Date: Fri, 18 Feb 2022 01:27:24 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: Nitin Jadhav <nitinjadhavpostgres@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	Bruce Momjian <bruce@momjian.us>, Tom Lane <tgl@sss.pgh.pa.us>,
	Magnus Hagander <magnus@hagander.net>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Report checkpoint progress with pg_stat_progress_checkpoint
 (was: Report checkpoint progress in server logs)
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Hi,

On Thu, Feb 17, 2022 at 10:39:02PM +0530, Nitin Jadhav wrote:
> > > Thank you for sharing the information.  'triggering backend PID' (int)
> > > - can be stored without any problem.
> >
> > There can be multiple processes triggering a checkpoint, or at least wanting it
> > to happen or happen faster.
> 
> Yes. There can be multiple processes but there will be one checkpoint
> operation at a time. So the backend PID corresponds to the current
> checkpoint operation. Let me know if I am missing something.

If there's a checkpoint timed triggered and then someone calls
pg_start_backup() which then wait for the end of the current checkpoint
(possibly after changing the flags), I think the view should reflect that in
some way.  Maybe storing an array of (pid, flags) is too much, but at least a
counter with the number of processes actively waiting for the end of the
checkpoint.

> > > 'checkpoint or restartpoint?'
> >
> > Do you actually need to store that?  Can't it be inferred from
> > pg_is_in_recovery()?
> 
> AFAIK we cannot use pg_is_in_recovery() to predict whether it is a
> checkpoint or restartpoint because if the system exits from recovery
> mode during restartpoint then any query to pg_stat_progress_checkpoint
> view will return it as a checkpoint which is ideally not correct. Please
> correct me if I am wrong.

Recovery ends with an end-of-recovery checkpoint that has to finish before the
promotion can happen, so I don't think that a restart can still be in progress
if pg_is_in_recovery() returns false.