Message-ID: <1553011697.9697.55.camel@credativ.de>
Subject: Re: Offline enabling/disabling of data checksums
From: Michael Banck <michael.banck@credativ.de>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>, Magnus Hagander
 <magnus@hagander.net>,  Fabien COELHO <coelho@cri.ensmp.fr>, Sergei
 Kornilov <sk@zsrv.org>, PostgreSQL Developers
 <pgsql-hackers@lists.postgresql.org>
Date: Tue, 19 Mar 2019 17:08:17 +0100
In-Reply-To: <20190319160037.baalicue7ovkhvb2@alap3.anarazel.de>
References: <20190314043936.GH3493@paquier.xyz>
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	 <1552577180.9697.4.camel@credativ.de> <20190315025027.GN3493@paquier.xyz>
	 <1552650841.9697.22.camel@credativ.de> <20190315122309.GA1434@paquier.xyz>
	 <1552653447.9697.24.camel@credativ.de> <20190318081301.GH1885@paquier.xyz>
	 <20190319153626.vgxcem5r65yhydck@alap3.anarazel.de>
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Organization: credativ GmbH
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Hi,

Am Dienstag, den 19.03.2019, 09:00 -0700 schrieb Andres Freund:
> On 2019-03-19 16:55:12 +0100, Michael Banck wrote:
> > Am Dienstag, den 19.03.2019, 08:36 -0700 schrieb Andres Freund:
> > > On 2019-03-18 17:13:01 +0900, Michael Paquier wrote:
> > > > +/*
> > > > + * Locations of persistent and temporary control files.  The control
> > > > + * file gets renamed into a temporary location when enabling checksums
> > > > + * to prevent a parallel startup of Postgres.
> > > > + */
> > > > +#define CONTROL_FILE_PATH		"global/pg_control"
> > > > +#define CONTROL_FILE_PATH_TEMP	CONTROL_FILE_PATH ".pg_checksums_in_progress"
> > > 
> > > I think this should be outright rejected. Again, you're making the
> > > control file into something it isn't. And there's no buyin for this as
> > > far as I can tell outside of Fabien and you. For crying out loud, if the
> > > server crashes during this YOU'VE CORRUPTED THE CLUSTER.
> > 
> > The cluster is supposed to be offline during this.  This is just an
> > additional precaution so that nobody starts it during the operation -
> > similar to how pg_upgrade disables the old data directory.
> 
> I don't see how that matters. Afterwards the cluster needs low level
> surgery to be recovered. That's a) undocumented b) likely to be done
> wrongly.  This is completely unacceptable *AND UNNECESSARY*.

Can you explain why low level surgery is needed and how that would look
like?

If pg_checksums successfully enables checksums, it will move back the
control file and update the checksum version - the cluster is ready to
be started again unless I am missing something?

If pg_checksums is interrupted by the admin, it will move back the
control file and the cluster is ready to be started again as well.

If pg_checksums aborts with a failure, the admin will have to move back
the control file before starting up the instance again, but I don't
think that counts?

If pg_checksums crashes due to I/O failures or other causes I can see
how possibly the block it was currently writing might need low level
surgery, but in that case we are in the domain of forensics already I
guess and that still does not pertain to the control file?

Sorry for being obtuse, I don't get it.


Michael

-- 
Michael Banck
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