Date: Tue, 28 Aug 2018 21:59:45 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Stephen Frost <sfrost@snowman.net>
Cc: Andres Freund <andres@anarazel.de>, pgsql-bugs@lists.postgresql.org,
	Alexander Kukushkin <cyberdemn@gmail.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>
Subject: Re: BUG #15346: Replica fails to start after the crash
Message-ID: <20180828125945.GG29157@paquier.xyz>
References: 
 <CA+q6zcVjv1Lp-3=prBbpq2CbBioK91SHarfw3F8FHuUN4EwcUA@mail.gmail.com>
 <20180822175034.lnvwk5js32k6mjyz@alvherre.pgsql>
 <20180823041420.GA1158@paquier.xyz>
 <CAFh8B==4gpwor6ODercru0iifqwd0nQrdRvXzmorWkMRQa4q=g@mail.gmail.com>
 <CAFh8B=nHdsX83hTZMqGqbht39ud+DRQOv4_Te+xu3DuPssQbZA@mail.gmail.com>
 <CAFh8B=mYGP1J8ueHKU40Dr4bNBUPJj1rTbZ6Wpg4gR30uCRunA@mail.gmail.com>
 <20180828024409.GB29157@paquier.xyz>
 <3B69F535-FD81-461B-97E9-657D4C99EDB4@anarazel.de>
 <20180828122311.GD3326@tamriel.snowman.net>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="OpLPJvDmhXTZE4Lg"
Content-Disposition: inline
In-Reply-To: <20180828122311.GD3326@tamriel.snowman.net>
Precedence: bulk


--OpLPJvDmhXTZE4Lg
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Tue, Aug 28, 2018 at 08:23:11AM -0400, Stephen Frost wrote:
> * Andres Freund (andres@anarazel.de) wrote:
>> Uh, where is that "control file last" bit coming from?
>=20
> pg_basebackup copies it last.  The comments should probably be improved
> as to *why* but my recollection is that it's, at least in part, to
> ensure the new cluster can't be used until it's actually a complete
> backup.

What we have now is mainly in basebackup.c.  See 8366c780 which
introduced that.  Stephen has that right, as we cannot rely on an
end-backup record when taking a backup from a standby, copying the
control file last ensures that the consistent point should be late
enough that no other pages are inconsistent.  Even with that, I think
that there is still a small race condition but I cannot put my finger on
it now.  I agree that the current comments do a bad job as to why this
happens.  That's actually something I discovered when discussing what
has resulted in f267c1c2.
--
Michael

--OpLPJvDmhXTZE4Lg
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----
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=BQlz
-----END PGP SIGNATURE-----

--OpLPJvDmhXTZE4Lg--