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

On Sun, 17 Jan 2021 11:33:52 +0530
Dilip Kumar <dilipbalaut@gmail.com> wrote:

> On Thu, Jan 14, 2021 at 6:49 PM Yugo NAGATA <nagata@sraoss.co.jp> wrote:
> >
> > On Wed, 13 Jan 2021 17:49:43 +0530
> > 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.
> >
> > Yes, there is no stuck on waiting for the WAL. However, it can be stuck during resolving
> > a recovery conflict. The process could wait for max_standby_streaming_delay or
> > max_standby_archive_delay at most before recovery get completely paused.
> 
> Okay, I agree that it is possible so for handling this we have a
> couple of options
> 1. pg_is_wal_replay_paused(), interface will wait for recovery to
> actually get paused, but user have an option to cancel that.  So I
> agree that there is currently no option to just know that recovery
> pause is requested without waiting for its actually get paused if it
> is requested.  So one option is we can provide an another interface as
> you mentioned pg_is_wal_replay_paluse_requeseted(), which can just
> return the request status.  I am not sure how useful it is.

If it is acceptable that pg_is_wal_replay_paused() makes users wait, 
I'm ok for the current interface. I don't feel the need of
pg_is_wal_replay_paluse_requeseted().

> 
> 2. Pass an option to pg_is_wal_replay_paused whether to wait for
> recovery to actually get paused or not.
> 
> 3. Pass an option to pg_wal_replay_pause(), whether to wait for
> recovery pause or just request and return.
> 
> I like the option 1, any other opinion on this?
> 
> > Also, it could wait for recovery_min_apply_delay if it has a valid value. It is possible
> > that a user set this parameter to a large value, so it could wait for a long time. However,
> > this will be avoided by calling recoveryPausesHere() or CheckAndSetRecoveryPause() in
> > recoveryApplyDelay().
> 
> Right

Is there any reason not to do it?

> 
> > > > > > > > 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.
> >
> > Thanks.  I confirmed that I can cancel pg_is_wal_repaly_paused() during stuck.
> 
> Thanks
> 
> > Although it is a very trivial comment, I think that the new line before
> > HandleStartupProcInterrupts() is unnecessary.
> >
> > @@ -6052,12 +6062,20 @@ recoveryPausesHere(bool endOfRecovery)
> >                                 (errmsg("recovery has paused"),
> >                                  errhint("Execute pg_wal_replay_resume() to continue.")));
> >
> > -       while (RecoveryIsPaused())
> > +       while (RecoveryPauseRequested())
> >         {
> > +
> >                 HandleStartupProcInterrupts();
> >
> >
> 
> I will fix in the next version.
> 
> -- 
> Regards,
> Dilip Kumar
> EnterpriseDB: http://www.enterprisedb.com


-- 
Yugo NAGATA <nagata@sraoss.co.jp>