operator is not necessary in
those cases.

The codes,

recoveryPause = (XLogCtl->recoveryPause == RECOVERY_PAUSED) ? true : false;
recoveryPause = (XLogCtl->recoveryPause != RECOVERY_IN_PROGRESS) ? true : false;

are equivalent with,

recoveryPause = (XLogCtl->recoveryPause == RECOVERY_PAUSED);
recoveryPause = (XLogCtl->recoveryPause != RECOVERY_IN_PROGRESS);

respectively.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/