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From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: cleanup patches for incremental backup
Date: Mon, 29 Jan 2024 15:13:21 -0600
Message-ID: <20240129211321.GA3676549@nathanxps13> (raw)
In-Reply-To: <CA+Tgmob50yte0TN-vsR8ZL-CqxsEVNKrN1QNpo7z=hr3kxfTNQ@mail.gmail.com>
References: <CAEze2Wjbf==0=v+Ck8B2vcCeqD_3_SfDV3HtpEQiP-r3e2fsSA@mail.gmail.com>
	<CA+TgmobTfzrnBesboXutZmrABdP79mO2HhXupY5YOdZ9VLWFog@mail.gmail.com>
	<CAEze2WjhdVCqEe_qqEok3NA6DwUdOGSBjAxzmYdAqiaaH1uRcg@mail.gmail.com>
	<CA+TgmoaQHtGNFczJzSO5sMobUpuikSHphtFPZpdLmjzwdmM1DQ@mail.gmail.com>
	<20240124170846.GA2643050@nathanxps13>
	<CA+TgmoZusu5g3rM1k=UB29Sf53c1OKjm1uFw9uo-sSXBLFZJiQ@mail.gmail.com>
	<8aed364e-f572-4561-b4fc-ff149067b670@gmail.com>
	<20240127163109.GA3047116@nathanxps13>
	<20240129182130.GA3670200@nathanxps13>
	<CA+Tgmob50yte0TN-vsR8ZL-CqxsEVNKrN1QNpo7z=hr3kxfTNQ@mail.gmail.com>

On Mon, Jan 29, 2024 at 03:18:50PM -0500, Robert Haas wrote:
> I'm wondering if what we need to do is run pg_walsummary on both
> summary files in that case. If we just pick one or the other, how do
> we know which one to pick?

Even if we do that, isn't it possible that none of the summaries will
include the change?  Presently, we get the latest summarized LSN, make a
change, and then wait for the next summary file with a greater LSN than
what we saw before the change.  But AFAICT there's no guarantee that means
the change has been summarized yet, although the chances of that happening
in a test are probably pretty small.

Could we get the LSN before and after making the change and then inspect
all summaries that include that LSN range?

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com





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