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In-Reply-To: <d9546a13-dd86-57d8-3bb4-32136fbb3673@gmail.com>
From: Robert Haas <robertmhaas@gmail.com>
Date: Tue, 15 Nov 2022 15:04:59 -0500
Message-ID: 
 <CA+TgmoaJEDirAMq_9q3B8PrXDf1h72S7hhPr4aotakYe9HLoLw@mail.gmail.com>
Subject: Re: Report checkpoint progress with pg_stat_progress_checkpoint (was:
 Report checkpoint progress in server logs)
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Nitin Jadhav <nitinjadhavpostgres@gmail.com>,
 Andres Freund <andres@anarazel.de>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Ashutosh Sharma <ashu.coek88@gmail.com>,
	Julien Rouhaud <rjuju123@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>
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On Fri, Nov 4, 2022 at 4:27 AM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
> Please find attached a rebase in v7.

I don't think it's a good thing that this patch is using the
progress-reporting machinery. The point of that machinery is that we
want any backend to be able to report progress for any command it
happens to be running, and we don't know which command that will be at
any given point in time, or how many backends will be running any
given command at once. So we need some generic set of counters that
can be repurposed for whatever any particular backend happens to be
doing right at the moment.

But none of that applies to the checkpointer. Any information about
the checkpointer that we want to expose can just be advertised in a
dedicated chunk of shared memory, perhaps even by simply adding it to
CheckpointerShmemStruct. Then you can give the fields whatever names,
types, and sizes you like, and you don't have to do all of this stuff
with mapping down to integers and back. The only real disadvantage
that I can see is then you have to think a bit harder about what the
concurrency model is here, and maybe you end up reimplementing
something similar to what the progress-reporting stuff does for you,
and *maybe* that is a sufficient reason to do it this way.

But I'm doubtful. This feels like a square-peg-round-hole situation.

--
Robert Haas
EDB: http://www.enterprisedb.com