Re: Is Recovery actually paused?

On Tue, 19 Jan 2021 at 8:12 AM, Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp> wrote:

> On Sun, 17 Jan 2021 11:33:52 +0530
> Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> > On Thu, Jan 14, 2021 at 6:49 PM Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp> wrote:
> > >
> > > On Wed, 13 Jan 2021 17:49:43 +0530
> > > Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > > On Wed, Jan 13, 2021 at 3:35 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com>
> wrote:
> > > > >
> > > > > On Wed, Jan 13, 2021 at 3:27 PM Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>
> wrote:
> > > > > >
> > > > > > On Thu, 10 Dec 2020 11:25:23 +0530
> > > > > > Dilip Kumar <dilipbalaut(at)gmail(dot)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?

I think I missed that.. I will do in the next version

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