Re: Offline enabling/disabling of data checksums

From: Michael Banck <michael(dot)banck(at)credativ(dot)de>
To: Magnus Hagander <magnus(at)hagander(dot)net>, Sergei Kornilov <sk(at)zsrv(dot)org>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Offline enabling/disabling of data checksums
Date: 2019-03-15 10:41:55
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Am Mittwoch, den 13.03.2019, 12:24 +0100 schrieb Magnus Hagander:
> On Wed, Mar 13, 2019 at 11:54 AM Sergei Kornilov <sk(at)zsrv(dot)org> wrote:
> > One new question from me: how about replication?
> > Case: primary+replica, we shut down primary and enable checksum, and
> > "started streaming WAL from primary" without any issue. I have
> > master with checksums, but replica without.
> > Or cluster with checksums, then disable checksums on primary, but
> > standby think we have checksums.
> Enabling or disabling the checksums offline on the master quite
> clearly requires a rebuild of the standby, there is no other way (this
> is one of the reasons for the online enabling in that patch, so I
> still hope we can get that done -- but not for this version).
> You would have the same with PITR backups for example.

I'd like to get back to PITR. 

I thought about this a bit and actually I think PITR might be fine in
the sense that if you enabled or disabled the cluster after the last
basebackup and then do PITR with the avaiable WAL beyond that, you would
get a working cluster, just with the checksum state the cluster had at
the time of the basebackup. I think that would be entirely accetable, so
long as nothing else breaks?

I made some quick tests and did see no errors, but maybe I am missing

> And especially if you have some tool that does block or segment level
> differential.

This might be the case, but not sure if PostgreSQL core must worry about
it? Obviously the documentation must be made explicit about these kinds
of cases.


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